What are type traits?

Learn to query type information with type traits.

What are type traits?

Type traits are templates that give information about types at compile time. They live in the <type_traits> header. std::is_integral<T>::value is a bool constant that is true when T is an integer type and false otherwise.

Traits drive branching, type selection and constraints in templates. The C++17 _v shortcut returns the value directly: std::is_integral_v<T> equals std::is_integral<T>::value. All results are computed at compile time.

_v and ::value

A trait result is read in two ways. Classic: std::is_integral<T>::value. C++17 shortcut: std::is_integral_v<T>. Both yield the same bool constant; the shortcut simply saves typing.

Similarly std::conditional<C, A, B>::type becomes std::conditional_t<C, A, B>. _v is for values, _t is for types. These shortcuts are the standard way to write readable template code.

Exercise Write the IsIntTag template

Write template <typename T> struct IsIntTag with static const bool value = std::is_integral_v<T>; inside. In main write std::cout << IsIntTag<int>::value;.

Show example solution

#include <iostream>
#include <type_traits>

template <typename T>
struct IsIntTag {
    static const bool value = std::is_integral_v<T>;
};

int main() {
    std::cout << IsIntTag<int>::value;
    return 0;
}

Test yourself

When are type traits evaluated?
- At compile time
- At run time
- At link time
- At load time
Show answer

At compile time — Type traits are computed at compile time.

Complete the trait shortcut

template <typename T>
constexpr bool is_int = std::____<T>;

Show answer

template <typename T>
constexpr bool is_int = std::is_integral_v<T>;

Match each trait to its purpose

std::is_integral_v<T>

Is integer?
std::is_floating_point_v<T>

Is floating?
std::is_pointer_v<T>

Is pointer?
std::is_class_v<T>

Is class?

Next lesson: is_integral

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