Microsoft Extensions to C and C++

Article
04/28/2015

Visual C++ extends the ANSI C and ANSI C++ standards as follows.

Keywords

Several keywords are added. In the list in C++ Keywords, the keywords that have two leading underscores are Visual C++ extensions.

Out of Class Definition of static const Integral (or enum) Members

Under the standard (/Za), you must make an out-of-class definition for data members, as shown here:

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 in a class; the initializing expression must be a const expression.

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

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

Casts

Both the C++ compiler and C compiler support these kinds of non-ANSI casts:

Non-ANSI casts to produce l-values. For example:
```
char *p;
(( int * ) p )++;
```
Note

This extension is available in the C language only. You can use the following ANSI C standard form in C++ code to modify a pointer as if it is a pointer to a different type.

The preceding example could be rewritten as follows to conform to the ANSI C standard.
```
p = ( char * )(( int * )p + 1 );
```
Non-ANSI casts of a function pointer to a data pointer. For example:
```
int ( * pfunc ) (); 
int *pdata;
pdata = ( int * ) pfunc;
```
To perform the same cast and also maintain ANSI compatibility, you can cast the function pointer to a uintptr_t before you cast it to a data pointer:
```
pdata = ( int * ) (uintptr_t) pfunc;
```

Variable-Length Argument Lists

Both the C++ compiler and C compiler support 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 )
{ }

Single-Line Comments

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

// This is a single-line comment.

Scope

The C compiler supports the following scope-related features.

Redefinitions of extern items as static:

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

Use of benign typedef redefinitions within the same scope:
```
typedef int INT;
typedef int INT;
```

Function declarators have file scope:

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 that are initialized by using nonconstant expressions:

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;
}

Data Declarations and Definitions

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

Mixed character and string constants in an initializer:
```
char arr[5] = {'a', 'b', "cde"};
```
Bit fields that have base types other than unsigned int or signed int.
Declarators that don't have a type:
```
x;
int main( void )
{
    x = 1;
}
```
Unsized arrays as the last field in structures and unions:
```
struct zero
{
    char *c;
    int zarray[];
};
```
Unnamed (anonymous) structures:
```
struct
{
    int i;
    char *s;
};
```
Unnamed (anonymous) unions:
```
union
{
    int i;
    float fl;
};
```

Unnamed members:

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

Intrinsic Floating-Point Functions

Both the C++ compiler and C compiler support 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 the C compiler, ANSI conformance is lost when these intrinsics are used, because they do not set the errno variable.

Passing a Non-Const Pointer Parameter to a Function that Expects a Reference to a Const Pointer Parameter

This is an extension to C++. This 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'
}

ISO646.H Not Enabled

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)

Address of String Literal Has Type const char [], not const char (*) []

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());
}

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