Microsoft Specific

The __fastcall calling convention specifies that arguments to functions are to be passed in registers, when possible. This calling convention only applies to the x86 architecture. The following list shows the implementation of this calling convention.

Using the /Gr compiler option causes each function in the module to compile as __fastcall unless the function is declared by using a conflicting attribute, or the name of the function is main.

The __fastcall keyword is accepted and ignored by the compilers that target ARM and x64 architectures; on an x64 chip, by convention, the first four arguments are passed in registers when possible, and additional arguments are passed on the stack. For more information, see Overview of x64 Calling Conventions. On an ARM chip, up to four integer arguments and eight floating-point arguments may be passed in registers, and additional arguments are passed on the stack.

For non-static class functions, if the function is defined out-of-line, the calling convention modifier does not have to be specified on the out-of-line definition. That is, for class non-static member methods, the calling convention specified during declaration is assumed at the point of definition. Given this class definition:

struct CMyClass {  
   void __fastcall mymethod();  
};  

this:

void CMyClass::mymethod() { return; }  

is equivalent to this:

void __fastcall CMyClass::mymethod() { return; }  

In the following example, the function DeleteAggrWrapper is passed arguments in registers:

// Example of the __fastcall keyword  
#define FASTCALL    __fastcall  
  
void FASTCALL DeleteAggrWrapper(void* pWrapper);  
// Example of the __ fastcall keyword on function pointer  
typedef BOOL (__fastcall *funcname_ptr)(void * arg1, const char * arg2, DWORD flags, ...);  

Argument Passing and Naming Conventions
Keywords