What does return a,b do and why?

Question

When we use "return 4,5" in c++ it does not give error but instead returns 5 (at least 4 would be understandable as it should return the first number it encounters) . Why does this happen and can we use this to return 2 values in any way?

Here is the code that i tried

#include<iostream>
using namespace std;
int something()
{
    return 4,5;
}
int main()
{
    int a=0,b=0;
    a,b = something();
    cout<<a<<b<<endl;
}

also in the above code for some reason 5 is assigned to b instead of a

_"it should return the first number it encounters"_ Why's that then? — Lightness Races in Orbit, Aug 14 '16 at 16:07

score 3 · Accepted Answer · answered Aug 14 '16 at 16:03

3

This is how comma operator works - it evaluates all the operands and returns the last one.

Unfortunately, C++ does not have built-in tuple (like int, double, etc.) type, so, it is impossible to return more than one value from the function. However, you could use wrapper type std::tuple and then unpack it using std::tie function:

#include <iostream>
#include <tuple>

std::tuple<int, int> something()
{
    return {1, 2};
}

int main()
{
    int a=0, b=0;
    std::tie(a, b) = something();

    std::cout << a << b << std::endl;
}

This is a little bit overhead for two variables, though.

answered Aug 14 '16 at 16:03

awesoon

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Or maybe `auto [a, b] = something();` :-) – Kerrek SB Aug 14 '16 at 16:09
@LightnessRacesinOrbit: That's C++17... – Kerrek SB Aug 14 '16 at 16:11
@KerrekSB: I'm interested and terrified at the same time. – Lightness Races in Orbit Aug 14 '16 at 16:11
I have next to no knowledge of what's coming up in C++17. Only just migrating our production environment to C++11! (Blame widely-used enterprise systems with support for many years yet whose default compiler is still GCC 4.4) – Lightness Races in Orbit Aug 14 '16 at 16:12
@LightnessRacesinOrbit: You should be. It's [dcl.decomp] in the WD. – Kerrek SB Aug 14 '16 at 16:13
niiice ,tie is really handy for making the code more readable ,instead of streaching it out on multiple lines – Humus Aug 14 '16 at 16:15
@KerrekSB: My Ctrl+F finds no match in N4594. Nor for "`auto [`". – Lightness Races in Orbit Aug 14 '16 at 17:50
@LightnessRacesinOrbit: What outdated nonsense is that -- look at [N4604](http://wg21.link/n4604)! – Kerrek SB Aug 14 '16 at 18:40
@KerrekSB: Oh dear lord. – Lightness Races in Orbit Aug 14 '16 at 18:55

Jesper Juhl · Answer 2 · 2016-08-14T16:13:06.620

2

It's using the built-in comma operator (since these are not user defined types). For user defined types it would be calling operator,() (the comma operator).

The comma operator evaluates both sides of the operator and returns the result of the latter. Which is why you get 5 and not 4 as the result.

As to why it is done here I cannot say - seems silly.

edited Aug 14 '16 at 16:13

answered Aug 14 '16 at 15:55

Jesper Juhl

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It most certainly isn't calling `operator,()`, as these are values of a built-in type. It's the comma operator, but it's definitely not resulting in a call to any `operator,()`. – Lightness Races in Orbit Aug 14 '16 at 16:07
No functions are called for the code in question. – Kerrek SB Aug 14 '16 at 16:08
Ok, ok, it's using the default, built-in comma operator in this case, but that's a detail. – Jesper Juhl Aug 14 '16 at 16:09
If by "a detail" you mean "the truth" then yes – Lightness Races in Orbit Aug 14 '16 at 16:10
No, that's a pretty big deal. The built-in comma operator has different ordering semantics than a function call, for example. – Kerrek SB Aug 14 '16 at 16:10
For the OP to understand what is going on it makes no difference. But you are of course correct. – Jesper Juhl Aug 14 '16 at 16:10
1

For the OP to understand what is going on it makes a huge difference. You're misteaching. Don't we have enough of that in our OPs' schools? :( – Lightness Races in Orbit Aug 14 '16 at 16:11
I'll edit the answer. Better now? – Jesper Juhl Aug 14 '16 at 16:11
@JesperJuhl: It would only be calling a function *if the user defined an overload*. – Kerrek SB Aug 14 '16 at 16:14

score 1 · Answer 3 · edited Aug 14 '16 at 16:08

1

If you want to return two values, return a std::vector with them. Maybe a std::pair, or a class. As far as why, this is just how C++ works. Comma is just an operator, like + or -. It discards its left operand and returns the right one. return returns the value of its expression from the function. The rest you can figure out yourself.

edited Aug 14 '16 at 16:08

Lightness Races in Orbit

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answered Aug 14 '16 at 15:58

Sam Varshavchik

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The comma operator also *evaluates* its left operand. – Kerrek SB Aug 14 '16 at 16:08

What does return a,b do and why?

3 Answers3