c++ - 为什么成员函数地址与自由函数相距如此远？

Question

以这个例子为例:https://godbolt.org/z/gHqCSA

#include<iostream>

template<typename Return, typename... Args>
std::ostream& operator <<(std::ostream& os, Return(*p)(Args...) ) {
    return os << (void*)p;
}

template <typename ClassType, typename Return, typename... Args>
std::ostream& operator <<(std::ostream& os, Return (ClassType::*p)(Args...) )
{
    unsigned char* internal_representation = reinterpret_cast<unsigned char*>(&p);
    os << "0x" << std::hex;

    for(int i = 0; i < sizeof p; i++) {
        os << (int)internal_representation[i];
    }

    return os;
}

struct test_debugger { void var() {} };
void fun_void_void(){};
void fun_void_double(double d){};
double fun_double_double(double d){return d;}

int main() {
    std::cout << "0. " << &test_debugger::var << std::endl;
    std::cout << "1. " << fun_void_void << std::endl;
    std::cout << "2. " << fun_void_double << std::endl;
    std::cout << "3. " << fun_double_double << std::endl;
}

// Prints:
//    0. 0x7018400100000000000
//    1. 0x100401080
//    2. 0x100401087
//    3. 0x100401093

我看到成员函数的地址是0x7018400100000000000，这是可以理解的，因为成员函数指针只有16个字节，而自由函数0x100401080只有8个字节。

但是，为什么成员函数地址0x7018400100000000000与自由函数地址0x100401080相距如此远？即|0x7018400100000000000 - 0x100401080| = 0x70184000FFEFFBFEF80？

为什么它不更靠近，即像0x100401...而不是0x701840...？还是我打印的成员函数地址错误？

Answer 1

您的体系结构是低端的。地址的低字节在p的第一个字节中，因此您的地址被向后打印。