Visual C++ What's New 2003 through 2015

Visual Studio 2015
 

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The latest version of this topic can be found at Visual C++ What's New 2003 through 2015.

In Visual C++ 2015, ongoing improvements to compiler conformance can sometimes change how the compiler understands your existing source code. When this happens, you might encounter new or different errors during your build, or even behavioral differences in code that previously built and seemed to run correctly.

Fortunately, these differences have little or no impact on most of your source code and when source code or other changes are needed to address these differences, fixes are usually small and straight-forward. We've included many examples of previously-acceptable source code that might need to be changed (before) and the fixes to correct them (after).

Although these differences can affect your source code or other build artifacts, they don't affect binary compatibility between updates to Visual C++ versions. A more-severe kind of change, the breaking change can affect binary compatibility, but these kinds of binary compatibility breaks only occur between major versions of Visual C++. For example, between Visual C++ 2013 and Visual C++ 2015. For information on the breaking changes that occurred between Visual C++ 2013 and Visual C++ 2015, see Visual C++ change history 2003 - 2015.

  • /Zc:forScope- option

    The compiler option /Zc:forScope- is deprecated and will be removed in a future release.

    Command line warning  D9035: option 'Zc:forScope-' has been deprecated and will be removed in a future release  
    
    

    The option was usually used in order to allow nonstandard code that uses loop variables after the point where, according to the standard, they should have gone out of scope. It was only necessary when you are compiling with the /Za option, since without /Za, using a for loop variable after the end of the loop is always allowed. If you don't care about standards conformance (for example, if your code isn't meant to portable to other compilers), you could turn off the /Za option (or set the Disable Language Extensions property to No). If you do care about writing portable, standards-compliant code, you should rewrite your code so that it conforms to the standard by moving the declaration of such variables to a point outside the loop.

    // zc_forScope.cpp  
    // compile with: /Zc:forScope- /Za  
    // C2065 expected  
    int main() {  
       // Uncomment the following line to resolve.  
       // int i;  
       for (int i =0; i < 1; i++)  
          ;  
       i = 20;   // i has already gone out of scope under /Za  
    }  
    
    
  • /Zg compiler option

    The /Zg compiler option (Generate Function Prototypes) is no longer available. This compiler option was previously deprecated.

  • You can no longer run unit tests with C++/CLI from the command-line with mstest.exe. Instead, use vstest.console.exe. See VSTest.Console.exe command-line options.

  • mutable keyword

    The mutable storage class specifier is no longer allowed in places where previously it compiled without error. Now, the compiler gives error C2071 (illegal storage class). According to the standard, the mutable specifier can be applied only to names of class data members, and cannot be applied to names declared const or static, and cannot be applied to reference members.

    For example, consider the following code:

    struct S {  
        mutable int &r;  
    };  
    
    

    Previous versions of the Visual C++ compiler accepted this, but now the compiler gives the following error:

    error C2071: 'S::r': illegal storage class  
    
    

    To fix the error, simply remove the redundant mutable keyword.

  • char_16_t and char32_t

    You can no longer use char16_t or char32_t as aliases in a typedef, because these types are now treated as built-in. It was common for users and library authors to define char16_t and char32_t as aliases of uint16_t and uint32_t, respectively.

    #include <cstdint>  
    
    typedef uint16_t char16_t; //C2628  
    typedef uint32_t char32_t; //C2628  
    
    int main(int argc, char* argv[])  
    {  
    uint16_t x = 1; uint32_t y = 2;  
    char16_t a = x;   
    char32_t b = y;   
    return 0;  
    }  
    
    

    To update your code, remove the typedef declarations and rename any other identifiers that collide with these names.

  • Non-type template parameters

    Certain code that involves non-type template parameters is now correctly checked for type compatibility when you provide explicit template arguments. For example, the following code compiled without error in previous versions of Visual C++.

    struct S1  
    {  
    void f(int);  
    void f(int, int);  
    };  
    
    struct S2  
    {  
    template <class C, void (C::*Function)(int) const> void f() {}  
    };  
    
    void f()  
    {  
    S2 s2;  
    s2.f<S1, &S1::f>();  
    }  
    
    
    

    The current compiler correctly gives an error, because the template parameter type doesn't match the template argument (the parameter is a pointer to a const member, but the function f is non-const):

    error C2893: Failed to specialize function template 'void S2::f(void)'note: With the following template arguments:note: 'C=S1'note: 'Function=S1::f'  
    
    

    To address this error in your code, make sure that the type of the template argument you use matches the declared type of the template parameter.

  • __declspec(align)

    The compiler no longer accepts __declspec(align) on functions. This was always ignored, but now it produces a compiler error.

    error C3323: 'alignas' and '__declspec(align)' are not allowed on function declarations  
    
    

    To fix this problem, remove __declspec(align) from the function declaration. Since it had no effect, removing it does not change anything.

  • Exception handling

    There are a couple of changes to exception handling. First, exception objects have to be either copyable or movable. The following code compiled in Visual C++ in Visual Studio 2013, but does not compile in Visual C++ in Visual Studio 2015:

    struct S {  
    public:  
        S();  
    private:  
        S(const S &);  
    };  
    
    int main()  
    {  
        throw S(); // error  
    }  
    
    
    

    The problem is that the copy constructor is private, so the object cannot be copied as happens in the normal course of handling an exception. The same applies when the copy constructor is declared explicit.

    struct S {  
        S();  
        explicit S(const S &);  
    };  
    
    int main()  
    {  
        throw S(); // error  
    }  
    
    
    

    To update your code, make sure that the copy constructor for your exception object is public and not marked explicit.

