Question

This example :

#include <iostream>
#include <cstring>

struct A
{
    int  a;
    bool b;
};

bool foo( const A a1, const A a2 )
{
    return ( 0 == std::memcmp( &a1, &a2, sizeof( A ) ) );
}

int main()
{
    A a1 = A();
    a1.a = 5;a1.b = true;
    A a2 = A();
    a2.a = 5;a2.b = true;

    std::cout<<std::boolalpha << foo( a1, a2 ) << std::endl;
}

is going to produce false, because of padding.

I do not have access to the foo function, and I can not change the way the comparison is done.

Assuming a bool occupies 1 byte (that is true on my system), if I change the struct A to this :

struct A
{
  int a;
  bool b;
  char dummy[3];
};

then it works fine on my system (the output is true).

Is there anything else I could do to fix the above problem (get the true output)?

Answer 1

The first one is not working because of padding in the struct. The padding is having different bit patterns for both objects.

If you use memset to set all the bits in the object before using it, then it will work:

A a1;
std::memset(&a1, 0, sizeof(A));
a1.a = 5;a1.b = true;

A a2;
std::memset(&a2, 0, sizeof(A));
a2.a = 5;a2.b = true;

Online demos:

• http://www.ideone.com/mVmsn (Original code written by you)
• http://www.ideone.com/Q13QO (My modification)

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By the way, you can write operator<, operator== etc, for PODs also.