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.