    Catching an exception by value also requires the exception object to be copyable. The following code compiled in Visual C++ in Visual Studio 2013, but does not compile in Visual C++ in Visual Studio 2015:

    struct B {  
    public:  
        B();  
    private:  
        B(const B &);  
    };  
    
    struct D : public B {  
    };  
    
    int main()  
    {  
        try  
        {  
        }  
        catch (D d) // error  
        {  
        }  
    }  
    
    
    

    You can fix this issue by changing the parameter type for the catch to a reference.

    catch(D& d)  
    {  
    }  
    
    
  • String literals followed by macros

    The compiler now supports user defined literals. As a consequence, string literals followed by macros without any intervening whitespace are interpreted as user-defined literals, which might produce errors or unexpected results. For example, in previous compilers the following code compiled successfully:

    #define _x "there"  
    char* func() {  
        return "hello"_x;  
    }  
    int main()  
    {  
        char * p = func();  
        return 0;  
    }  
    
    

    The compiler interpreted this as a string literal "hello" followed by a macro, which is expanded "there", and then the two string literals were concatenated into one. In Visual C++ in Visual Studio 2015, the compiler interprets this as a user-defined literal, but since there is no matching user-defined literal _x defined, it gives an error.

    error C3688: invalid literal suffix '_x'; literal operator or literal operator template 'operator ""_x' not found  
    note: Did you forget a space between the string literal and the prefix of the following string literal?  
    
    
    

    To fix this problem, add a space between the string literal and the macro.

  • Adjacent string literals

    Similarly to the previous, due to related changes in string parsing, adjacent string literals (either wide or narrow character string literals) without any whitespace were interpreted as a single concatenated string in previous releases of Visaul C++. In Visual C++ in Visual Studio 2015, you must now add whitespace between the two strings. For example, the following code must be changed:

    char * str = "abc""def";  
    
    

    Simply add a space in between the two strings.

    char * str = "abc" "def";  
    
    
  • Placement new and delete

    A change has been made to the delete operator in order to bring it into conformance with C++14 standard. Details of the standards change can be found at C++ Sized Deallocation. The changes add a form of the global delete operator that takes a size parameter. The breaking change is that if you were previously using an operator delete with the same signature (to correspond with a placement new operator), you will receive a compiler error (C2956, which occurs at the point where the placement new is used, since that's the position in code where the compiler tries to identify an appropriate matching delete operator).

    The function void operator delete(void *, size_t) was a placement delete operator corresponding to the placement new function "void * operator new(size_t, size_t)" in C++11. With C++14 sized deallocation, this delete function is now a usual deallocation function (global delete operator). The standard requires that if the use of a placement new looks up a corresponding delete function and finds a usual deallocation function, the program is ill-formed.

    For example, suppose your code defines both a placement new and a placement delete:

    void * operator new(std::size_t, std::size_t);  
    void operator delete(void*, std::size_t) noexcept;  
    
    
    

    The problem occurs because of the match in function signatures between a placement delete operator you've defined, and the new global sized delete operator. Consider whether you can use a different type other than size_t for any placement new and delete operators. Note that the type of the size_t typedef is compiler-dependent; it is a typedef for unsigned int in Visual C++. A good solution is to use an enumerated type such as this:

    enum class my_type : size_t {};  
    
    
    

    Then, change your definition of placement new and delete to use this type as the second argument instead of size_t. You’ll also need to update the calls to placement new to pass the new type (for example, by using static_cast<my_type> to convert from the integer value) and update the definition of new and delete to cast back to the integer type. You don’t need to use an enum for this; a class type with a size_t member would also work.

    An alternative solution is that you might be able to eliminate the placement new altogether. If your code uses placement new to implement a memory pool where the placement argument is the size of the object being allocated or deleted, then sized deallocation feature might be suitable to replace your own custom memory pool code, and you can get rid of the placement functions and just use your own two-argument delete operator instead of the placement functions.

    If you don't want to update your code immediately, you can revert to the old behavior by using the compiler option /Zc:sizedDealloc-. If you use this option, the two-argument delete functions don’t exist and won't cause a conflict with your placement delete operator.

  • Union data members

    Data members of unions can no longer have reference types. The following code compiled successfully in Visual C++ in Visual Studio 2013, but produces an error in Visual C++ in Visual Studio 2015.

    union U1 {  
        const int i;  
    };  
    union U2 {  
       int &i;  
    };  
    union U3 {  
        struct {int &i;};  
    };  
    
    

    The preceding code produces the following errors:

    test.cpp(67): error C2625: 'U2::i': illegal union member; type 'int &' is reference type  
    test.cpp(70): error C2625: 'U3::i': illegal union member; type 'int &' is reference type  
    
    

    To address this issue, change reference types either to a pointer or a value. Changing the type to a pointer requires changes in the code that uses the union field. Changing the code to a value would change the data stored in the union, which affects other fields since fields in union types share the same memory. Depending on the size of the value, it might also change the size of the union.

  • Anonymous unions are now more conformant to the standard. Previous versions of the compiler generated an explicit constructor and destructor for anonymous unions. These are deleted in Visual C++ in Visual Studio 2015.

    struct S {  
      S();  
     };  
    
     union {  
      struct {  
       S s;  
      };  
     } u; // C2280  
    
    

    The preceding code generates the following error in Visual C++ in Visual Studio 2015:

    error C2280: '<unnamed-type-u>::<unnamed-type-u>(void)': attempting to reference a deleted function  
    note: compiler has generated '<unnamed-type-u>::<unnamed-type-u>' here  
    
    

    To resolve this issue, provide your own definitions of the constructor and/or destructor.

    struct S {  
    // Provide a default constructor by adding an empty function body.  
    S() {}   
    };  
    
    union {  
    struct {  
    S s;  
    };  
    } u;  
    
    
  • Unions with anonymous structs

    In order to conform with the standard, the runtime behavior has changed for members of anonymous structures in unions. The constructor for anonymous structure members in a union is no longer implicitly called when such a union is created. Also, the destructor for anonymous structure members in a union is no longer implicitly called when the union goes out of scope. Consider the following code, in which a union U contains an anonymous structure that contains a member which is a named structure S that has a destructor.

