为什么成员函数地址离自由函数这么远?

Why member function address are so far away from free functions?

本文关键字：函数 成员 地址 为什么 自由      更新时间：2023-10-16

举这个例子：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，而自由函数作为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这么远？即|0x7018400100000000000 - 0x100401080| = 0x70184000FFEFFBFEF80？

为什么它不更接近，即像0x100401...而不是0x701840...？还是我打印了错误的会员函数地址？

你的架构是小端序的。 地址的低字节位于p的第一个字节中，因此您的地址被反向打印出来。

修复了自动检测小端/大端的代码：https://godbolt.org/z/XSvT5R

#include <iostream>
#include <iomanip>
#include <sstream>
inline bool is_big_endian() {
long int longvalue = 1;
// https://stackoverflow.com/questions/8978935/detecting-endianness
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
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