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

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The trait can then be used in constructs such as [static_assert](<http://stackoverflow.com/documentation/c%2b%2b/3822/static-assert#t=201607281007230765701>) or [std::enable_if](<http://stackoverflow.com/documentation/c%2b%2b/1169/sfinae-substitution-failure-is-not-an-error/3777/enable-if#t=201607281006475141537>). 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
}