    #include <stdio.h>  
    struct S {  
        S() { printf("Creating S\n"); }  
        ~S(){ printf("Destroying S\n"); }  
    };  
    union U {  
        struct {  
        S s;  
    };  
        U() {}  
        ~U(){}  
    };  
    
    void f()  
    {  
        U u;  
        // Destructor implicitly called here.  
    }  
    
    int main()  
    {  
        f();  
    
        char s[1024];  
        printf("Press any key.\n");  
        gets_s(s);  
        return 0;  
    }  
    
    

    In Visual C++ in Visual Studio 2013, the constructor for S is called when the union is created, and the destructor for S is called when the stack for function f is cleaned up. But in Visual C++ in Visual Studio 2015, the constructor and destructor are not called. The compiler gives a warning about this behavior change.

    warning C4587: 'U::s': behavior change: constructor is no longer implicitly calledwarning C4588: 'U::s': behavior change: destructor is no longer implicitly called  
    
    

    To restore the original behavior, give the anonymous structure a name. The runtime behavior of non-anonymous structures is the same, regardless of the compiler version.

    #include <stdio.h>  
    
    struct S {  
        S() { printf("Creating S.\n"); }  
        ~S() { printf("Destroying S\n"); }  
    };  
    union U {  
        struct {  
            S s;  
        } namedStruct;  
        U() {}  
        ~U() {}  
    };  
    
    void f()  
    {  
        U u;  
    }  
    
    int main()  
    {  
        f();  
    
        char s[1024];  
        printf("Press any key.\n");  
        gets_s(s);  
        return 0;  
    }  
    
    

    Alternatively, try moving the constructor and destructor code into new functions, and add calls to these functions from the constructor and destructor for the union.

    #include <stdio.h>  
    
    struct S {  
        void Create() { printf("Creating S.\n"); }  
        void Destroy() { printf("Destroying S\n"); }  
    };  
    union U {  
        struct {  
            S s;  
        };  
        U() { s.Create();  }  
        ~U() { s.Destroy(); }  
    };  
    
    void f()  
    {  
        U u;  
    }  
    
    int main()  
    {  
        f();  
    
    char s[1024];  
    printf("Press any key.\n");  
    gets_s(s);  
    return 0;  
    }  
    
    
  • Template resolution

    Changes have been made to name resolution for templates. In C++, when considering candidates for the resolution of a name, it can be the case that one or more names under consideration as potential matches produces an invalid template instantiation. These invalid instantiations do not normally cause compiler errors, a principle which is known as SFINAE (Substitution Failure Is Not An Error).

    Now, if SFINAE requires the compiler to instantiate the specialization of a class template, then any errors that occur during this process are compiler errors. In previous versions, the compiler would ignore such errors. For example, consider the following code:

    #include <type_traits>  
    
    template<typename T>  
    struct S  
    {  
    S() = default;  
    S(const S&);  
    S(S&&);  
    
    template<typename U, typename = typename std::enable_if<std::is_base_of<T, U>::value>::type>  
    S(S<U>&&);  
    };  
    
    struct D;  
    
    void f1()  
    {  
    S<D> s1;  
        S<D> s2(s1);  
    }  
    
    struct B  
    {  
    };  
    
    struct D : public B  
    {  
    };  
    
    void f2()  
    {  
    S<D> s1;  
        S<D> s2(s1);  
    }  
    
    
    

    If you compile with the current compiler, you get the following error:

    type_traits(1110): error C2139: 'D': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_base_of'  
    ..\t331.cpp(14): note: see declaration of 'D'  
    ..\t331.cpp(10): note: see reference to class template instantiation 'std::is_base_of<T,U>' being compiled  
            with  
            [  
                T=D,  
                U=D  
            ]  
    
    

    This is because at the point of the first invocation of the is_base_of the class 'D' has not yet been defined.

    In this case, the fix is not to use such type traits until the class has been defined. If you move the definitions of B and D to the beginning of the code file, the error is resolved. If the definitions are in header files, check the order of the include statements for the header files to make sure that any class definitions are compiled before the problematic templates are used.

  • Copy constructors

    In both Visual Studio 2013 and Visual Studio 2015, the compiler generates a copy constructor for a class if that class has a user-defined move constructor but no user-defined copy constructor. In Dev14, this implicitly generated copy constructor is also marked "= delete".

  • Private virtual base classes and indirect inheritance

    Previous versions of the compiler allowed a derived class to call member functions of its indirectly-derivedprivate virtual base classes. This old behavior was incorrect and does not conform to the C++ standard. The compiler no longer accepts code written in this way and issues compiler error C2280 as a result.

    error C2280: 'void *S3::__delDtor(unsigned int)': attempting to reference a deleted function  
    
    

    Example (before)

    class base  
    {  
    protected:  
        base();  
        ~base();  
    };  
    
    class middle: private virtual base {};class top: public virtual middle {};  
    
    void destroy(top *p)  
    {  
        delete p;  //   
    }  
    
    

    Example (after)

    class base;  // as above  
    
    class middle: protected virtual base {};  
    class top: public virtual middle {};  
    
    void destroy(top *p)  
    {  
        delete p;  
    }  
    
    

    -or-

    class base;  // as above  
    
    class middle: private virtual base {};  
    class top: public virtual middle, private virtual bottom {};  
    
    void destroy(top *p)  
    {  
        delete p;  
    }  
    
    
  • Overloaded operator new and operator delete

    Previous versions of the compiler allowed non-member operator new and non-member operator delete to be declared static, and to be declared in namespaces other than the global namespace. This old behavior created a risk that the program would not call the new or delete operator implementation that the programmer intended, resulting in silent bad runtime behavior. The compiler no longer accepts code written in this way and issues compiler error C2323 instead.

    error C2323: 'operator new': non-member operator new or delete functions may not be declared static or in a namespace other than the global namespace.  
    
