How to check if a template parameter is a struct/class?

Question

For a small example like this, I want to only accept T if T is a struct/class and reject builtin types like 'int', 'char', 'bool' etc.

template<typename T>
struct MyStruct
{
   T t;
};

Answer 1

You are looking for std::is_class traits from <type_traits> header. Which

Checks whether T is a non-union class type. Provides the member constant value which is equal to true, if T is a class type (but not union). Otherwise, value is equal to false.

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For instance, you can static_assert for the template type T like follows:

#include <type_traits> // std::is_class

template<typename T>
struct MyStruct
{
   static_assert(std::is_class<T>::value, " T must be struct/class type!");
   T t;
};

(See a demo)

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c++20 concept Updates

In C++20, one can provide a concept using std::is_class as follows too.

#include <type_traits> // std::is_class

template <class T> // concept
concept is_class = std::is_class<T>::value;

template<is_class T> // use the concept
struct MyStruct
{
   T t;
};

(See a demo)