CODEWITHSUNDEEP
c++functionreference
b3hn4m
b3hn4m

Reputation: 75

returning by reference vs returning by value in C++

Given the following vector and functions:

std::vector<int> param;
std::vector<int>& getparam_1() {
return param;
}

std::vector<int> getparam_2() {
return param;
}

I understand that both following expressions will be compiled:

std::vector<int> param_copy1 = getparam_1();
std::vector<int> param_copy2 = getparam_2();

But what are their differences?

Upvotes: 1

Views: 418

Answers (3)

enumerator
enumerator

Reputation: 197

The most obvious difference is you can change the global variable if you return by reference.

EXAMPLE

#include <iostream>
#include <vector>

std::vector<int> param{ 1,2,3,4,5 };
std::vector<int>& getparam_1()
{
    return param;
}

std::vector<int> getparam_2()
{
    return param;
}

int main()
{
    getparam_1().push_back(9);
    for (const auto& i : param)
    {
        std::cout << i;
    }
}

This will print "123459".

#include <iostream>
#include <vector>

std::vector<int> param{ 1,2,3,4,5 };
std::vector<int>& getparam_1()
{
    return param;
}

std::vector<int> getparam_2()
{
    return param;
}

int main()
{
    getparam_2().push_back(9);
    for (const auto& i : param)
    {
        std::cout << i;
    }
}

This will print "12345".

Upvotes: 2

user13088490
user13088490

Reputation:

Return by value

Copy of that value is returned to the caller.

Return by reference

Reference to the variable is passed back to the caller. The caller can then use this reference to continue modifying the vector.

If you are using int in the vector you do not have to bother return by reference it does not make big different. If you are using class or struct you should take consideration of using return by reference or pointer.

Upvotes: 1

eerorika
eerorika

Reputation: 238311

But what are their differences?

The outcome is identical, so there is no difference in that regard.

From perspective of the abstract machine, prior to C++17, both examples have one copy and the second example has a move from that copy. But the move can be optimised away, so there's still no practical difference necessarily. Since C++17, there is just one copy and no move in either case.

There is minor difference for the abstract machine in where that copy happens. In first example the copy is outside the function while in the second example the copy is inside the function.

Upvotes: 1

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