    

    Example (before)

    
    static inline void * __cdecl operator new(size_t cb, const std::nothrow_t&)  // error C2323  
    
    

    Example (after)

    
    void * __cdecl operator new(size_t cb, const std::nothrow_t&)  // removed 'static inline'  
    
    

    Additionally, although the compiler doesn't give a specific diagnostic, inline operator new is considered ill-formed.

  • Calling 'operator type()' (user-defined conversion) on non-class types

    Previous versions of the compiler allowed 'operator type()' to be called on non-class types while silently ignoring it. This old behavior created a risk of silent bad code generation, resulting in unpredictable runtime behavior. The compiler no longer accepts code written in this way and issues compiler error C2228 instead.

    error C2228: left of '.operator type' must have class/struct/union  
    
    

    Example (before)

    typedef int index_t;  
    
    void bounds_check(index_t index);  
    
    void login(int column)  
    {  
        bounds_check(column.operator index_t());  // error C2228  
    }  
    
    

    Example (after)

    typedef int index_t;  
    
    void bounds_check(index_t index);  
    
    void login(int column)  
    {  
        bounds_check(column);  // removed cast to 'index_t', 'index_t' is an alias of 'int'  
    }  
    
    
  • Redundant typename in elaborated type specifiers

    Previous versions of the compiler allowed typename in an elaborated type specifiers; code written in this way is semantically incorrect. The compiler no longer accepts code written in this way and issues compiler error C3406 instead.

    error C3406: 'typename' cannot be used in an elaborated type specifier  
    
    

    Example (before)

    template <typename class T>  
    class container;  
    
    

    Example (after)

    template <class T>  // alternatively, could be 'template <typename T>'; 'typename' is not elaborating a type specifier in this case  
    class container;  
    
    
  • Type deduction of arrays from an initializer list

    Previous versions of the compiler did not support type deduction of arrays from an initializer list. The compiler now supports this form of type deduction and, as a result, calls to function templates using initializer lists might now be ambiguous or a different overload might be chosen than in previous versions of the compiler. To resolve these issues, the program must now explicitly specify the overload that the programmer intended.

    When this new behavior causes overload resolution to consider an additional candidate that is equally as good as the historic candidate, the call becomes ambiguous and the compiler issues compiler error C2668 as a result.

    error C2668: 'function' : ambiguous call to overloaded function.  
    
    

    Example 1: Ambiguous call to overloaded function (before)

    // In previous versions of the compiler, code written in this way would unambiguously call f(int, Args...)  
    template <typename... Args>  
    void f(int, Args...);  //   
    
    template <int N, typename... Args>  
    void f(const int (&)[N], Args...);  
    
    int main()  
    {  
        // The compiler now considers this call ambiguous, and issues a compiler error  
        f({3});  error C2668: 'f' ambiguous call to overloaded function  
    }  
    
    

    Example 1: ambiguous call to overloaded function (after)

    template <typename... Args>  
    void f(int, Args...);  //   
    
    template <int N, typename... Args>  
    void f(const int (&)[N], Args...);  
    
    int main()  
    {  
        // To call f(int, Args...) when there is just one expression in the initializer list, remove the braces from it.  
        f(3);  
    }  
    
    

    When this new behavior causes overload resolution to consider an additional candidate that is a better match than the historic candidate, the call resolves unambiguously to the new candidate, causing a change in program behavior that is probably different than the programmer intended.

    Example 2: change in overload resolution (before)

    // In previous versions of the compiler, code written in this way would unambiguously call f(S, Args...)  
    struct S  
    {  
        int i;  
        int j;  
    };  
    
    template <typename... Args>  
    void f(S, Args...);  
    
    template <int N, typename... Args>  
    void f(const int *&)[N], Args...);  
    
    int main()  
    {  
        // The compiler now resolves this call to f(const int (&)[N], Args...) instead  
        f({1, 2});  
    }  
    
    

    Example 2: change in overload resolution (after)

    struct S;  // as before  
    
    template <typename... Args>  
    void f(S, Args...);  
    
    template <int N, typename... Args>  
    void f(const int *&)[N], Args...);  
    
    int main()  
    {  
        // To call f(S, Args...), perform an explicit cast to S on the initializer list.  
        f(S{1, 2});  
    }  
    
    
  • Restoration of switch statement warnings

    A Previous version of the compiler removed previously-existing warnings related to switch statements; these warnings have now been restored. The compiler now issues the restored warnings, and warnings related to specific cases (including the default case) are now issued on the line containing the offending case, rather than on the last line of the switch statement. As a result of now issuing those warnings on different lines than in the past, warnings previously suppressed by using #pragma warning(disable:####) may no longer be suppressed as intended. To suppress these warnings as intended, it might be necessary to move the #pragma warning(disable:####) directive to a line above the first potentially-offending case. The following are the restored warnings.

