CODEWITHSUNDEEP
c++
void.pointer
void.pointer

Reputation: 26355

Passing reference types into variable argument list

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?

Upvotes: 1

Views: 473

Answers (3)

Vaughn Cato
Vaughn Cato

Reputation: 64308

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.

Upvotes: 2

scientiaesthete
scientiaesthete

Reputation: 929

No, it is not valid to pass in a reference because it is a non-POD type.

From the spec:

If the argument has a non-POD class type, the behavior is undefined.

See this related question on passing std::string, which is non-POD.
Code for reference: http://codepad.org/v7cVm4ZW

Upvotes: 0

bhuwansahni
bhuwansahni

Reputation: 1844

Yes, you can pass a reference to an int as here. You will be changing the global variable g_myInt if you changed that parameter in the function myVarArffunction...

Upvotes: -1

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