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c++type-traits

what is the difference between std::conditional and std::conditional_t c++

I cannot find in c++ the difference between std::conditional< >::type and std::conditional_t< > .

When I compile

 using A = typename conditional< true, int, char>::type;                     
 using B = typename conditional_t< true, int, char>::type;

an error: expected nested-name-specifier gets out. I was unable to use conditional and nest, while conditional_t seems to nest.

Solution

  • std::conditional_t is just a helper alias. From cppreference:

    template< bool B, class T, class F >
using conditional_t = typename conditional<B,T,F>::type;

    On the same link you can also find the statement:

    The behavior of a program that adds specializations for std::conditional is undefined.

    The same is not true for type traits you write or that come with libraries. And then the crux is that the fact that some_trait::type is a type is only a convention. Nothing in the language forbids you to specialize some_trait such that type is a static member of type int whose value is 42.

    And thats why you need to disambiguate the member alias type via typename (see When is the "typename" keyword necessary?). std::conditional_t on the other hand is a type alias. It can only be a type. Its a helper that makes the use of the trait more convenient.

    std::conditional_t (usually) has no member alias type. It is an alias for the member alias type of std::conditional.