    warning C4060: switch statement contains no 'case' or 'default' labels  
    
    
    warning C4061: enumerator 'bit1' in switch of enum 'flags' is not explicitly handled by a case label  
    
    
    warning C4062: enumerator 'bit1' in switch of enum 'flags' is not handled  
    
    
    warning C4063: case 'bit32' is not a valid value for switch of enum 'flags'  
    
    
    warning C4064: switch of incomplete enum 'flags'  
    
    
    warning C4065: switch statement contains 'default' but no 'case' labels  
    
    
    warning C4808: case 'value' is not a valid value for switch condition of type 'bool'  
    
    
    Warning C4809: switch statement has redundant 'default' label; all possible 'case' labels are given  
    
    

    Example of C4063 (before)

    class settings  
    {  
    public:  
        enum flags  
        {  
            bit0 = 0x1,  
            bit1 = 0x2,  
            ...  
        };  
        ...  
    };  
    
    int main()  
    {  
        auto val = settings::bit1;  
    
        switch (val)  
        {  
        case settings::bit0:  
            break;  
    
        case settings::bit1:  
            break;  
    
        case settings::bit0 | settings::bit1:  // warning C4063  
            break;  
        }  
    };  
    
    

    Example of C4063 (after)

    class settings {...};  // as above  
    
    int main()  
    {  
        // since C++11, use std::underlying_type to determine the underlying type of an enum  
        typedef std::underlying_type<settings::flags>::type flags_t;  
    
        auto val = settings::bit1;  
    
        switch (static_cast<flags_t>(val))  
        {  
        case settings::bit0:  
            break;  
    
        case settings::bit1:  
            break;  
    
        case settings::bit0 | settings::bit1:  // ok  
            break;  
        }  
    };  
    
    

    Examples of the other restored warnings are provided in their documentation.

  • #include: use of parent-directory specifier '..' in pathname (only affects /Wall /WX)

    Previous versions of the compiler did not detect the use of the parent-directory specifier '..' in the pathname of #include directives. Code written in this way is usually intended to include headers that exist outside of the project by incorrectly using project-relative paths. This old behavior created a risk that the program could be compiled by including a different source file than the programmer intended, or that these relative paths would not be portable to other build environments. The compiler now detects and notifies the programmer of code written in this way and issues an optional compiler warning C4464, if enabled.

    warning C4464: relative include path contains '..'  
    
    

    Example (before)

    #include "..\headers\C4426.h"  // emits warning C4464  
    
    

    Example (after)

    #include "C4426.h"  // add absolute path to 'headers\' to your project's include directories  
    
    

    Additionally, although the compiler does not give a specific diagnostic, we also recommend that the parent-directory specifier ".." should note be used to specify your project's include directories.

  • #pragma optimize() extends past end of header file (only affects /Wall /WX)

    Previous versions of the compiler did not detect changes to optimization flag settings that escape a header file included within a translation unit. The compiler now detects and notifies the programmer of code written in this way and issues an optional compiler warning C4426 at the location of the offending #include, if enabled. This warning is only issued if the changes conflict with the optimization flags set by command-line arguments to the compiler.

    warning C4426: optimization flags changed after including header, may be due to #pragma optimize()  
    
    

    Example (before)

    // C4426.h  
    #pragma optimize("g", off)  
    ...  
    // C4426.h ends  
    
    // C4426.cpp  
    #include "C4426.h"  // warning C4426  
    
    

    Example (after)

    // C4426.h  
    #pragma optimize("g", off)  
    ...  
    #pragma optimize("", on)  // restores optimization flags set via command-line arguments  
    // C4426.h ends  
    
    // C4426.cpp  
    #include "C4426.h"  
    
    
  • Mismatched #pragma warning(push) and #pragma warning(pop) (only affects /Wall /WX)

    Previous versions of the compiler did not detect #pragma warning(push) state changes being paired with #pragma warning(pop) state changes in a different source file, which is rarely intended. This old behavior created a risk that the program would be compiled with a different set of warnings enabled than the programmer intended, possibly resulting in silent bad runtime behavior. The compiler now detects and notifies the programmer of code written in this way and issues an optional compiler warning C5031 at the location of the matching #pragma warning(pop), if enabled. This warning includes a note referencing the location of the corresponding #pragma warning(push).

    warning C5031: #pragma warning(pop): likely mismatch, popping warning state pushed in different file  
    
    

    Example (before)

    // C5031_part1.h  
    #pragma warning(push)  
    #pragma warning(disable:####)  
    ...  
    // C5031_part1.h ends without #pragma warning(pop)  
    
    // C5031_part2.h  
    ...  
    #pragma warning(pop)  // pops a warning state not pushed in this source file   
    ...  
    // C5031_part1.h ends  
    
    // C5031.cpp  
    #include "C5031_part1.h" // leaves #pragma warning(push) 'dangling'  
    ...  
    #include "C5031_part2.h" // matches 'dangling' #pragma warning(push), resulting in warning C5031  
    ...  
    
    
    

    Example (after)

    // C5031_part1.h  
    #pragma warning(push)  
    #pragma warning(disable:####)  
    ...  
    #pragma warning(pop)  // pops the warning state pushed in this source file  
    // C5031_part1.h ends without #pragma warning(pop)  
    
    // C5031_part2.h  
    #pragma warning(push)  // pushes the warning state pushed in this source file  
    #pragma warning(disable:####)  
    ...  
    #pragma warning(pop)  
    // C5031_part1.h ends  
    
    // C5031.cpp  
    #include "C5031_part1.h" // #pragma warning state changes are self-contained and independent of other source files or their #include order.  
    ...  
    #include "C5031_part2.h"  
    ...  
    
    
    

    Though uncommon, code written in this way is sometimes intentional. Code written in this way is sensitive to changes in #include order; when possible, we recommend that source code files manage warning state in a self-contained way.

