[ACCEPTED]-Parallel assignment in C++-c++

Accepted answer
Score: 16

That's not possible. Your code example

a, b = b, a;

is 8 interpreted in the following way:

a, (b = b), a

It does 7 nothing. The comma operator makes it return 6 the value of a (the right most operand). Because 5 assignment binds tighter, b = b is in parens.

The 4 proper way doing this is just

std::swap(a, b);

Boost includes 3 a tuple class with which you can do

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

It internally 2 creates a tuple of references to a and b, and 1 then assigned to them a tuple of b and a.

Score: 4

Parallel assignment is not supported in 5 C++. Languages that support this usually 4 treat a,b,c as a list on either side of the assignment 3 operator, this isn't the way the comma operator 2 works in C++. In C++, a, b evaluates a and 1 then b, so a, b = b, a is the same as a; b = b; a;.

Score: 1

Or Perl. But no, it's not possible (as far 6 as I'm aware), you need to use a temporary 5 variable, as in:

int a = 4;
int b = 5;

    int tmp = a;
    a = b;
    b = tmp;

FYI, internally those languages 4 (or Perl atleast) create a temporary list 3 { a, b }, then assign the list to the two variables. In 2 other words, this is atleast as performant, if 1 only a little more messy.

Score: 1

This question is very old – I 25 just stumbled into it today...
...and wondered 24 why nobody gave this answer before...

I think, it's 23 possible to do it in C++11 similar like 22 Python does it (under the hood):

#include <iostream>
using namespace std;

int main()
  int a = 4, b = 5;
  cout << "Before assignment: a: " << a << ", b: " << b << endl;
  pair<int&, int&> ba(b, a);
  ba = make_pair(a, b); // <===: (b, a) = (a, b)
  cout << "After assignment : a: " << a << ", b: " << b << endl;
  return 0;

I tried 21 this out on ideone.com. The output was:

Before assignment: a: 4, b: 5
After assignment : a: 5, b: 4

If I remember 20 right (I'm not a Python expert), in Python, a, b denotes 19 a pair. (Python Doc.: 5.3. Tuples and Sequences)

Such pair can be done in C++ 11 18 easily e.g. with std::pair. In this case, I made 17 a pair of references and assigned the pair 16 of values. It works as the make_pair() loads both variables 15 before the right pair (of values) is assigned 14 to the left pair of references.

Scrolling 13 again, I realize that this answer is close 12 to the boost based solution of Johannes answer.

May be, the 11 reason is that it didn't work in C++03. I 10 tried in coliru.stacked-crooked.com: With 9 -std=c++03 it yields terrible to read compiler errors 8 &ndash; changing to -std=c++11 and it compiles 7 and executes fine as described above.


I just 6 cannot imagine what this solution is good 5 for nor what practical worth it may have. This 4 is not what I tried to do. As many other 3 answers states "It does not work." IMHO 2 it does (spelling it right according to 1 the C++ language)...

Score: 0

Or Lua...
There are tricks with C/C++, like 3 using xor or operations, but with risk of 2 overflow and such. Just do it the painful 1 way, with three assignments. Not a big deal.

Score: 0

There is no such function in the Standard 3 Library. You could write a set of template 2 functions :

template <typename T1> void ParAssign(T1& Lhs_1, T1 const& Rhs1);
template <typename T1, typename T2> void ParAssign(T1& Lhs1, T2& Lhs2, T1 const& Rhs1, T2 const& Rhs2);
// etc.

That's non-trivial if there is 1 aliasing, as in your swap example.

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