Compiler Warnings That Are Off by Default
The compiler includes warnings that are turned off by default because most users don't want to see them. However, you can enable these warnings by using one of the following options.
- #pragma warning(default : warning_number)
The specified warning (warning_number) is enabled at its default level. Documentation for the warning contains the default level of the warning.
- #pragma warning(warning_level : warning_number)
The specified warning (warning_number) is enabled at the specified level (warning_level).
/Wall enables all warnings that are off by default.
The following warnings are turned off by default.
C4061 (level 4)
enumerator 'identifier' in a switch of enum 'enumeration' is not explicitly handled by a case label
C4062 (level 4)
enumerator 'identifier' in a switch of enum 'enumeration' is not handled
C4191 (level 3)
'operator/operation': unsafe conversion from 'type of expression' to 'type required'
C4242 (level 4)
'identifier': conversion from 'type1' to 'type2', possible loss of data
C4254 (level 4)
'operator': conversion from 'type1' to 'type2', possible loss of data
C4255 (level 4)
'function': no function prototype given: converting '()' to '(void)'
C4263 (level 4)
'function': member function does not override any base class virtual member function
C4264 (level 1)
'virtual_function': no override available for virtual member function from base 'class'; function is hidden
C4265 (level 3)
'class': class has virtual functions, but destructor is not virtual
C4266 (level 4)
'function': no override available for virtual member function from base 'type'; function is hidden
C4287 (level 3)
'operator': unsigned/negative constant mismatch
C4289 (level 4)
nonstandard extension used : 'var' : loop control variable declared in the for-loop is used outside the for-loop scope
C4296 (level 4)
'operator': expression is always false
C4302 (level 2)
'conversion': truncation from 'type1' to 'type2'
C4311 (level 1)
'variable' : pointer truncation from 'type' to 'type'
C4312 (level 1)
'operation' : conversion from 'type1' to 'type2' of greater size
C4339 (level 4)
'type' : use of undefined type detected in CLR meta-data - use of this type may lead to a runtime exception
C4342 (level 1)
behavior change: 'function' called, but a member operator was called in previous versions
C4350 (level 1)
behavior change: 'member1' called instead of 'member2'
'this' : used in base member initializer list
C4365 (level 4)
'action': conversion from 'type_1' to 'type_2', signed/unsigned mismatch
C4370 (level 3)
layout of class has changed from a previous version of the compiler due to better packing
C4371 (level 3)
layout of class may have changed from a previous version of the compiler due to better packing of member 'member'
C4388 (level 4)
C4412 (level 2)
'function': function signature contains type 'type'; C++ objects are unsafe to pass between pure code and mixed or native
C4431 (level 4)
missing type specifier - int assumed. Note: C no longer supports default-int
C4435 (level 4)
'class1' : Object layout under /vd2 will change due to virtual base 'class2'
C4437 (level 4)
dynamic_cast from virtual base 'class1' to 'class2' could fail in some contexts
C4444 (level 3)
top level '__unaligned' is not implemented in this context
C4471 (level 4)
a forward declaration of an unscoped enumeration must have an underlying type (int assumed)
C4472 (level 1)
'identifier' is a native enum: add an access specifier (private/public) to declare a managed enum
C4514 (level 4)
'function': unreferenced inline function has been removed
C4536 (level 4)
'type name': type-name exceeds meta-data limit of 'limit' characters
C4545 (level 1)
expression before comma evaluates to a function which is missing an argument list
C4546 (level 1)
function call before comma missing argument list
C4547 (level 1)
'operator': operator before comma has no effect; expected operator with side-effect
C4548 (level 1)
expression before comma has no effect; expected expression with side-effect
C4549 (level 1)
'operator': operator before comma has no effect; did you intend 'operator'?
C4555 (level 1)
expression has no effect; expected expression with side-effect
C4557 (level 3)
'__assume' contains effect 'effect'
C4571 (level 4)
informational: catch(…) semantics changed since Visual C++ 7.1; structured exceptions (SEH) are no longer caught
C4574 (level 4)
'identifier' is defined to be '0': did you mean to use '#if identifier'?
C4608 (level 3)
'symbol1' has already been initialized by another union member in the initializer list, 'symbol2'
C4619 (level 3)
#pragma warning: there is no warning number 'number'
C4623 (level 4)
'derived class': default constructor could not be generated because a base class default constructor is inaccessible
C4625 (level 4)
'derived class': copy constructor could not be generated because a base class copy constructor is inaccessible
C4626 (level 4)
'derived class': assignment operator could not be generated because a base class assignment operator is inaccessible
C4628 (level 1)
digraphs not supported with -Ze. Character sequence 'digraph' not interpreted as alternate token for 'char'
C4640 (level 3)
'instance': construction of local static object is not thread-safe
C4668 (level 4)
'symbol' is not defined as a preprocessor macro, replacing with '0' for 'directives'
C4682 (level 4)
'symbol' : no directional parameter attribute specified, defaulting to [in]
C4686 (level 3)
'user-defined type': possible change in behavior, change in UDT return calling convention
C4692 (level 1)
'function': signature of non-private member contains assembly private native type 'native_type'
C4710 (level 4)
'function': function not inlined
C4738 (level 3)
storing 32-bit float result in memory, possible loss of performance
C4767 (level 4)
section name 'symbol' is longer than 8 characters and will be truncated by the linker
C4786 (level 3)
'symbol' : object name was truncated to 'number' characters in the debug information
C4820 (level 4)
'bytes' bytes padding added after construct 'member_name'
C4826 (level 2)
conversion from 'type1' to 'type2' is sign-extended. This may cause unexpected runtime behavior
C4837 (level 4)
trigraph detected: '??%c' replaced by '%c'
C4905 (level 1)
wide string literal cast to 'LPSTR'
C4906 (level 1)
string literal cast to 'LPWSTR'
C4917 (level 1)
'declarator': a GUID can only be associated with a class, interface, or namespace
C4928 (level 1)
illegal copy-initialization; more than one user-defined conversion has been implicitly applied
C4931 (level 4)
we are assuming the type library was built for number-bit pointers
C4946 (level 1)
reinterpret_cast used between related classes: 'class1' and 'class2'
'function': profile-guided optimizations disabled because optimizations caused profile data to become inconsistent
C4986 (level 4)
'symbol': exception specification does not match previous declaration
C4987 (level 4)
nonstandard extension used: 'throw (...)'
C4988 (level 4)
'symbol': variable declared outside class/function scope