  • Unmatched #pragma warning(push) (only affects /Wall /WX)

    Previous versions of the compiler did not detect unmatched #pragma warning(push) state changes at the end of a translation unit. The compiler now detects and notifies the programmer of code written in this way and issues an optional compiler warning C5032 at the location of the unmatched #pragma warning(push), if enabled. This warning is only issued if there are no compilation errors in the translation unit.

    warning C5032: detected #pragma warning(push) with no corresponding #pragma warning(pop)  
    
    

    Example (before)

    // C5032.h  
    #pragma warning(push)  
    #pragma warning(disable:####)  
    ...  
    // C5032.h ends without #pragma warning(pop)  
    
    // C5032.cpp  
    #include "C5032.h"  
    ...  
    // C5032.cpp ends -- the translation unit is completed without #pragma warning(pop), resulting in warning C5032 on line 1 of C5032.h  
    
    

    Example (after)

    // C5032.h  
    #pragma warning(push)  
    #pragma warning(disable:####)  
    ...  
    #pragma warning(pop) // matches #pragma warning (push) on line 1  
    // C5032.h ends  
    
    // C5032.cpp  
    #include "C5032.h"  
    ...  
    // C5032.cpp ends -- the translation unit is completed without unmatched #pragma warning(push)  
    
    
  • Additional warnings might be issued as a result of improved #pragma warning state tracking

    Previous versions of the compiler tracked #pragma warning state changes insufficiently well to issue all intended warnings. This behavior created a risk that certain warnings would be effectively suppressed in circumstances different than the programmer intended. The compiler now tracks #pragma warning state more robustly -- especially related to #pragma warning state changes inside of templates -- and optionally issues new warnings C5031 and C5032 which are intended to help the programmer locate unintended uses of #pragma warning(push) and #pragma warning(pop).

    As a result of improved #pragma warning state change tracking, warnings formerly incorrectly suppressed or warnings related to issues formerly misdiagnosed might now be issued.

  • Improved identification of unreachable code

    C++ Standard Library changes and improved ability to inline function calls over previous versions of the compiler might allow the compiler to prove that certain code is now unreachable. This new behavior can result in new and more-frequently issued instances of warning C4720.

    warning C4720: unreachable code  
    
    

    In many cases, this warning might only be issued when compiling with optimizations enabled, since optimizations may inline more function calls, eliminate redundant code, or otherwise make it possible to determine that certain code is unreachable. We have observed that new instances of warning C4720 have frequently occurred in try/catch blocks, especially in relation to use of std::find.

    Example (before)

    try   
    {   
        auto iter = std::find(v.begin(), v.end(), 5);   
    }   
    catch(…)   
    {   
        do_something();  // ok   
    }  
    
    

    Example (after)

    try   
    {   
        auto iter = std::find(v.begin(), v.end(), 5);   
    }   
    catch(…)   
    {   
        do_something();  // warning C4702: unreachable code  
    }  
    
    
  • Additional warnings and errors might be issued as a result of partial support for expression SFINAE

    Previous versions of the compiler did not parse certain kinds of expressions inside decltype specifiers due to lack of support for expression SFINAE. This old behavior was incorrect and does not conform to the C++ standard. The compiler now parses these expressions and has partial support for expression SFINAE due to ongoing conformance improvements. As a result, the compiler now issues warnings and errors found in expressions that previous versions of the compiler did not parse.

    When this new behavior parses a decltype expression that includes a type that has not yet been declared, the compiler issues compiler error C2039 as a result.

    error C2039: 'type': is not a member of '`global namespace''  
    
    

    Example 1: use of an undeclared type (before)

    struct s1  
    {  
      template <typename T>  
      auto f() -> decltype(s2<T>::type::f());  // error C2039  
    
      template<typename>  
      struct s2 {};  
    }  
    
    

    Example 1 (after)

    struct s1  
    {  
      template <typename>  // forward declare s2struct s2;  
    
      template <typename T>  
      auto f() -> decltype(s2<T>::type::f());  
    
      template<typename>  
      struct s2 {};  
    }  
    
    

    When this new behavior parses a decltype expression that is missing a necessary use of the typename keyword to specify that a dependent name is a type, the compiler issues compiler warning C4346 together with compiler error C2923.

    warning C4346: 'S2<T>::Type': dependent name is not a type  
    
    
    error C2923: 's1': 'S2<T>::Type' is not a valid template type argument for parameter 'T'  
    
    

    Example 2: dependent name is not a type (before)

    template <typename T>  
    struct s1  
    {  
      typedef T type;  
    };  
    
    template <typename T>  
    struct s2  
    {  
      typedef T type;  
    };  
    
    template <typename T>  
    T declval();  
    
    struct s  
    {  
      template <typename T>  
      auto f(T t) -> decltype(t(declval<S1<S2<T>::type>::type>()));  // warning C4346, error C2923  
    };  
    
    

    Example 2 (after)

    template <typename T> struct s1 {...};  // as above  
    template <typename T> struct s2 {...};  // as above  
    
    template <typename T>  
    T declval();  
    
    struct s  
    {  
      template <typename T>  
      auto f(T t) -> decltype(t(declval<S1<typename S2<T>::type>::type>()));  
    };  
    
    
  • volatile member variables prevent implicitly defined constructors and assignment operators

    Previous versions of the compiler allowed a class that has volatile member variables to have default copy/move constructors and default copy/move assignment operators automatically generated. This old behavior was incorrect and does not conform to the C++ standard. The compiler now considers a class that has volatile member variables to have non-trivial construction and assignment operators which prevents default implementations of these operators from being automatically generated. When such a class is a member of a union (or an anonymous union inside of a class), the copy/move constructors and copy/move assignment operators of the union (or the class containing the unonymous union) will be implicitly defined as deleted. Attempting to construct or copy the union (or class containing the anonymous union) without explicitly defining them is an error and the compiler issues compiler error C2280 as a result.

