The Type-Traits Library: Type Comparisons

In my last post, “The Type-Traits Library: Type Checks” I introduced type checks at compile time with the type-traits library. Today, I write about type comparisons at compile time.

The type-traits library empowers you to compare types at compile time. Compile time means that there are no costs involved at run time.

Comparing types

The type-traits library supports in C++11 three kinds of comparisons:

is_base_of<Base, Derived>
is_convertible<From, To>
is_same<T, U>

With C++20, we additionally got

is_pointer_interconvertible_with_class<From, To>
is_pointer_interconvertible_base_of<Base, Derived>

For simplicity reasons, I write only about the C++11 metafunctions.

Thanks to its member value, each class template returns true or false and is the optimal fit for static_assert.

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// compare.cpp

#include <cstdint>
#include <iostream>
#include <type_traits>

class Base{};
class Derived: public Base{};

int main(){
  
  std::cout << std::boolalpha << '\n';
  
  std::cout << "std::is_base_of<Base,Derived>::value: " << std::is_base_of<Base,Derived>::value << '\n';
  std::cout << "std::is_base_of<Derived,Base>::value: " << std::is_base_of<Derived,Base>::value << '\n';
  std::cout << "std::is_base_of<Derived,Derived>::value: " << std::is_base_of<Derived,Derived>::value << '\n';
  
  // static_assert(std::is_base_of<Derived,Base>::value,"Derived is not base of Base");                  // (1) 
  
  std::cout << '\n';
  
  std::cout << "std::is_convertible<Base*,Derived*>::value: " << std::is_convertible<Base*,Derived*>::value << '\n';
  std::cout << "std::is_convertible<Derived*,Base*>::value: " << std::is_convertible<Derived*,Base*>::value << '\n';
  std::cout << "std::is_convertible<Derived*,Derived*>::value: " << std::is_convertible<Derived*,Derived*>::value << '\n';
  
  // static_assert(std::is_convertible<Base*,Derived*>::value,"Base* can not be converted to Derived*");  // (2)
  
  std::cout << '\n';
  
  std::cout << "std::is_same<int, int32_t>::value: " << std::is_same<int, int32_t>::value << '\n';
  std::cout << "std::is_same<int, int64_t>::value: " << std::is_same<int, int64_t>::value << '\n';
  std::cout << "std::is_same<long int, int64_t>::value: " << std::is_same<long int, int64_t>::value << '\n';
  
  // static_assert(std::is_same<int, int64_t>::value,"int is not the same type as int64_t");              // (3)
  
  std::cout << '\n';
  
}

The output of the program should not surprise you.

If I use the static_assert in lines (1), (2), and (3), the assertion fires at compile time:

The website cppreference.com contains possible implementations of all the metafunctions std::is_base_of,std::is_convertible,, and std::is_same. It’s pretty interesting and challenging to study them.

Possible Implementations

First of all, here are possible implementations of the three metafunctions. Let me start with the simplest one, based on std::is_same.

`std::is_same`

I use in the following example the namespace rgr to distinguish my implementation from the C++ standard implementation.

// isSame.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;                      // (2)              
  typedef integral_constant<bool, false> false_type;
  
  template<class T, class U>
  struct is_same : false_type {};                                       // (3)
 
  template<class T>
  struct is_same<T, T> : true_type {};

}

int main() {

    std::cout << '\n';

    std::cout << std::boolalpha;

    std::cout << "rgr::is_same<int, const int>::value: " 
              << rgr::is_same<int, const int>::value << '\n';          // (1)
    std::cout << "rgr::is_same<int, volatile int>::value: " 
              << rgr::is_same<int, volatile int>::value << '\n';
    std::cout << "rgr::is_same<int, int>::value: "  
              << rgr::is_same<int, int>::value << '\n';

    std::cout << '\n';

    std::cout << "std::is_same<int, const int>::value: " 
              << std::is_same<int, const int>::value << '\n';
    std::cout << "std::is_same<int, volatile int>::value: " 
              << std::is_same<int, volatile int>::value << '\n';
    std::cout << "std::is_same<int, int>::value: "  
              << std::is_same<int, int>::value << '\n';

    std::cout << '\n';

}

A short reminder: The invocation of the function template rgr::is_same<int, const int> (line 1) causes the invocation of the expression rgr::false_type::value (line 2), because std::is_same<> is derived from false_type (line 3). rgr::false_type::value is an alias for rgr::integral_constant<bool, false>::value (line 2). I use in the example the static constexpr value of the class integral_constant. integral_constant is the base class of the type-traits functions.

