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c++c++14thissizeof

sizeof behavior, fit a class within a memory space?

I am trying to verify that an instance of a class will fit in a queue (defined by a C library), and noticed that sizeof() didn't do what I thought it did.

#include <iostream>
using namespace std;

class A {
    int x, y, z;
public:
    A() {}
    void PrintSize() { cout << sizeof(this) << endl; }
};

class B : public A {
    int a, b, c;
public:
    B() {}

    void PrintSize() { cout << sizeof(this) << endl; }
};

int main() {
    A a;
    B b;
    a.PrintSize(); // is 8
    b.PrintSize(); // is 8

    cout << sizeof(a) << endl;
    cout << sizeof(b) << endl;
}

returns:

8
8
12
24

I did not expect the "8" values - why is it different when taken within the class? How can verify that the class that I define fits within a memory space, especially at compile time?

Solution

  • This expression

    sizeof(this)

    gives the size of the pointer this. It seems you mean the size of the corresponding class

    sizeof( *this )