Ist es möglich, den Typ einer Variablen in Standard-C++ zu drucken?

Question

Zum Beispiel:

int a = 12;
cout << typeof(a) << endl;

Erwartetes Ergebnis:

int

Answer 1

Hier ist eine Konvertierung des Typs "ins Englische", die jeden Qualifizierer, jedes Ausmaß, jedes Argument usw. dekonstruiert und rekursiv die Zeichenkette aufbaut, die den Typ beschreibt. Ich denke, der Vorschlag "deduced this" würde helfen, viele der Spezialisierungen zu reduzieren. Auf jeden Fall war dies eine unterhaltsame Morgenübung, ungeachtet der übermäßigen Aufblähung :)

struct X {
    using T = int *((*)[10]);
    T f(T, const unsigned long long * volatile * );
};

int main() {

    std::cout << describe<decltype(&X::f)>() << std::endl;
}

Output:

pointer to member function of class 1X taking (pointer to array[10]
of pointer to int, pointer to volatile pointer to const unsigned 
long long), and returning pointer to array[10] of pointer to int

Hier ist der Code: https://godbolt.org/z/7jKK4or43

Note : Die aktuellste Version ist in meinem Github: https://github.com/cuzdav/type_to_string

// Print types as strings, including functions, member 

#include <type_traits>
#include <typeinfo>
#include <string>
#include <utility>

namespace detail {

template <typename T> struct Describe;

template <typename T, class ClassT> 
struct Describe<T (ClassT::*)>  {
    static std::string describe();
};
template <typename RetT, typename... ArgsT> 
struct Describe<RetT(ArgsT...)>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...)>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) volatile>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const volatile>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) volatile noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const volatile noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...)&>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const &>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) volatile &>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) & noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const volatile &>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const & noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) volatile & noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const volatile & noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) &&>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const &&>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) volatile &&>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) && noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const volatile &&>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const && noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) volatile && noexcept>  {
    static std::string describe();
};
template <typename RetT, class ClassT, typename... ArgsT> 
struct Describe<RetT(ClassT::*)(ArgsT...) const volatile && noexcept>  {
    static std::string describe();
};

template <typename T>
std::string describe()
{
    using namespace std::string_literals;
    auto terminal = [&](char const * desc) {
        return desc + " "s + typeid(T).name();
    };
    if constexpr(std::is_const_v<T>) {
        return "const " + describe<std::remove_const_t<T>>();
    }
    else if constexpr(std::is_volatile_v<T>) {
        return "volatile " + describe<std::remove_volatile_t<T>>();
    }
    else if constexpr (std::is_same_v<bool, T>) {
        return "bool";
    }
    else if constexpr(std::is_same_v<char, T>) {
        return "char";
    }
    else if constexpr(std::is_same_v<signed char, T>) {
        return "signed char";
    }
    else if constexpr(std::is_same_v<unsigned char, T>) {
        return "unsigned char";
    }
    else if constexpr(std::is_unsigned_v<T>) {
        return "unsigned " + describe<std::make_signed_t<T>>();
    }
    else if constexpr(std::is_void_v<T>) {
        return "void";
    }
    else if constexpr(std::is_integral_v<T>) {
        if constexpr(std::is_same_v<short, T>) 
            return "short";
        else if constexpr(std::is_same_v<int, T>) 
            return "int";
        else if constexpr(std::is_same_v<long, T>) 
            return "long";
        else if constexpr(std::is_same_v<long long, T>) 
            return "long long";
    }
    else if constexpr(std::is_same_v<float, T>) {
        return "float";
    }
    else if constexpr(std::is_same_v<double, T>) {
        return "double";
    }
    else if constexpr(std::is_same_v<long double, T>) {
        return "long double";
    }
    else if constexpr(std::is_same_v<std::nullptr_t, T>) { 
        return "nullptr_t";
    }
    else if constexpr(std::is_class_v<T>) {
        return terminal("class");
    }
    else if constexpr(std::is_union_v<T>) {
        return terminal("union");
    }
    else if constexpr(std::is_enum_v<T>) {
        std::string result;
        if (!std::is_convertible_v<T, std::underlying_type_t<T>>) {
            result += "scoped ";
        }
        return result + terminal("enum");
    }  
    else if constexpr(std::is_pointer_v<T>) {
        return "pointer to " + describe<std::remove_pointer_t<T>>();
    }
    else if constexpr(std::is_lvalue_reference_v<T>) {
        return "lvalue-ref to " + describe<std::remove_reference_t<T>>();
    }
    else if constexpr(std::is_rvalue_reference_v<T>) {
        return "rvalue-ref to " + describe<std::remove_reference_t<T>>();
    }
    else if constexpr(std::is_bounded_array_v<T>) {
        return "array[" + std::to_string(std::extent_v<T>) + "] of " +
            describe<std::remove_extent_t<T>>();
    }
    else if constexpr(std::is_unbounded_array_v<T>) {
        return "array[] of " + describe<std::remove_extent_t<T>>();
    }
    else if constexpr(std::is_function_v<T>) {
        return Describe<T>::describe();
    }
    else if constexpr(std::is_member_object_pointer_v<T>) {
        return Describe<T>::describe();
    }
    else if constexpr(std::is_member_function_pointer_v<T>) {
        return Describe<T>::describe();
    }
}

