Why std::make_shared has to be provided type info but std::make_pair doesn't have to?

Question

For example, when we want to create a std::pair, we can simply call:

std::make_pair(5, "foo");

But when we intend to create a shared_ptr, we have to provide type info:

std::make_shared<Foo>("Boo", 1024);

Is there any reason that makes them different?

Answer 1

The function std::make_pair<T1, T2> creates a pair std::pair<T1, T2> where the types T1 and T2 are the types of arguments of the function. As such they can be deduced by the compiler and normally you do not need to provide them explicitly.

The function std::make_shared<T> creates a new object of type T and returns shared_ptr<T> storing this object. The function passes all its arguments to the constructor of T. There is no way for the compiler to deduce the type T from the arguments. Even if the argument is e.g. const T& the compiler cannot know if you want to create shared_ptr<T> or shared_ptr<SomeOtherTypeWithConstructorAcceptingT>.

Answer 2

std::make_shared should be called emplace_shared -- it calls an arbitrary ctor.

make_pair constructs a copy or move of its types.

They have different purposes: one is for type deduction help, the other for emplace construction. They are both factories, hence the make name.