PRB: Using References with va_* Macros from stdarg.hLast reviewed: July 31, 1997Article ID: Q119394 |
The information in this article applies to:
SYMPTOMSIn Microsoft C++, if you use functions that accept a variable number of arguments, you may encounter problems when trying to use the va_* family of functions to access the parameters if the second parameter used for the va_start macro is a reference type.
CAUSEThis problem is caused by the way that the va_start macro is defined and the way that the C++ language handles taking the address of a reference. Applying the "address of" operator to a reference type results in a pointer to the object that is being referred to. The va_start macro takes the address of the last named parameter to locate subsequent parameters. When the last named parameter is a reference, this causes problems because the macro is no longer referring to the current call stack but whatever follows the object being referred to, which could be a previous call stack or a global memory object.
RESOLUTIONThe workaround is to redefine the va_start macro to use inline assembly to subvert the C++ language. NOTE: This solution is not portable and will require changing if you intend your source code to be used on non-Intel platforms.
MORE INFORMATIONThe va_start macro is used in conjunction with the va_arg macro to "walk" the stack to get the parameters passed to the variable argument list. The va_start macro is defined as follows:
#define va_start(ap,v) ( ap = (va_list)&v + _INTSIZEOF(v) )where va_list is defined as a char * on Intel platforms. The macro parameter "ap" is of type va_list. The problem arises from taking the address of the second parameter, "v", if v is a reference type. The net result of this macro being expanded is that ap is supposed to point to the first of the variable parameters. Casting v to a non-reference type intuitively seems like the logical solution, but because the result of a cast is not an l-value, the compiler returns an error message. NOTE: The only way to get an l-value from a cast is to cast the value to a reference type, which results in the same problem.
Sample CodeThe sample code below demonstrates a solution for this problem:
/* Compile options needed: none */ #include <stdio.h> #include <stdarg.h> // Uncomment the following lines to work-around the problem: // // #ifdef va_start // #undef va_start // // #ifdef _WIN32 // #define va_start(ap,v) {int var= _INTSIZEOF(v); \ // __asm lea eax,v __asm add eax,var __asm mov ap,eax \ // } // #else // #define va_start(ap,v) { int var=_INTSIZEOF(v);\ // __asm lea ax,v __asm add ax,var __asm mov ap,ax\ // } // #endif // #endif void numprint( int &first ... ) { va_list ap; va_start( ap, first ); printf("%d\n", first ); int ival = va_arg( ap, int ); printf("%d\n", ival ); double dval = va_arg( ap, double ); printf( "%.2f\n", dval ); va_end(ap); } void main() { int i=100,j=1000; float f=999.99; numprint( i,j,f ); } |
Additional query words: ellipsis
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