template <typename RetT, typename... ArgsT> 
std::string Describe<RetT(ArgsT...)>::describe() {
    std::string result = "function taking (";
    ((result += detail::describe<ArgsT>(", ")), ...);
    return result + "), returning " + detail::describe<RetT>();
}

template <typename T, class ClassT> 
std::string Describe<T (ClassT::*)>::describe() {
    return "pointer to member of " + detail::describe<ClassT>() +
        " of type " + detail::describe<T>();
}

struct Comma {
    char const * sep = "";
    std::string operator()(std::string const& str) {
        return std::exchange(sep, ", ") + str;
    }
};
enum Qualifiers {NONE=0, CONST=1, VOLATILE=2, NOEXCEPT=4, LVREF=8, RVREF=16};

template <typename RetT, typename ClassT, typename... ArgsT>
std::string describeMemberPointer(Qualifiers q) {
    std::string result = "pointer to ";
    if (NONE != (q & CONST)) result += "const ";
    if (NONE != (q & VOLATILE)) result += "volatile ";
    if (NONE != (q & NOEXCEPT)) result += "noexcept ";
    if (NONE != (q & LVREF)) result += "lvalue-ref ";
    if (NONE != (q & RVREF)) result += "rvalue-ref ";
    result += "member function of " + detail::describe<ClassT>() + " taking (";
    Comma comma;
    ((result += comma(detail::describe<ArgsT>())), ...);
    return result + "), and returning " + detail::describe<RetT>();
}

template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...)>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(NONE);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(CONST);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) volatile>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(VOLATILE);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) volatile noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(VOLATILE | NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const volatile>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(CONST | VOLATILE);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(CONST | NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const volatile noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(CONST | VOLATILE | NOEXCEPT);
}

template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) &>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(LVREF);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const &>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(LVREF | CONST);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) & noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(LVREF | NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) volatile &>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(LVREF | VOLATILE);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) volatile & noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(LVREF | VOLATILE | NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const volatile &>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(LVREF | CONST | VOLATILE);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const & noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(LVREF | CONST | NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const volatile & noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(LVREF | CONST | VOLATILE | NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...)&&>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(RVREF);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const &&>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(RVREF | CONST);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) && noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(RVREF | NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) volatile &&>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(RVREF | VOLATILE);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) volatile && noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(RVREF | VOLATILE | NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const volatile &&>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(RVREF | CONST | VOLATILE);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const && noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(RVREF | CONST | NOEXCEPT);
}
template <typename RetT, class ClassT, typename... ArgsT> 
std::string Describe<RetT(ClassT::*)(ArgsT...) const volatile && noexcept>::describe() {
    return describeMemberPointer<RetT, ClassT, ArgsT...>(RVREF | CONST | VOLATILE | NOEXCEPT);
}

} // detail

///////////////////////////////////
// Main function
///////////////////////////////////
template <typename T>
std::string describe() {
    return detail::describe<T>();
}

///////////////////////////////////
// Sample code
///////////////////////////////////
#include <iostream>

struct X {
    using T = int *((*)[10]);
    T f(T, const unsigned long long * volatile * );
};

int main() {
    std::cout << describe<decltype(&X::f)>() << std::endl;
}

Answer 2

Aufbauend auf einigen der vorherigen Antworten habe ich diese Lösung gefunden, die das Ergebnis von __PRETTY_FUNCTION__ in der Binärdatei. Es verwendet ein statisches Array, um die String-Repräsentation des Typnamens zu speichern.

Sie erfordert C++23.

#include <iostream>
#include <string_view>
#include <array>

template <typename T>
constexpr auto type_name() {
    auto gen = [] <class R> () constexpr -> std::string_view  {
        return __PRETTY_FUNCTION__;
    };
    constexpr std::string_view search_type = "float";
    constexpr auto search_type_string = gen.template operator()<float>();
    constexpr auto prefix = search_type_string.find(search_type);
    constexpr auto suffix = search_type_string.size() - prefix - search_type.size();
    constexpr auto str = gen.template operator()<T>();
    constexpr int size = str.size() - prefix - suffix;
    constexpr auto static arr = [&]<std::size_t... I>(std::index_sequence<I...>) constexpr {
        return std::array<char, size>{str[prefix + I]...};
    } (std::make_index_sequence<size>{});

    return std::string_view(arr.data(), size);
}

Answer 3

Ich kopiere von dieser Antwort: https://stackoverflow.com/a/56766138/11502722

Ich konnte dies erhalten etwas Arbeiten für C++ static_assert() . Der Haken an der Sache ist, dass static_assert() akzeptiert nur String-Literale; constexpr string_view wird nicht funktionieren. Sie müssen zusätzlichen Text um den Typennamen akzeptieren, aber es funktioniert:

template<typename T>
constexpr void assertIfTestFailed()
{
#ifdef __clang__
    static_assert(testFn<T>(), "Test failed on this used type: " __PRETTY_FUNCTION__);
#elif defined(__GNUC__)
    static_assert(testFn<T>(), "Test failed on this used type: " __PRETTY_FUNCTION__);
#elif defined(_MSC_VER)
    static_assert(testFn<T>(), "Test failed on this used type: " __FUNCSIG__);
#else
    static_assert(testFn<T>(), "Test failed on this used type (see surrounding logged error for details).");
#endif
    }
}

MSVC-Ausgabe:

error C2338: Test failed on this used type: void __cdecl assertIfTestFailed<class BadType>(void)
... continued trace of where the erroring code came from ...