C++ type system

The concept of type is important in C++. Every variable, function argument, and function return value must have a type in order to be compiled. Also, all expressions (including literal values) are implicitly given a type by the compiler before they're evaluated. Some examples of types include built-in types such as int to store integer values, double to store floating-point values, or Standard Library types such as class std::basic_string to store text. You can create your own type by defining a class or struct. The type specifies the amount of memory that's allocated for the variable (or expression result). The type also specifies the kinds of values that may be stored, how the compiler interprets the bit patterns in those values, and the operations you can perform on them. This article contains an informal overview of the major features of the C++ type system.

Terminology

Scalar type: A type that holds a single value of a defined range. Scalars include arithmetic types (integral or floating-point values), enumeration type members, pointer types, pointer-to-member types, and std::nullptr_t. Fundamental types are typically scalar types.

Compound type: A type that isn't a scalar type. Compound types include array types, function types, class (or struct) types, union types, enumerations, references, and pointers to non-static class members.

Variable: The symbolic name of a quantity of data. The name can be used to access the data it refers to throughout the scope of the code where it's defined. In C++, variable is often used to refer to instances of scalar data types, whereas instances of other types are typically called objects.

Object: For simplicity and consistency, this article uses the term object to refer to any instance of a class or structure. When it's used in the general sense, it includes all types, even scalar variables.

POD type (plain old data): This informal category of data types in C++ refers to types that are scalar (see the Fundamental types section) or are POD classes. A POD class has no static data members that aren't also PODs, and has no user-defined constructors, user-defined destructors, or user-defined assignment operators. Also, a POD class has no virtual functions, no base class, and no private or protected non-static data members. POD types are often used for external data interchange, for example with a module written in the C language (which has POD types only).

Specifying variable and function types

C++ is both a strongly typed language and a statically typed language; every object has a type and that type never changes. When you declare a variable in your code, you must either specify its type explicitly, or use the auto keyword to instruct the compiler to deduce the type from the initializer. When you declare a function in your code, you must specify the type of its return value and of each argument. Use the return value type void if no value is returned by the function. The exception is when you're using function templates, which allow for arguments of arbitrary types.

After you first declare a variable, you can't change its type at some later point. However, you can copy the variable's value or a function's return value into another variable of a different type. Such operations are called type conversions, which are sometimes necessary but are also potential sources of data loss or incorrectness.

When you declare a variable of POD type, we strongly recommend you initialize it, which means to give it an initial value. Until you initialize a variable, it has a "garbage" value that consists of whatever bits happened to be in that memory location previously. It's an important aspect of C++ to remember, especially if you're coming from another language that handles initialization for you. When you declare a variable of non-POD class type, the constructor handles initialization.

The following example shows some simple variable declarations with some descriptions for each. The example also shows how the compiler uses type information to allow or disallow certain subsequent operations on the variable.

int result = 0;              // Declare and initialize an integer.
double coefficient = 10.8;   // Declare and initialize a floating
                             // point value.
auto name = "Lady G.";       // Declare a variable and let compiler
                             // deduce the type.
auto address;                // error. Compiler cannot deduce a type
                             // without an intializing value.
age = 12;                    // error. Variable declaration must
                             // specify a type or use auto!
result = "Kenny G.";         // error. Can't assign text to an int.
string result = "zero";      // error. Can't redefine a variable with
                             // new type.
int maxValue;                // Not recommended! maxValue contains
                             // garbage bits until it is initialized.

Fundamental (built-in) types

Unlike some languages, C++ has no universal base type from which all other types are derived. The language includes many fundamental types, also known as built-in types. These types include numeric types such as int, double, long, bool, plus the char and wchar_t types for ASCII and UNICODE characters, respectively. Most integral fundamental types (except bool, double, wchar_t, and related types) all have unsigned versions, which modify the range of values that the variable can store. For example, an int, which stores a 32-bit signed integer, can represent a value from -2,147,483,648 to 2,147,483,647. An unsigned int, which is also stored as 32 bits, can store a value from 0 to 4,294,967,295. The total number of possible values in each case is the same; only the range is different.

