Compiler warnings that are off by default

The compiler includes warnings that are turned off by default, because most developers don't want to see them. In some cases, they represent a stylistic choice, or are common idioms in older code, or take advantage of a Microsoft extension to the language. In other cases, they indicate an area where programmers often make incorrect assumptions, which may lead to unexpected or undefined behavior. Some of these warnings may be very noisy in library headers. The C runtime libraries and the C++ standard libraries are intended to emit no warnings only at warning level /W4.

Enable warnings that are off by default

You can enable warnings that are normally off by default 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

    /Wall enables all warnings that are off by default. If you use this option, you can turn off individual warnings by using the /wd option.

  • /wLnnnn

    This enables warning nnnn at level L.

Warnings that are off by default

The following warnings are turned off by default in Visual Studio 2015 and later versions:

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': 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
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'
C4355 '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) 'classname': layout of class may have changed from a previous version of the compiler due to better packing of member 'member'
C4388 (level 4) signed/unsigned mismatch
C4412 (level 2) 'function': function signature contains type 'type'; C++ objects are unsafe to pass between pure code and mixed or native
C4426 (level 1) optimization flags changed after including header, may be due to #pragma optimize() 14.1
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
C4464 (level 4) relative include path contains '..'
C4471 (level 4) a forward declaration of an unscoped enumeration must have an underlying type (int assumed) Perm
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) 'operator1': operator before comma has no effect; did you intend 'operator2'?
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'?
C4577 (level 1) 'noexcept' used with no exception handling mode specified; termination on exception is not guaranteed. Specify /EHsc
C4582 (level 4) 'type': constructor is not implicitly called
C4583 (level 4) 'type': destructor is not implicitly called
C4587 (level 1) 'anonymous_structure': behavior change: constructor is no longer implicitly called
C4588 (level 1) 'anonymous_structure': behavior change: destructor is no longer implicitly called
C4596 (level 4) 'identifier': illegal qualified name in member declaration 14.3 Perm
C4598 (level 1 and level 3) '#include "header"': header number number in the precompiled header does not match current compilation at that position 14.3
C4599 (level 3) 'option path': command-line argument number number does not match pre-compiled header 14.3
C4605 (level 1) '/Dmacro' specified on current command line, but was not specified when precompiled header was built
C4608 (level 3) 'union_member' has already been initialized by another union member in the initializer list, 'union_member' Perm
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
C4647 (level 3) behavior change: __is_pod(type) has different value in previous versions
C4654 (level 4) Code placed before include of precompiled header line will be ignored. Add code to precompiled header. 14.1
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
C4746 volatile access of 'expression' is subject to /volatile:<iso|ms> setting; consider using __iso_volatile_load/store intrinsic functions
C4749 (level 4) conditionally supported: offsetof applied to non-standard-layout type 'type'
C4767 (level 4) section name 'symbol' is longer than 8 characters and will be truncated by the linker
C4768 (level 3) __declspec attributes before linkage specification are ignored
C4774 (level 4) 'string' : format string expected in argument number is not a string literal
C4777 (level 4) 'function' : format string 'string' requires an argument of type 'type1', but variadic argument number has type 'type2'
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: '??character' replaced by 'character'
C4841 (level 4) non-standard extension used: compound member designator used in offsetof
C4842 (level 4) the result of 'offsetof' applied to a type using multiple inheritance is not guaranteed to be consistent between compiler releases
C4868 (level 4) 'file(line_number)' compiler may not enforce left-to-right evaluation order in braced initialization list
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'
C4962 '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
C5022 'type': multiple move constructors specified
C5023 'type': multiple move assignment operators specified
C5024 (level 4) 'type': move constructor was implicitly defined as deleted
C5025 (level 4) 'type': move assignment operator was implicitly defined as deleted
C5026 (level 1 and level 4) 'type': move constructor was implicitly defined as deleted
C5027 (level 1 and level 4) 'type': move assignment operator was implicitly defined as deleted
C5029 (level 4) nonstandard extension used: alignment attributes in C++ apply to variables, data members and tag types only
C5031 (level 4) #pragma warning(pop): likely mismatch, popping warning state pushed in different file 14.1
C5032 (level 4) detected #pragma warning(push) with no corresponding #pragma warning(pop) 14.1
C5034 use of intrinsic 'intrinsic' causes function function to be compiled as guest code 15.3
C5035 use of feature 'feature' causes function function to be compiled as guest code 15.3
C5036 (level 1) varargs function pointer conversion when compiling with /hybrid:x86arm64 'type1' to 'type2' 15.3
C5038 (level 4) data member 'member1' will be initialized after data member 'member2' 15.3
C5039 (level 4) 'function': pointer or reference to potentially throwing function passed to extern C function under -EHc. Undefined behavior may occur if this function throws an exception. 15.5
C5042 (level 3) 'function': function declarations at block scope cannot be specified 'inline' in standard C++; remove 'inline' specifier 15.5
C5045 Compiler will insert Spectre mitigation for memory load if /Qspectre switch specified 15.7

14.1 This warning is available starting in Visual Studio 2015 Update 1.
14.3 This warning is available starting in Visual Studio 2015 Update 3.
15.3 This warning is available starting in Visual Studio 2017 version 15.3.
15.5 This warning is available starting in Visual Studio 2017 version 15.5.
15.7 This warning is available starting in Visual Studio 2017 version 15.7.
Perm This warning is off unless the /permissive- compiler option is set.

Warnings off by default in earlier versions

These warnings were off by default in versions of the compiler before Visual Studio 2015:

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
C4319 (level 1) 'operator': zero extending 'type1' to 'type2' of greater size

This warning was off by default in versions of the compiler before Visual Studio 2012:

C4431 (level 4) missing type specifier - int assumed. Note: C no longer supports default-int

See Also

warning