Microsoft Extensions to C and C++

Article
06/30/2006

The following are Visual C++ extensions to the ANSI C and ANSI C++ standards:

Keywords

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

Windows CE .NET supports the __based, __finally, and __try keywords.

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

Under the standard /Za - Disable Language Extensions, 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 - Enable Language Extensions, the out-of-class definition is optional for static, constintegral, and constenum 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 a header file provides an out-of-class definition and when multiple source files include the header file, you should use selectany. For example:

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

Casts

The compiler supports the following two non-ANSI casts:

Non-ANSI casts to produce l-values:
```
char *p;
(( int * ) p )++;
```
Non-ANSI casting of a function pointer to a data pointer:
```
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:
```
pdata = ( int * ) (int) pfunc;
```

Variable-Length Argument Lists

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 )
{ }

Single-Line Comments

The C compiler supports single-line comments introduced with 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;
```

File scope for function declarators:

void func1()
{
    extern int func2( double );
}
void 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:

int clip( int );
int bar( int );
void 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 with base types other than unsigned int or signed int.
Declarators without either a storage class or a type:
```
x;
void 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

The compiler supports inline generation of the atan, atan2, cos, exp, log, log10, sin, sqrt, and tan functions. Function generation is specific with /Oi - Generate Intrinsic Functions. For C, ANSI conformance is lost when these intrinsics are used.

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

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'
}

new Operator Does Not Throw Exception

If the new operator fails to allocate requested memory, no exception will be thrown; a null pointer is returned instead.

Scope of Variable in for Loop Stays in Effect After Loop

The following code will compile under /Za. However, to keep compatibility with older code bases, it will not compile under /Ze:

int main () {
   for (int i = 0; i<1; ++i);
   for (int i = 0; i<1; ++i);
   return 0;
}

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)

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Microsoft Extensions to C and C++

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