Question

int g_myInt = 0;
int& getIntReference() { return g_myInt; }

void myVarArgFunction( int a, ... ) {
  // .........
}

int main() {
  myVarArgFunction( 0, getIntReference() );
  return 0;
}

In the (uncompiled and untested) C++ code above, is it valid to pass in an int& into a variable argument list? Or is it only safe to pass-by-value?

Answer 1

Section 5.2.2.7 of the C++03 standard says that lvalue-to-rvalue conversion is first performed, so your function should receive a copy of the int instead of a reference.

I tried it with this program using g++ 4.6.3:

#include <iostream>
#include <stdarg.h>

int g_myInt = 7;
int& getIntReference() { return g_myInt; }

void myVarArgFunction( int a, ... )
{
  va_list ap;
  va_start(ap,a);
  int value = va_arg(ap,int);
  va_end(ap);
  std::cerr << value << "\n";
}

int main(int,char **)
{
  myVarArgFunction( 0, getIntReference() );
  return 0;
}

And got an output of 7.