Macros for generating concept-like type traits to check for member functions of objects. More...

#include <type_traits>

Macros
#define	KAMPING_MAKE_HAS_MEMBER(Member)
	Macro for generating has_member_xxx and has_member_xxx_v templates. They return true if the type given as template parameter has a member (template) function provided name.

Detailed Description

Macros for generating concept-like type traits to check for member functions of objects.

Macro Definition Documentation

◆ KAMPING_MAKE_HAS_MEMBER

#define KAMPING_MAKE_HAS_MEMBER ( Member )

Value:

    template <typename Type, typename... MemberArgs>                                                            \
    class has_member_##Member {                                                                                 \
        template <                                                                                              \
            typename C,                                                                                         \
            typename = std::void_t<decltype(std::declval<C>().Member(std::declval<MemberArgs>()...))>>          \
        static auto test(int) -> std::true_type;                                                                \
        template <typename C>                                                                                   \
        static auto test(long) -> std::false_type;                                                              \
        template <                                                                                              \
            typename C,                                                                                         \
            typename... TemplateParams,                                                                         \
            typename = std::void_t<                                                                             \
                decltype(std::declval<C>().template Member<TemplateParams...>(std::declval<MemberArgs>()...))>> \
        static auto test_with_template_params(int) -> std::true_type;                                           \
        template <typename C, typename... TemplateParams>                                                       \
        static auto test_with_template_params(long) -> std::false_type;                                         \
                                                                                                                \
    public:                                                                                                     \
        static constexpr bool value = decltype(test<Type>(0))::value;                                           \
                                                                                                                \
        template <typename... TemplateParams>                                                                   \
        static constexpr bool value_with_template_params =                                                      \
            decltype(test_with_template_params<Type, TemplateParams...>(0))::value;                             \
    };                                                                                                          \
                                                                                                                \
    template <typename Type, typename... MemberArgs>                                                            \
    [[maybe_unused]] static constexpr bool has_member_##Member##_v = has_member_##Member<Type, MemberArgs...>::value;

Macro for generating has_member_xxx and has_member_xxx_v templates. They return true if the type given as template parameter has a member (template) function provided name.

If the function has no template parameters or they can be auto-deduced, use has_member_xxx::value or has_member_xxx_v, if not use has_member_xxx::value_with_template_params<...>.

If the member function takes arguments, pass their types as additional template parameters to has_member_xxx.

See the examples for details.

Example:

// Add templates has_member_bar and has_member_bar_v
KAMPING_MAKE_HAS_MEMBER(bar)
 
// Add templates has_member_baz and has_member_baz_v
KAMPING_MAKE_HAS_MEMBER(baz)
 
// Add templates has_member_fizz and has_member_fizz_v
KAMPING_MAKE_HAS_MEMBER(fizz)
 
struct Foo {
  int bar();
  int baz(char);
  template<typename T>
  int fizz(T);
};
 
// check if Foo.bar() is callable
static_assert(has_member_bar_v<Foo>)
 
// check if Foo.bar(char) is callable
static_assert(!has_member_bar_v<Foo, char>)
 
// check if Foo.baz(char) is callable
static_assert(has_member_baz_v<Foo, char>)
 
// check if Foo.baz() is callable
static_assert(!has_member_baz_v<Foo>)
 
// check if Foo.fizz(int) is callable
static_assert(has_member_fizz_v<Foo, int>)
 
// check if Foo.fizz<int>(int) is callable
static_assert(has_member_fizz<Foo, int>::value_with_template_params<int>)
 
// check if Foo.fizz<int, double>() is callable
static_assert(!has_member_fizz<Foo>::value_with_template_params<int, double>)

Explanation:

To obtain value, the static member function test is instantiated using the given type Type.
Using declval, we get an instance of Type and try to call the expected member function with instances of the passed MemberArgs.
- Optionally, test_with_template_params also instantiates the functions template parameters with the passed types.
If that member does not exist, we can not obtain the decltype, and cannot instantiate std::void_t<...>, which fails to initialize the whole function test(int).
Then, the next best instantiation is test(long), which returns std::false_type.
If we find the requested member, we get std::true_type.
test has int and long overloads to resolve ambiguity. Passing 0 to test ensure that we first try to instantiate the true variant.