Check Types

Template Metaprogramming is programming at compile time. But what has template metaprogramming in common with the type-traits library? A lot! The type-traits library is pure template metaprogramming, tamed in a library. With this post, my presentation of the type-traits library becomes more structured.

Check type properties

The type-trait library supports primary and composite type categories. You get the answer with the attribute value.

Primary type categories

C++ has 14 primary type categories. They are complete and orthogonal. This means that each type is exactly a member of one type category. The check for the type categories is independent of the type qualifiers const or volatile.

The 14 primary type categories:

template <class T> struct is_void;
template <class T> struct is_integral;
template <class T> struct is_floating_point;
template <class T> struct is_array;
template <class T> struct is_pointer;
template <class T> struct is_reference;
template <class T> struct is_member_object_pointer;
template <class T> struct is_member_function_pointer;
template <class T> struct is_enum;
template <class T> struct is_union;
template <class T> struct is_class;
template <class T> struct is_function;
template <class T> struct is_lvalue_reference;
template <class T> struct is_rvalue_reference;

And here is the application of the primary type categories:

//  primaryTypeCategories.cpp

#include <iostream>
#include <type_traits>

struct A{
  int a;
  int f(int){return 2011;}
};

enum E{
  e= 1,
};

union U{
  int u;
};


int main(){
  
  std::cout <<  std::boolalpha <<  std::endl;

  std::cout << std::is_void<void>::value << std::endl;
  std::cout << std::is_integral<short>::value << std::endl;
  std::cout << std::is_floating_point<double>::value << std::endl;
  std::cout << std::is_array<int [] >::value << std::endl;
  std::cout << std::is_pointer<int*>::value << std::endl;
  std::cout << std::is_reference<int&>::value << std::endl;
  std::cout << std::is_member_object_pointer<int A::*>::value <<  std::endl;
  std::cout << std::is_member_function_pointer<int (A::*)(int)>::value << std::endl;
  std::cout << std::is_enum<E>::value << std::endl;
  std::cout << std::is_union<U>::value << std::endl;
  std::cout << std::is_class<std::string>::value << std::endl;
  std::cout << std::is_function<int * (double)>::value << std::endl;	
  std::cout << std::is_lvalue_reference<int&>::value << std::endl;
  std::cout << std::is_rvalue_reference<int&&>::value << std::endl;
  
  std::cout <<  std::endl;

}

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Thanks to the use of the flag std::boolalpha in line 22, the program displays true or false instead of 1 or 0. Each call of the 14 primary type categories returns true.

How does the magic work?

The technique’s key is based on templates and template specialization, a few conventions, and a lot of typing. I wrote a possible implementation of the function template std::integral. std::integral will check if the type is integral.

// integral.cpp

#include <iostream>
#include <type_traits>

namespace rgr{

  template<class T, T v>
  struct integral_constant {
      static constexpr T value = v;
      typedef T value_type;
      typedef integral_constant type;
      constexpr operator value_type() const noexcept { return value; }
      constexpr value_type operator()() const noexcept { return value; } //since c++14
  };

  typedef integral_constant<bool, true> true_type;
  typedef integral_constant<bool, false> false_type;

  template <class T>
  struct is_integral : public false_type{};

  template <>
  struct is_integral<bool> : public true_type{};

  template <>
  struct is_integral<char> : public true_type{};

  template <>
  struct is_integral<signed char> : public true_type{};

  template <>
  struct is_integral<unsigned char> : public true_type{};

  template <>
  struct is_integral<wchar_t> : public true_type{};

  template <>
  struct is_integral<short> : public true_type{};

  template <>
  struct is_integral<int> : public true_type{};

  template <>
  struct is_integral<long> : public true_type{};

  template <>
  struct is_integral<long long> : public true_type{};

  template <>
  struct is_integral<unsigned short> : public true_type{};

  template <>
  struct is_integral<unsigned int> : public true_type{};

  template <>
  struct is_integral<unsigned long> : public true_type{};

  template <>
  struct is_integral<unsigned long long> : public true_type{};
  