    error C2280: 'B::B(const B &)': attempting to reference a deleted function  
    
    

    Example (before)

    struct A  
    {  
      volatile int i;  
      volatile int j;  
    };  
    
    extern A* pa;  
    
    struct B  
    {  
      union  
      {  
        A a;  
        int i;  
      };  
    };  
    
    B b1 {*pa};  
    B b2 (b1);  // error C2280  
    
    

    Example (after)

    struct A  
    {  
      int i;int j;  
    };  
    
    extern volatile A* pa;  
    
    A getA()  // returns an A instance copied from contents of pa  
    {  
      A a;  
      a.i = pa->i;  
      a.j = pa->j;  
      return a;  
    }  
    
    struct B;  // as above  
    
    B b1 {GetA()};  
    B b2 (b1);  // error C2280  
    
    
  • Static member functions do not support cv-qualifiers.

    Previous versions of Visual C++ 2015 allowed static member functions to have cv-qualifiers. This behavior is due to a regression in Visual C++ 2015 and Visual C++ 2015 Update 1; Visual C++ 2013 and previous versions of Visual C++ reject code written in this way. The behavior of Visual C++ 2015 and Visual C++ 2015 Update 1 is incorrect and does not conform to the C++ standard. Visual Studio 2015 Update 2 rejects code written in this way and issues compiler error C2511 instead.

    error C2511: 'void A::func(void) const': overloaded member function not found in 'A'  
    
    

    Example (before)

    struct A  
    {  
      static void func();  
    };  
    
    void A::func() const {}  // C2511  
    
    
    

    Example (after)

    struct A  
    {  
      static void func();  
    };  
    
    void A::func() {}  // removed const  
    
    
    
  • Forward declaration of enum is not allowed in WinRT code (affects /ZW only)

    Code compiled for the Windows Runtime (WinRT) doesn't allow enum types to be forward declared, similarly to when managed C++ code is compiled for the .Net Framework using the /clr compiler switch. This behavior is ensures that the size of an enumeration is always known and can be correctly projected to the WinRT type system. The compiler rejects code written in this way and issues compiler error C2599 together with compiler error C3197.

    error C2599: 'CustomEnum': the forward declaration of a WinRT enum is not allowed  
    
    
    error C3197: 'public': can only be used in definitions  
    
    

    Example (before)

    namespace A {  
      public enum class CustomEnum: int32;  // forward declaration; error C2599, error C3197  
    }  
    
    namespace A {  
      public enum class CustomEnum: int32  
      {  
        Value1  
      };  
    }  
    
    public ref class Component sealed  
    {  
    public:  
      CustomEnum f()  
      {  
        return CustomEnum::Value1;  
      }  
    };  
    
    

    Example (after)

    
              // forward declaration of CustomEnum removed  
    
    namespace A {  
      public enum class CustomEnum: int32  
      {  
        Value1  
      };  
    }  
    
    public ref class Component sealed  
    {  
    public:  
      CustomEnum f()  
      {  
        return CustomEnum::Value1;  
      }  
    };  
    
    
  • Overloaded non-member operator new and operator delete may not be declared inline (Level 1 (/W1) on-by-default)

    Previous versions of the compiler do not issue a warning when non-member operator new and operator delete functions are declared inline. Code written in this way is ill-formed (no diagnostic required) and can cause memory issues resulting from mismatched new and delete operators (especially when used together with sized deallocation) that can be difficult to diagnose. The compiler now issues compiler warning C4595 to help identify code written in this way.

    warning C4595: 'operator new': non-member operator new or delete functions may not be declared inline  
    
    

    Example (before)

    
              inline void* operator new(size_t sz)  // warning C4595  
    {  
      ...  
    }  
    
    

    Example (after)

    
              void* operator new(size_t sz)  // removed inline  
    {  
      ...  
    }  
    
    

    Fixing code that's written in this way might require that the operator definitions be moved out of a header file and into a corresponding source file.

  • std::is_convertable now detects self-assignment (standard library)

    Previous versions of the std::is_convertable type-trait did not correctly detect self-assignment of a class type when its copy constructor is deleted or private. Now, std::is_convertable<>::value is correctly set to false when applied to a class type with a deleted or private copy constructor.

    There is no compiler diagnostic associated with this change.

    Example

    #include <type_traits>  
    
    class X1  
    {  
    public:  
        X1(const X1&) = delete;  
    };  
    
    class X2  
    {  
    private:  
        X2(const X2&);  
    };  
    
    static_assert(std::is_convertible<X1&, X1>::value, "BOOM");static_assert(std::is_convertible<X2&, X2>::value, "BOOM");  
    
    

    In previous versions of Visual C++, the static assertions at the bottom of this example pass because std::is_convertable<>::value was incorrectly set to true. Now, std::is_convertable<>::value is correctly set to false, causing the static assertions to fail.

