Question

struct BLA
{
    int size_;

    int size()const{ return size_; }
}

int x;
BLA b[ 2 ];
BLA * p = &b[ 0 ];

b[ 0 ].size_ = 4;
b[ 1 ].size_ = 6;

When I compile this line:

x = p->size_ + (p++)->size_;

I receive the expected result. But, when I compile this line (without the previous one):

x = p->size() + (p++)->size();

Then I get different result. The ‘p’ is not incremented at the same time as in the previous line. Can someone explain this, please? Tried on VS 2008 and VS 2010.

Answer 1

It’s undefined behaviour to seperately read and modify a variable without an intervening sequence point. You’ve seen a good example of the consequences of that.