}

int main(){
  
  std::cout << std::boolalpha << std::endl;
  
  std::cout << "std::is_integral<int>::value: " << std::is_integral<int>::value << std::endl;
  std::cout << "rgr::is_integral<int>::value: " << rgr::is_integral<int>::value << std::endl;
  
  std::cout << "std::is_integral<double>::value: " << std::is_integral<double>::value << std::endl;
  std::cout << "rgr::is_integral<double>::value: " << rgr::is_integral<double>::value << std::endl;
  
  std::cout << std::endl;
  
  std::cout << "std::true_type::value: " << std::true_type::value << std::endl;
  std::cout << "rgr::true_type::value: " << rgr::true_type::value << std::endl;
  
  std::cout << "std::false_type::value: " << std::false_type::value << std::endl;
  std::cout << "rgr::false_type::value: " << rgr::false_type::value << std::endl;
  
  std::cout << std::endl;
  
  std::cout << "std::integral_constant<bool, true>::value: " << std::integral_constant<bool, true>::value << std::endl;
  std::cout << "rgr::integral_constant<bool, true>::value: " << rgr::integral_constant<bool, true>::value << std::endl;
  
  std::cout << "std::integral_constant<bool, false>::value: " << std::integral_constant<bool, false>::value << std::endl;
  std::cout << "rgr::integral_constant<bool, false>::value: " << rgr::integral_constant<bool, false>::value << std::endl;  
  
  std::cout << std::endl;
  
}

I use in my implementation the namespace rgr and compare my implementation with type-traits implementation in the namespace std. The invocation of the function template rgr::is_integral<int>::value (line 69) causes under the hood the invocation of the expression rgr::true_type::value (line 77) because integral<int> is derived from true_type (line 42). rgr::true_type::value is an alias for rgr::integral_constant<bool, true>::value (line 17). I use only in the example the static constexpr value of the class integral_constant. integral_constant is the base class of the type-traits functions.

For completeness, the output of the program. My implementation behaves like the type-traits library.

Based on the 14 primary type categories, there are seven composite type categories in C++.

Composite type categories

The is_fundamental type category uses the function template is_same. More about I in the next post, in which I will write about type comparisons with the type-traits library.

There are more type checks possible with the type-traits.

Type properties

In addition to the primary and composite type categories, you can check the type properties.

    template <class T> struct is_const;
    template <class T> struct is_volatile;
    template <class T> struct is_trivial;
    template <class T> struct is_trivially_copyable;
    template <class T> struct is_standard_layout;
    template <class T> struct is_pod;
    template <class T> struct is_literal_type;
    template <class T> struct is_empty;
    template <class T> struct is_polymorphic;
    template <class T> struct is_abstract;
    template <class T> struct is_signed;
    template <class T> struct is_unsigned;
    template <class T, class... Args> struct is_constructible;
    template <class T> struct is_default_constructible;
    template <class T> struct is_copy_constructible;
    template <class T> struct is_move_constructible;
    template <class T, class U> struct is_assignable;
    template <class T> struct is_copy_assignable;
    template <class T> struct is_move_assignable;
    template <class T> struct is_destructible;
    template <class T, class... Args> struct is_trivially_constructible;
    template <class T> struct is_trivially_default_constructible;
    template <class T> struct is_trivially_copy_constructible;
    template <class T> struct is_trivially_move_constructible;
    template <class T, class U> struct is_trivially_assignable;
    template <class T> struct is_trivially_copy_assignable;
    template <class T> struct is_trivially_move_assignable;
    template <class T> struct is_trivially_destructible;
    template <class T, class... Args> struct is_nothrow_constructible;
    template <class T> struct is_nothrow_default_constructible;
    template <class T> struct is_nothrow_copy_constructible;
    template <class T> struct is_nothrow_move_constructible;
    template <class T, class U> struct is_nothrow_assignable;
    template <class T> struct is_nothrow_copy_assignable;
    template <class T> struct is_nothrow_move_assignable;
    template <class T> struct is_nothrow_destructible;
    template <class T> struct has_virtual_destructor;

Many of the function templates like is_trivially_copyable, have the name component trivially. That means these methods must be generated by the compiler and not by the developer. A method you explicitly request from the compiler with the keyword default is also trivial.

What’s next?

The type-traits library has a lot to offer. I will write in the next post about type comparison and type modifications at compile time.

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