For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

(Microsoft Specific) This keyword is obsolete. In versions of Visual Studio earlier than Visual Studio 2013, this lets you mark variables, so that when you compile with /Wp64 the compiler will report any warnings that would be reported if you were compiling with a 64-bit compiler.

type __w64 identifier  


One of the three types that could cause problems in code being ported from a 32-bit to a 64-bit compiler: int, long, or a pointer.

The identifier for the variable you are creating.

System_CAPS_ICON_important.jpg Important

The /Wp64 compiler option and __w64 keyword are deprecated in Visual Studio 2010 and Visual Studio 2013 and removed starting in Visual Studio 2013. If you use the /Wp64 compiler option on the command line, the compiler issues Command-Line Warning D9002. The __w64 keyword is silently ignored. Instead of using this option and keyword to detect 64-bit portability issues, use a Visual C++ compiler that targets a 64-bit platform. For more information, see Configuring Programs for 64-Bit.

Any typedef that has __w64 on it must be 32 bits on x86 and 64 bits on x64.

To detect portability issues by using versions of the Visual C++ compiler earlier than Visual Studio 2010, the __w64 keyword should be specified on any typedefs that change size between 32 bit and 64 bit platforms. For any such type, __w64 must appear only on the 32-bit definition of the typedef.

The __w64 keyword is ignored if the compilation does not use /Wp64.

For more information about porting to 64-bit, see the following topics:

// __w64.cpp  
// compile with: /W3 /Wp64  
typedef int Int_32;  
#ifdef _WIN64  
typedef __int64 Int_Native;  
typedef int __w64 Int_Native;  
int main() {  
   Int_32 i0 = 5;  
   Int_Native i1 = 10;  
   i0 = i1;   // C4244 64-bit int assigned to 32-bit int  
   // char __w64 c;  error, cannot use __w64 on char