What is the purpose of std::make_pair vs the constructor of std::pair?

Question

What is the purpose of std::make_pair?

Why not just do std::pair(0, \'a\')?

Is there any difference between the tw

Answer 1

As @MSalters replied above, you can now use curly braces to do this in C++11 (just verified this with a C++11 compiler):

pair<int, int> p = {1, 2};

Answer 2

make_pair creates an extra copy over the direct constructor. I always typedef my pairs to provide simple syntax.
This shows the difference (example by Rampal Chaudhary):

class Sample
{
    static int _noOfObjects;

    int _objectNo;
public:
    Sample() :
        _objectNo( _noOfObjects++ )
    {
        std::cout<<"Inside default constructor of object "<<_objectNo<<std::endl;
    }

    Sample( const Sample& sample) :
    _objectNo( _noOfObjects++ )
    {
        std::cout<<"Inside copy constructor of object "<<_objectNo<<std::endl;
    }

    ~Sample()
    {
        std::cout<<"Destroying object "<<_objectNo<<std::endl;
    }
};
int Sample::_noOfObjects = 0;


int main(int argc, char* argv[])
{
    Sample sample;
    std::map<int,Sample> map;

    map.insert( std::make_pair( 1, sample) );
    //map.insert( std::pair<int,Sample>( 1, sample) );
    return 0;
}

Answer 3

Class template arguments could not be inferred from the constructor before C++17

Before C++17 you could not write something like:

std::pair p(1, 'a');

since that would infer template types from the constructor arguments, you had to write it explicitly as:

std::pair<int,char> p(1, 'a');

C++17 makes that syntax possible, and therefore make_pair redundant.

Before C++17, std::make_pair allowed us to write less verbose code:

MyLongClassName1 o1;
MyLongClassName2 o2;
auto p = std::make_pair(o1, o2);

instead of the more verbose:

std::pair<MyLongClassName1,MyLongClassName2> p{o1, o2};

which repeats the types, and can be very long.

Type inference works in that pre-C++17 case because make_pair is not a constructor.

make_pair is essentially equivalent to:

template<class T1, class T2>
std::pair<T1, T2> my_make_pair(T1 t1, T2 t2) {
    return std::pair<T1, T2>(t1, t2);
}

The same concept applies to inserter vs insert_iterator.

See also:

• Why not infer template parameter from constructor?
• https://en.wikibooks.org/wiki/More_C++_Idioms/Object_Generator

Minimal example

To make things more concrete, we can observe the problem minimally with:

main.cpp

template <class MyType>
struct MyClass {
    MyType i;
    MyClass(MyType i) : i(i) {}
};

template<class MyType>
MyClass<MyType> make_my_class(MyType i) {
    return MyClass<MyType>(i);
}

int main() {
    MyClass<int> my_class(1);
}

then:

g++-8 -Wall -Wextra -Wpedantic -std=c++17 main.cpp

compiles happily, but:

g++-8 -Wall -Wextra -Wpedantic -std=c++14 main.cpp

fails with:

main.cpp: In function ‘int main()’:
main.cpp:13:13: error: missing template arguments before ‘my_class’
     MyClass my_class(1);
             ^~~~~~~~

and requires instead to work:

MyClass<int> my_class(1);

or the helper:

auto my_class = make_my_class(1);

which uses a regular function instead of a constructor.

Difference for std::reference_wrapper

This comment mentions that std::make_pair unwraps std::reference_wrapper while the constructor does not, so that's one difference. TODO example.

Tested with GCC 8.1.0, Ubuntu 16.04.

Answer 4

The difference is that with std::pair you need to specify the types of both elements, whereas std::make_pair will create a pair with the type of the elements that are passed to it, without you needing to tell it. That's what I could gather from various docs anyways.

See this example from http://www.cplusplus.com/reference/std/utility/make_pair/

pair <int,int> one;
pair <int,int> two;

one = make_pair (10,20);
two = make_pair (10.5,'A'); // ok: implicit conversion from pair<double,char>

Aside from the implicit conversion bonus of it, if you didn't use make_pair you'd have to do

one = pair<int,int>(10,20)

every time you assigned to one, which would be annoying over time...

Answer 5

starting from c++11 just use uniform initialization for pairs. So instead of:

std::make_pair(1, 2);

or

std::pair<int, int>(1, 2);

just use

{1, 2};

Answer 6

It's worth noting that this is a common idiom in C++ template programming. It's known as the Object Generator idiom, you can find more information and a nice example here.

Edit As someone suggested in the comments (since removed) the following is a slightly modified extract from the link in case it breaks.

An Object Generator allows creation of objects without explicitly specifying their types. It is based on a useful property of function templates which class templates don't have: The type parameters of a function template are deduced automatically from its actual parameters. std::make_pair is a simple example that returns an instance of the std::pair template depending on the actual parameters of the std::make_pair function.

template <class T, class U>
std::pair <T, U> 
make_pair(T t, U u)
{
  return std::pair <T, U> (t,u);
}