The compiler recognizes these built-in types, and it has built-in rules that govern what operations you can perform on them, and how they can be converted to other fundamental types. For a complete list of built-in types and their size and numeric limits, see Built-in types.

The following illustration shows the relative sizes of the built-in types in the Microsoft C++ implementation:

The following table lists the most frequently used fundamental types, and their sizes in the Microsoft C++ implementation:

Other C++ implementations may use different sizes for certain numeric types. For more information on the sizes and size relationships that the C++ standard requires, see Built-in types.

The void type

The void type is a special type; you can't declare a variable of type void, but you can declare a variable of type void * (pointer to void), which is sometimes necessary when allocating raw (untyped) memory. However, pointers to void aren't type-safe and their use is discouraged in modern C++. In a function declaration, a void return value means that the function doesn't return a value; using it as a return type is a common and acceptable use of void. While the C language required functions that have zero parameters to declare void in the parameter list, for example, fn(void), this practice is discouraged in modern C++; a parameterless function should be declared fn(). For more information, see Type conversions and type safety.

const type qualifier

Any built-in or user-defined type may be qualified by the const keyword. Additionally, member functions may be const-qualified and even const-overloaded. The value of a const type can't be modified after it's initialized.

const double PI = 3.1415;
PI = .75; //Error. Cannot modify const variable.

The const qualifier is used extensively in function and variable declarations and "const correctness" is an important concept in C++; essentially it means to use const to guarantee, at compile time, that values aren't modified unintentionally. For more information, see const.

A const type is distinct from its non-const version; for example, const int is a distinct type from int. You can use the C++ const_cast operator on those rare occasions when you must remove const-ness from a variable. For more information, see Type conversions and type safety.

String types

Strictly speaking, the C++ language has no built-in string type; char and wchar_t store single characters - you must declare an array of these types to approximate a string, adding a terminating null value (for example, ASCII '\0') to the array element one past the last valid character (also called a C-style string). C-style strings required much more code to be written or the use of external string utility library functions. But in modern C++, we have the Standard Library types std::string (for 8-bit char-type character strings) or std::wstring (for 16-bit wchar_t-type character strings). These C++ Standard Library containers can be thought of as native string types because they're part of the standard libraries that are included in any conformant C++ build environment. Use the #include <string> directive to make these types available in your program. (If you're using MFC or ATL, the CString class is also available, but isn't part of the C++ standard.) The use of null-terminated character arrays (the C-style strings previously mentioned) is discouraged in modern C++.

User-defined types

When you define a class, struct, union, or enum, that construct is used in the rest of your code as if it were a fundamental type. It has a known size in memory, and certain rules about how it can be used apply to it for compile-time checking and, at runtime, for the life of your program. The primary differences between the fundamental built-in types and user-defined types are as follows:

• The compiler has no built-in knowledge of a user-defined type. It learns of the type when it first encounters the definition during the compilation process.

• You specify what operations can be performed on your type, and how it can be converted to other types, by defining (through overloading) the appropriate operators, either as class members or non-member functions. For more information, see Function overloading

Pointer types

As in the earliest versions of the C language, C++ continues to let you declare a variable of a pointer type by using the special declarator * (asterisk). A pointer type stores the address of the location in memory where the actual data value is stored. In modern C++, these pointer types are referred to as raw pointers, and they're accessed in your code through special operators: * (asterisk) or -> (dash with greater-than, often called arrow). This memory access operation is called dereferencing. Which operator you use depends on whether you're dereferencing a pointer to a scalar, or a pointer to a member in an object.

Working with pointer types has long been one of the most challenging and confusing aspects of C and C++ program development. This section outlines some facts and practices to help use raw pointers if you want to. However, in modern C++, it's no longer required (or recommended) to use raw pointers for object ownership at all, due to the evolution of the smart pointer (discussed more at the end of this section). It's still useful and safe to use raw pointers for observing objects. However, if you must use them for object ownership, you should do so with caution and with careful consideration of how the objects owned by them are created and destroyed.

