function pointer with cout prints 1

ostream 에 1이 출력되는 이유는, pointer가 bool 로 바껴서 그렇다

bool check 로 생각하면됨

전역 lambda capture로 1로 변환됨;

https://lunapiece.net/Article/14007902

https://stackoverflow.com/questions/21958261/outputstream-prints-1-for-some-reason

https://stackoverflow.com/questions/2064692/how-to-print-function-pointers-with-cout

    // 27.7.2.6 Formatted output:
    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
    basic_ostream& operator<<(basic_ostream& (*__pf)(basic_ostream&))
    { return __pf(*this); }

Why C++ Member Function Pointers Are 16 Bytes Wide

http://lazarenko.me/wide-pointers/

void call_by_ptr(const C &obj, void (C::*mem_func)() const)
{
    void *data[2];
    std::memcpy(data, &mem_func, sizeof(mem_func));
    std::cout << "------------------------------\n"
        "Object ptr:\t" << &obj <<
        "\nFunction ptr:\t" << data[0] <<
        "\nPointer adj:\t" << data[1] << std::endl;
    (obj.*mem_func)();
}

#include <iostream>
#include <cstdint>
#include <vector>
#include <array>
#include <sstream>
#include <string>
#include <iostream>

namespace function_display
{
    template<class Ret, class... Args>
    std::ostream& operator <<(std::ostream& os, Ret(*p)(Args...))
    {
        return os << "funptr " << (void*)p;
    }
}

// example code:
void fun_void_void() {};
void fun_void_double(double d) {};
double fun_double_double(double d) { return d; }

using Callback = void(*)(void* buffer, size_t size, void* user);
static const auto FREE_CALLBACK = [](void* mem, size_t size, void* user) { free(mem); };

struct stream
{
    stream& operator<<(stream& (* f)(stream&)) noexcept {
        return f(*this);
    }

    stream& operator<<(const void* value) noexcept {
        return *this;
    }

    stream& operator<<(bool value) noexcept {
        return *this;
    }

    template<class Ret, class... Args>
    stream& operator<<(Ret(*p)(Args...))
    {
        *this << "funptr " << (void*)p;
        return *this;
    }
};

int main()
{
    using namespace function_display;

    // ampersands & are optional
    std::cout << "1. " << &fun_void_void << std::endl; // prints "1. funptr 0x40cb58"
    std::cout << "2. " << &fun_void_double << std::endl; // prints "2. funptr 0x40cb5e"
    std::cout << "3. " << &fun_double_double << std::endl; // prints "3. funptr 0x40cb69"
    std::cout << "4. " << FREE_CALLBACK << std::endl; // prints "4. 1"
    std::cout << "5. " << &FREE_CALLBACK << std::endl; // prints "5. 0x100001f57"

    Callback callback = FREE_CALLBACK;
    std::cout << "6. " << callback << std::endl; // prints "6. funptr 0x1000001240"

    stream out;
    out << FREE_CALLBACK; // bool value
    out << &FREE_CALLBACK; // const void*
    out << callback; // funptr
    out << &callback; // const void*

    return 0;
}