Why the size of class showing 12 and 16 byte?-CodePudding

I have five classes here, class A B C I can explain the size of them.

class D I expect the result is 12 bytes, but the output is 16 bytes, I found the reason is that after adding the virtual function, the alignment will become 8 bytes,

So I created the Test class again, according to the above reasoning, my expected result is 16 bytes, but after running, the result is 12 bytes,

Should not the Test class is also 8 bytes alignment, and the result should be 16 bytes? Or what causes the class to perform 8 bytes alignment?

Code:

#include <iostream>
using namespace std;

class A
{
    //empty
};

class B
{
    int a = 123;
};

class C
{
    public:
        void print(){
            cout << "C" << endl;
        }
    private:
        int i = 123;
};

class D
{
    public:
        virtual void print(){
            std::cout << "D" << std::endl;
        }
        virtual int d(){
            return 0;
        }
        void add(){
            std::cout << "D add" << std::endl;
        }
    private:
        int i;
};

class Test
{
    private:
        int i;
        int j;
        int l;
};
int main(){
    cout << sizeof(A) << endl;//1 byte:avoid null pointer
    
    cout << sizeof(B) << endl;//4 bytes:one int
    
    cout << sizeof(C) << endl;//4 bytes:one int

    cout << sizeof(D) << endl;//16 bytes(using 12byte):one int   one pointer and 8 alignment
    
    cout << sizeof(Test) << endl;//12 bytes:Why not 16 bytes?

    return 0;
}

CodePudding user response：

class D I expect the result is 12 bytes, but the output is 16 bytes

Your expectation is misguided.

I found the reason is that after adding the virtual function, the alignment will become 8 bytes,

That's the reason. 12 is not aligned to 8 bytes. 16 is.

So I created the Test class again, according to the above reasoning, my expected result is 16 bytes, but after running, the result is 12 bytes,

You didn't add virtual functions to Test, so your expectation is wrong.

Should not the Test class is also 8 bytes alignment,

There's no reason to expect that.

Or what causes the class to perform 8 bytes alignment?

In this case, it was caused by having a virtual member function. In other cases it can also be caused by having a sub object with alignment of 8.