A funny code snippet of "sizeof()"

Here is the code

Code:

---

struct A1{
        char b;
        struct {
                char c;
                char a;
        };
};

        cout<<sizeof(A1)<<endl;

struct A2{
        char b;
        struct B{
                char c;
                char a;
        };
};
        cout<<sizeof(A2)<<endl;

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It's interesting to see the size of A1 and A2 are different just because you give A2.B a name! Why is THAT?

You didn't actually declare struct B as a member of A2, so it's members are inaccessible and are probably eliminated by an optimizer. This would produce a smaller struct.

On the other hand, the anonymous struct in A1 is meaningless, so it has no impact on the accessability of its members. The optimizer probably scraps the anonymous wrapper.

The output is

Code:

---

3
1

---

I don't understand why a byte alignment is applied to A1?

Quote:

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Originally Posted by citizen

You didn't actually declare struct B as a member of A2, so it's members are inaccessible and are probably eliminated by an optimizer. This would produce a smaller struct.

On the other hand, the anonymous struct in A1 is meaningless, so it has no impact on the accessability of its members. The optimizer probably scraps the anonymous wrapper.

---

It's not an alignment issue. struct B is not a member of A2. Therefore, its members are not a part of the object, producing something of a smaller size.

Code:

---

#include <iostream>

struct foo
{
    char stub[3];
};

struct bar
{
    char stub[2];
};

struct quz
{
    char c;
    bar b;
};

int main ( )
{
    std::cout<<"sizeof (quz) = "<<sizeof (quz)<<"\n";
    std::cout<<"sizeof (foo) = "<<sizeof (foo)<<"\n";
}

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If you run that, foo and quz might be the same size, but this is only because quz actually has a bar.

Code:

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~/formus $ g++ -o anonstruct -ansi -pedantic -Wall anonstruct.cpp
anonstruct.cpp:9: error: ISO C++ prohibits anonymous structs

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Simply put, you've hit a compiler extension, so you shouldn't be surprised that it does weird things.