为什么成员函数地址离自由函数这么远？

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离自由函数地址0x100401080那么远呢？即，|0x7018400100000000000 - 0x100401080| = 0x70184000FFEFFBFEF80?

为什么它不更近，例如 0x100401... 而不是 0x701840...？还是我打印成员函数地址错了？

Answer 1

您的架构是小端架构。地址的低字节在p的第一个字节，所以你的地址是倒着打印出来的。

Answer 2

自动检测little/big字节序的固定代码：https://godbolt.org/z/XSvT5R

#include <iostream>
#include <iomanip>
#include <sstream>

inline bool is_big_endian() {
    long int longvalue = 1;

    // 
    unsigned char* representation = reinterpret_cast<unsigned char*>(&longvalue);
    return ( (unsigned) representation[sizeof(long int) - 1] ) == 1;
}

template<typename Pointer>
std::ostream& print_pointer(std::ostream& os, const Pointer& pointer) {
    const unsigned char* representation = (unsigned char*) &pointer;

    int precision = 0;
    bool haszeros = false;

    unsigned firsthexdigit;
    unsigned secondhexdigit;

    std::ostringstream stream;
    stream.flags( os.flags() );
    stream << std::hex;

    #define print_pointer_HEX_DIGIT \
        firsthexdigit = (unsigned) representation[index] >> 4 & 0xf; \
        secondhexdigit = (unsigned) representation[index] & 0xf; \
        if( haszeros || firsthexdigit ) { \
            precision++; \
            haszeros = true ; \
            stream << firsthexdigit; \
        } \
        if( haszeros || secondhexdigit ) { \
            precision++; \
            haszeros = true ; \
            stream << secondhexdigit; \
        }

    if( is_big_endian() ) {
        for(int index = 0; index < static_cast<int>(sizeof pointer); index++) {
            print_pointer_HEX_DIGIT
        }
    }
    else {
        for(int index = static_cast<int>(sizeof pointer - 1); index >= 0 ; index--) {
            print_pointer_HEX_DIGIT
        }
    }

    if( os.precision() - ++precision > 0 ) {
        return os << "0x" + std::string( os.precision() - ++precision, '0' ) + stream.str();
    }

    return os << "0x" + stream.str();
}

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

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

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;
    std::cout << "4. " << std::setfill('0') << std::setw(16) << fun_void_void << std::endl;
    std::cout << "5. " << std::setprecision(16) << fun_void_double << std::endl;
}

// Prints:
//    0. 0x100402e80
//    1. 0x100401118
//    2. 0x10040111f
//    3. 0x10040112b
//    4. 000000x100401118
//    5. 0x0000010040111f