Parallel assignment in C++

Question

Is there any way of doing parallel assignment in C++? Currently, the below compiles (with warnings)

#include <iostream> 

int main() { 
  int a = 4;
  int b = 5;
  a, b = b, a;
  std::cout << "a: " << a << endl
            << "b: " << b << endl;

  return 0;
}

and prints:

a: 4
b: 5

What I'd like it to print ... if it weren't obvious, is:

a: 5
b: 4

As in, say, ruby, or python.

Answer 1

That's not possible. Your code example

a, b = b, a;

is interpreted in the following way:

a, (b = b), a

It does nothing. The comma operator makes it return the value of a (the right most operand). Because assignment binds tighter, b = b is in parens.

The proper way doing this is just

std::swap(a, b);

Boost includes a tuple class with which you can do

tie(a, b) = make_tuple(b, a);

It internally creates a tuple of references to a and b, and then assigned to them a tuple of b and a.