What is the purpose of std::make_pair?
Why not just do std::pair<int, char>(0, 'a')?
Is there any difference between the two methods?
47 Answers
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
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...
4As @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};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:
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.cppcompiles happily, but:
g++-8 -Wall -Wextra -Wpedantic -std=c++14 main.cppfails 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.
4There is no difference between using make_pair and explicitly calling the pair constructor with specified type arguments. std::make_pair is more convenient when the types are verbose because a template method has type deduction based on its given parameters.
For example,
std::vector< std::pair< std::vector<int>, std::vector<int> > > vecOfPair;
std::vector<int> emptyV;
// shorter
vecOfPair.push_back(std::make_pair(emptyV, emptyV)); // longer
vecOfPair.push_back(std::pair< std::vector<int>, std::vector<int> >(emptyV, emptyV));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);
}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;
}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};
