Difference between make_pair and curly brackets { } for assigning a pair in C++?

Question

I didn't find anyone answering this, is there any difference between the following :

v.push_back({x, y});

and :

v.push_back(make_pair(x, y));

Assuming that v was declared this way :

vector<pair<int,int> > v;

Answer 1

I think you might have accepted that answer a little too quickly. The commonly accepted way to do this is like this:

vec.emplace_back (x, y);

And if you look at Godbolt, you can see that this inlines everything (which may or may not be what you want):

https://godbolt.org/z/aCl02d

Run it at Wandbox:

https://wandbox.org/permlink/uo3OqlS2X4s5YpB6

Code:

#include <vector>
#include <iostream>

int x = 1;
int y = 2;
std::vector<std::pair<int,int>> vec;

int main () {
    vec.push_back(std::make_pair(x, y));
    std::cout << "make_pair done\n";
    vec.push_back({x, y});
    std::cout << "push_back done\n";
    vec.emplace_back (x, y);
    std::cout << "emplace_back done\n";

    for (std::pair <int, int> p : vec)
    {
        std::cout << p.first << ", " << p.second << "\n";
    }
}

Output:

make_pair done
push_back done
emplace_back done
1, 2
1, 2
1, 2

Of course, everything runs faster if you reserve the appropriate number of elements in the vector up front. Maybe that's what the people posing this question are really wanting you to say.