成员函数指针

参考

为什么 C++ 中成员函数指针是 16 字节？

demo

#include <iostream>
using namespace std;

class _Undefined_class{
public:
    void func(void);
};

void*       _M_object;
const void* _M_const_object;
void (*_M_function_pointer)();
void (_Undefined_class::*p4)();

int main() {
    cout << sizeof(_M_object) << "\n"
         << sizeof(_M_const_object) << "\n"
         << sizeof(_M_function_pointer) << "\n"
         << sizeof(&_Undefined_class::func) << "\n"
         << sizeof(p4) << "\n";
}

8
8
8
16
16

成员函数中是可以使用this指针的，这个以前是知道的，但不够明确的是this指针的存放位置。按目前看的文章，this指针是占用成员函数指针大小的：

A pointer to member function is a pair as follows:

ptr:

For a non-virtual function, this field is a simple function pointer. For a virtual function, it is 1 plus the virtual table offset (in bytes) of the function, represented as a ptrdiff_t. The value zero represents a NULL pointer, independent of the adjustment field value below.

adj:

The required adjustment to this, represented as a ptrdiff_t.

#include <iostream>
#include <cstring>

using namespace std;
struct A {
    void foo() const {
        std::cout << "A's this:\t" << this << std::endl;
    }
    char pad0[32];
};

struct B {
    void bar() const {
        std::cout << "B's this:\t" << this << std::endl;
    }
    char pad2[64];
};

struct C : A, B
{ };

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)();
}

int main()
{
    C obj;
    call_by_ptr(obj, &C::foo);
    call_by_ptr(obj, &C::bar);
}

------------------------------
Object ptr:     0x7ffd3b46ec80
Function ptr:   0x400a56
Pointer adj:    0
A's this:       0x7ffd3b46ec80
------------------------------
Object ptr:     0x7ffd3b46ec80
Function ptr:   0x400a90
Pointer adj:    0x20
B's this:       0x7ffd3b46eca0

ending