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Thinking in C++
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Operator comma

The comma operator is called when it appears next to an object of the type the comma is defined for. However, “operator, is not called for function argument lists, only for objects that are out in the open, separated by commas. There doesn’t seem to be a lot of practical uses for this operator; it’s in the language for consistency. Here’s an example showing how the comma function can be called when the comma appears before an object, as well as after:

//: C12:OverloadingOperatorComma.cpp
#include <iostream>
using namespace std;

class After {
  const After& operator,(const After&) const {
    cout << "After::operator,()" << endl;
    return *this;

class Before {};

Before& operator,(int, Before& b) {
  cout << "Before::operator,()" << endl;
  return b;

int main() {
  After a, b;
  a, b;  // Operator comma called

  Before c;
  1, c;  // Operator comma called
} ///:~

The global function allows the comma to be placed before the object in question. The usage shown is fairly obscure and questionable. Although you would probably use a comma-separated list as part of a more complex expression, it’s too subtle to use in most situations.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire