Empty Classes

#include <iostream>

#include <cstdlib>

//定义一个空类

class A {};

using namespace std;

int main(int argc, char* argv[])

{

// A is emptyclass

A* p1 = new A;

A* p2 = new A;

cout << "sizeof(A) = " << sizeof(A) << " or " << sizeof(*p1) << endl;

cout << "address p1 = " << p1 << endl;

cout << "address p2 = " << p2 << endl;

//我机器上的结果

//sizeof(A) = 1

//address p1 = 00372AD0 //address p2 = 00372B08 //p1 != p2

#endif

return 0;

}

////以下资料摘自 www.glenmccl.com

In C, an empty struct like:

struct A {};

is invalid, whereas in C++ usage like:

struct A {};

or:

class B {};

is perfectly legal. This type of construct is useful when developing a skeleton or placeholder for a class.

An empty class has size greater than zero. Two class objects of empty classes will have distinct addresses,

There are still one or two C++ compilers that generate C code as their "assembly" language. To handle an empty class, they will generate a dummy member, so for example:

class A {};

becomes:

struct A {

char __dummy;

};

in the C output.