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

Question

Taking this example: 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
I see the address of the member function is 0x7018400100000000000, which is understandable because member functions pointers have 16 bytes while free function as 0x100401080 have only 8 bytes.
However, why the member function address 0x7018400100000000000 is so far away from free function address 0x100401080? i.e., |0x7018400100000000000 - 0x100401080| = 0x70184000FFEFFBFEF80?
Why it is not closer i.e., something like 0x100401... instead of 0x701840...? Or I am printing the member function address wrong?

Answer 1

Your architecture is little-endian. The low byte of the address is in the first byte of p, so your address is being printed out backwards.

Answer 2

Fixed code which automatically detects little/big endian: 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