Microsoft Extensions to C and C++

Microsoft extends the C++ language with several additional keywords. For a complete list, see C++ Keywords in the C++ Language Reference. Keywords with two leading underscores are Microsoft extensions.

Under the standard (/Za), you need to make an out-of-class definition for data members. For example,

class CMyClass  {
   static const int max = 5;
   int m_array[max];
}
...
const int CMyClass::max;   // out of class definition

Under /Ze, the out-of-class definition is optional for static, const integral, and const enum data members. Only integrals and enums that are static and const can have initializers inside a class; the initializing expression must be a const expression.

To avoid errors when an out-of-class definition is provided (when the out-of-class definition is provided in a header file and the header file is included in multiple source files), you should use selectany. For example:

__declspec(selectany) const int CMyClass::max = 5;

The compiler supports the following two non-ANSI casts:

• Use of non-ANSI casts to produce l-values:

  Copy

  char *p;
  (( int * ) p )++;
  

  The preceding example could be rewritten to conform with the ANSI C standard as follows:

  Copy

  p = ( char * )(( int * )p + 1 );
  

• Non-ANSI casting of a function pointer to a data pointer:

  Copy

  int ( * pfunc ) (); 
  int *pdata;
  pdata = ( int * ) pfunc;
  

  To perform the same cast while maintaining ANSI compatibility, you must cast the function pointer to an int before casting it to a data pointer:

  Copy

  pdata = ( int * ) (int) pfunc;
  

The compiler supports use of a function declarator that specifies a variable number of arguments, followed by a function definition that provides a type instead:

void myfunc( int x, ... );
void myfunc( int x, char * c )
{ }

The C compiler supports single-line comments, which are introduced with two forward slash (//) characters:

// This is a single-line comment.

The C compiler supports the following scope-related features:

• Redefinitions of extern items as static:

  Copy

  extern int clip();
  static int clip()
  {}
  

• Use of benign typedef redefinitions within the same scope:

  Copy

  typedef int INT;
  typedef int INT;
  

• Function declarators have file scope:

  Copy

  void func1()
  {
      extern int func2( double );
  }
  int main( void )
  {
      func2( 4 );    //  /Ze passes 4 as type double
  }                  //  /Za passes 4 as type int
  

• Use of block-scope variables initialized with nonconstant expressions:

  Copy

  int clip( int );
  int bar( int );
  int main( void )
  {
      int array[2] = { clip( 2 ), bar( 4 ) };
  }
  int clip( int x )
  {
      return x;
  }
  int bar( int x )
  {
      return x;
  }
  

The C compiler supports the following data declaration and definition features:

• Mixed character and string constants in an initializer:

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  char arr[5] = {'a', 'b', "cde"};
  

• Bit fields with base types other than unsigned int or signed int.

• Declarators without either a storage class or a type:

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  x;
  int main( void )
  {
      x = 1;
  }
  

• Unsized arrays as the last field in structures and unions:

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  struct zero
  {
      char *c;
      int zarray[];
  };
  

• Unnamed (anonymous) structures:

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  struct
  {
      int i;
      char *s;
  };
  

• Unnamed (anonymous) unions:

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  union
  {
      int i;
      float fl;
  };
  

• Unnamed members:

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  struct s
  {
     unsigned int flag : 1;
     unsigned int : 31;
  }
  

The compiler supports inline generation x86 Specific > of the atan, atan2, cos, exp, log, log10, sin, sqrt, and tan functions END x86 Specific when /Oi is specified. For C, ANSI conformance is lost when these intrinsics are used, because they do not set the errno variable.

This is an extension to C++. The following code will compile with /Ze:

typedef   int   T;

const T  acT = 9;      // A constant of type 'T'
const T* pcT = &acT;   // A pointer to a constant of type 'T'

void func2 ( const T*& rpcT )   // A reference to a pointer to a constant of type 'T'
{
   rpcT = pcT;
}

T*   pT;               // A pointer to a 'T'

void func ()
{
   func2 ( pT );      // Should be an error, but isn't detected
   *pT   = 7;         // Invalidly overwrites the constant 'acT'
}

Under /Ze, you have to include iso646.h if you want to use text forms of the following operators:

• && (and)

• &= (and_eq)

• & (bitand)

• | (bitor)

• ~ (compl)

• ! (not)

• != (not_eq)

• || (or)

• |= (or_eq)

• ^ (xor)

• ^= (xor_eq)

The following example will output char const (*)[4] under /Za, but char const [4] under /Ze.

#include <stdio.h>
#include <typeinfo>

int main()
{
    printf_s("%s\n", typeid(&"abc").name());
}

Reference