CODEWITHSUNDEEP
c++c++11std-pairstatic-assert
rubenvb
rubenvb

Reputation: 76805

Why does std::make_pair not return a pair? Or does it?

My internal sanity check failed so I'm rerunning it on Stackoverflow.

The following code:

#include <iostream>
#include <typeinfo>
#include <utility>

int main()
{
    constexpr auto pair_of_ints = std::make_pair(1, 2);
    std::cerr << typeid(pair_of_ints).name();
    //static_assert(std::is_same<decltype(pair_of_ints), std::pair<int, int>>::value, "WTF");
}

produces the mangled symbol name for std::__1::pair<int, int> on my system (XCode Clang 8.x).

If I then enable the static_assert, it fails. I have no idea why. How can I make this work? I have a function that returns a pair or tuple depending on the arguments passed to it and would like to verify it actually returns a pair or tuple in the correct cases.

Upvotes: 20

Views: 3294

Answers (1)

Holt
Holt

Reputation: 37706

You declared pair_of_ints as constexpr which implies const:

[dcl.constexpr]#9

A constexpr specifier used in an object declaration declares the object as const.

So the type of pair_of_ints is actually:

const std::pair<int, int>

typeid ignores cv-qualifiers, which is why this information does not appear in the name:

[expr.typeid]#5

If the type of the expression or type-id is a cv-qualified type, the result of the typeid expression refers to a std::type_info object representing the cv-unqualified type.

You could either test against the const-qualified type, or drop the const-qualifier using std::remove_const_t:

static_assert(std::is_same<decltype(pair_of_ints), 
                           const std::pair<int, int>>::value);
static_assert(std::is_same<std::remove_const_t<decltype(pair_of_ints)>, 
                           std::pair<int, int>>::value);

Upvotes: 37

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