Two facts are interesting when you study the following output. My implementations rgr::is_same behaves such as std::is_same, and const and volatile are part of the type.

It’s pretty straightforward to implement the metafunction isSameIgnoringConstVolatile based on the metafunction is_same.

// isSameIgnoringConstVolatile.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, class U>
  struct is_same : false_type {};
 
  template<class T>
  struct is_same<T, T> : true_type {};
  
  template<typename T, typename U>                                    // (1)
  struct isSameIgnoringConstVolatile: rgr::integral_constant<
         bool,
         rgr::is_same<typename std::remove_cv<T>::type, 
                      typename std::remove_cv<U>::type>::value  
     > {};

}

int main() {

    std::cout << '\n';

    std::cout << std::boolalpha;

    std::cout << "rgr::isSameIgnoringConstVolatile<int, const int>::value: " 
              << rgr::isSameIgnoringConstVolatile<int, const int>::value << '\n';
    std::cout << "rgr::is_same<int, volatile int>::value: " 
              << rgr::isSameIgnoringConstVolatile<int, volatile int>::value << '\n';
    std::cout << "rgr::isSameIgnoringConstVolatile<int, int>::value: "  
              << rgr::isSameIgnoringConstVolatile<int, int>::value << '\n';

    std::cout << '\n';

}

The meta function isSameIgnoringConstVolatile derives from rgr::integral_constant and uses the function std::remove_cv to remove const or volatile from its types. As you may assume, std::remove_cv is a function from the type-traits library and allows you the modify types at compile time. I will write more about type modification in my next post.

Finally, here is the output of the program:

Let’s have a closer look at the two metafunctions std::is_base_of, and std::is_convertible. Here are the possible implementations from cppreference.com.

`std::is_base_of`

namespace details {
    template <typename B>
    std::true_type test_pre_ptr_convertible(const volatile B*);
    template <typename>
    std::false_type test_pre_ptr_convertible(const volatile void*);
 
    template <typename, typename>
    auto test_pre_is_base_of(...) -> std::true_type;
    template <typename B, typename D>
    auto test_pre_is_base_of(int) ->
        decltype(test_pre_ptr_convertible<B>(static_cast<D*>(nullptr)));
}
 
template <typename Base, typename Derived>
struct is_base_of :
    std::integral_constant<
        bool,
        std::is_class<Base>::value && std::is_class<Derived>::value &&
        decltype(details::test_pre_is_base_of<Base, Derived>(0))::value
    > { };

`std::is_convertible`

namespace detail {
 
template<class T>
auto test_returnable(int) -> decltype(
    void(static_cast<T(*)()>(nullptr)), std::true_type{}
);
template<class>
auto test_returnable(...) -> std::false_type;
 
template<class From, class To>
auto test_implicitly_convertible(int) -> decltype(
    void(std::declval<void(&)(To)>()(std::declval<From>())), std::true_type{}
);
template<class, class>
auto test_implicitly_convertible(...) -> std::false_type;
 
} // namespace detail
 
template<class From, class To>
struct is_convertible : std::integral_constant<bool,
    (decltype(detail::test_returnable<To>(0))::value &&
     decltype(detail::test_implicitly_convertible<From, To>(0))::value) ||
    (std::is_void<From>::value && std::is_void<To>::value)
> {};

Now, you know why I explained std::is_same. Here is my challenge.

My Challenge

Explain the previous implementation of the type-traits functions std::is_base_of and std::is_convertible. Send your explanation to Rainer.Grimm@ModernesCpp.de until Thursday (December 2th). The best answer for each function gets a coupon for my LeanPub bundle Modern C++ Collection.

I will publish the best answer to each function in my next post and name your first name. If I should name your full name, please write it.

What’s next?

Thanks to the type-traits library, you can modify types at compile time. I write about it in my next post.

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