C++ Type Traits Standard type traits


Example

C++11

The type_traits header contains a set of template classes and helpers to transform and check properties of types at compile-time.

These traits are typically used in templates to check for user errors, support generic programming, and allow for optimizations.


Most type traits are used to check if a type fulfils some criteria. These have the following form:

template <class T> struct is_foo;

If the template class is instantiated with a type which fulfils some criteria foo, then is_foo<T> inherits from std::integral_constant<bool,true> (a.k.a. std::true_type), otherwise it inherits from std::integral_constant<bool,false> (a.k.a. std::false_type). This gives the trait the following members:

Constants

static constexpr bool value

true if T fulfils the criteria foo, false otherwise

Functions

operator bool

Returns value

C++14

bool operator()

Returns value

Types

NameDefinition
value_typebool
typestd::integral_constant<bool,value>

The trait can then be used in constructs such as static_assert or std::enable_if. An example with std::is_pointer:

template <typename T>
void i_require_a_pointer (T t) {
    static_assert(std::is_pointer<T>::value, "T must be a pointer type");
}

//Overload for when T is not a pointer type
template <typename T>
typename std::enable_if<!std::is_pointer<T>::value>::type
does_something_special_with_pointer (T t) {
    //Do something boring
}

//Overload for when T is a pointer type
template <typename T>
typename std::enable_if<std::is_pointer<T>::value>::type 
does_something_special_with_pointer (T t) {
    //Do something special
}

There are also various traits which transform types, such as std::add_pointer and std::underlying_type. These traits generally expose a single type member type which contains the transformed type. For example, std::add_pointer<int>::type is int*.