  • Defaulted or deleted trivial copy and move constructors respect access specifiers

    Previous versions of the compiler did not check the access specifier of defaulted or deleted trivial copy and move constructors before allowing them to be called. This old behavior was incorrect and does not conform to the C++ standard. In some cases, this old behavior created a risk of silent bad code generation, resulting in unpredictable runtime behavior. The compiler now checks the access specifier of defaulted or deleted trivial copy and move constructors to determine whether it can be called, and if not, issues compiler warning C2248 as a result.

    error C2248: 'S::S' cannot access private member declared in class 'S'  
    
    

    Example (before)

    class S {  
    public:  
           S() = default;  
    private:  
        S(const S&) = default;  
    };  
    
    void f(S);  // pass S by value  
    
    int main()  
    {  
        S s;  
        f(s);  // error C2248, can't invoke private copy constructor  
    }  
    
    

    Example (after)

    class S {  
    public:  
           S() = default;  
    private:  
        S(const S&) = default;  
    };  
    
    void f(const S&);  // pass S by reference  
    
    int main()  
    {  
        S s;  
        f(s);  
    }  
    
    
  • Deprecation of attributed ATL code support (Level 1 (/W1) on-by-default)

    Previous versions of the compiler supported attributed ATL code. As the next phase of removing support for attributed ATL code that began in Visual C++ 2008, attributed ATL code has been deprecated. The compiler now issues compiler warning C4467 to help identify this kind of deprecated code.

    warning C4467: Usage of ATL attributes is deprecated  
    
    

    If you want to continue using attributed ATL code until support is removed from the compiler, you can disable this warning by passing the /Wv:18 or /wd:4467 command line arguments to the compiler, or by adding #pragma warning(disable:4467) in your source code.

    Example 1 (before)

    
              [uuid("594382D9-44B0-461A-8DE3-E06A3E73C5EB")]  
    class A {};  
    
    

    Example 1 (after)

    
    __declspec(uuid("594382D9-44B0-461A-8DE3-E06A3E73C5EB")) A {};  
    
    
    

    Sometimes you might need or want to create an IDL file to avoid the use deprecated ATL attributes, as in the example code below

    Example 2 (before)

    [emitidl];  
    [module(name="Foo")];  
    
    [object, local, uuid("9e66a290-4365-11d2-a997-00c04fa37ddb")]  
    __interface ICustom {  
        HRESULT Custom([in] long l, [out, retval] long *pLong);  
        [local] HRESULT CustomLocal([in] long l, [out, retval] long *pLong);  
    };  
    
    [coclass, appobject, uuid("9e66a294-4365-11d2-a997-00c04fa37ddb")]  
    class CFoo : public ICustom  
    {  
        // ...  
    };  
    
    

    First, create the *.idl file; the vc140.idl generated file can be used to obtain an *.idl file containing the interfaces and annotations.

    Next, add a MIDL step to your build to make sure that the C++ interface definitions are generated.

    Example 2 IDL (after)

    import "docobj.idl";  
    
    [  
        object,local,uuid(9e66a290-4365-11d2-a997-00c04fa37ddb)  
    ]  
    
    interface ICustom : IUnknown {  
        HRESULT  Custom([in] long l, [out,retval] long *pLong);  
        [local] HRESULT  CustomLocal([in] long l, [out,retval] long *pLong);  
    };  
    
    [ version(1.0), uuid(29079a2c-5f3f-3325-99a1-3ec9c40988bb) ]  
    library Foo  
    {  
        importlib("stdole2.tlb");  
        importlib("olepro32.dll");  
            [  
                version(1.0),  
                appobject,uuid(9e66a294-4365-11d2-a997-00c04fa37ddb)  
            ]  
    
        coclass CFoo {  
            interface ICustom;  
        };  
    }  
    
    

    Then, use ATL directly in the implementation file, as in the example code below.

    Example 2 Implementation (after)

    #include <idl.header.h>  
    #include <atlbase.h>  
    
    class ATL_NO_VTABLE CFooImpl :  
        public ICustom,  
        public ATL::CComObjectRootEx<CComMultiThreadModel>  
    {  
    public:  
        BEGIN_COM_MAP(CFooImpl)  
        COM_INTERFACE_ENTRY(ICustom)  
        END_COM_MAP()  
    };  
    
    
  • Precompiled header (PCH) files and mismatched #include directives (only affects /Wall /WX)

    Previous versions of the compiler accepted mismatched #include directives in source files between -Yc and -Yu compilations when using precompiled header (PCH) files. Code written in this way is no longer accepted by the compiler. The compiler now issues compiler warning CC4598 to help identify mismatched #include directives when using PCH files.

    warning C4598: 'b.h': included header file specified for Ycc.h at position 2 does not match Yuc.h at that position  
    
    

    Example (before):

    X.cpp (-Ycc.h)

    #include "a.h"  
    #include "b.h"  
    #include "c.h"  
    
    

    Z.cpp (-Yuc.h)

    #include "b.h"  
    #include "a.h"  // mismatched order relative to X.cpp  
    #include "c.h"  
    
    

    Example (after)

    X.cpp (-Ycc.h)

    #include "a.h"  
    #include "b.h"   
    #include "c.h"  
    
    

    Z.cpp (-Yuc.h)

    #include "a.h"  
    #include "b.h" // matched order relative to X.cpp  
    #include "c.h"  
    
    
  • Precompiled header (PCH) files and mismatched include directories (only affects /Wall /WX)

    Previous versions of the compiler accepted mismatched include directory (-I) command line arguments to the compiler between -Yc and -Yu compilations when using precompiled header (PCH) files. Code written in this way is no longer accepted by the compiler. The compiler now issues compiler warning CC4599 to help identify mismatched include directory (-I) command line arguments when using PCH files.

    warning C4599: '-I..' : specified for Ycc.h at position 1 does not match Yuc.h at that position  
    
    

    Example (before)

    cl /c /Wall /Ycc.h -I.. X.cpp  
    cl /c /Wall /Yuc.h Z.cpp  
    
    

    Example (after)

    cl /c /Wall /Ycc.h -I.. X.cpp  
    cl /c /Wall /Yuc.h -I.. Z.cpp  
    
    
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