The first thing that you should know is that a raw pointer variable declaration only allocates enough memory to store an address: the memory location that the pointer refers to when it's dereferenced. The pointer declaration doesn't allocate the memory needed to store the data value. (That memory is also called the backing store.) In other words, by declaring a raw pointer variable, you're creating a memory address variable, not an actual data variable. If you dereference a pointer variable before you've made sure that it contains a valid address for a backing store, it causes undefined behavior (usually a fatal error) in your program. The following example demonstrates this kind of error:

int* pNumber;       // Declare a pointer-to-int variable.
*pNumber = 10;      // error. Although this may compile, it is
                    // a serious error. We are dereferencing an
                    // uninitialized pointer variable with no
                    // allocated memory to point to.

The example dereferences a pointer type without having any memory allocated to store the actual integer data or a valid memory address assigned to it. The following code corrects these errors:

    int number = 10;          // Declare and initialize a local integer
                              // variable for data backing store.
    int* pNumber = &number;   // Declare and initialize a local integer
                              // pointer variable to a valid memory
                              // address to that backing store.
...
    *pNumber = 41;            // Dereference and store a new value in
                              // the memory pointed to by
                              // pNumber, the integer variable called
                              // "number". Note "number" was changed, not
                              // "pNumber".

The corrected code example uses local stack memory to create the backing store that pNumber points to. We use a fundamental type for simplicity. In practice, the backing stores for pointers are most often user-defined types that are dynamically allocated in an area of memory called the heap (or free store) by using a new keyword expression (in C-style programming, the older malloc() C runtime library function was used). Once allocated, these variables are normally referred to as objects, especially if they're based on a class definition. Memory that is allocated with new must be deleted by a corresponding delete statement (or, if you used the malloc() function to allocate it, the C runtime function free()).

However, it's easy to forget to delete a dynamically allocated object- especially in complex code, which causes a resource bug called a memory leak. For this reason, the use of raw pointers is discouraged in modern C++. It's almost always better to wrap a raw pointer in a smart pointer, which automatically releases the memory when its destructor is invoked. (That is, when the code goes out of scope for the smart pointer.) By using smart pointers, you virtually eliminate a whole class of bugs in your C++ programs. In the following example, assume MyClass is a user-defined type that has a public method DoSomeWork();

void someFunction() {
    unique_ptr<MyClass> pMc(new MyClass);
    pMc->DoSomeWork();
}
  // No memory leak. Out-of-scope automatically calls the destructor
  // for the unique_ptr, freeing the resource.

For more information about smart pointers, see Smart pointers.

For more information about pointer conversions, see Type conversions and type safety.

For more information about pointers in general, see Pointers.

Windows data types

In classic Win32 programming for C and C++, most functions use Windows-specific typedefs and #define macros (defined in windef.h) to specify the types of parameters and return values. These Windows data types are mostly special names (aliases) given to C/C++ built-in types. For a complete list of these typedefs and preprocessor definitions, see Windows Data Types. Some of these typedefs, such as HRESULT and LCID, are useful and descriptive. Others, such as INT, have no special meaning and are just aliases for fundamental C++ types. Other Windows data types have names that are retained from the days of C programming and 16-bit processors, and have no purpose or meaning on modern hardware or operating systems. There are also special data types associated with the Windows Runtime Library, listed as Windows Runtime base data types. In modern C++, the general guideline is to prefer the C++ fundamental types unless the Windows type communicates some extra meaning about how the value is to be interpreted.

More information

For more information about the C++ type system, see the following articles.

Value types
Describes value types along with issues relating to their use.

Type conversions and type safety
Describes common type conversion issues and shows how to avoid them.

See also

Welcome back to C++
C++ language reference
C++ Standard Library