if..else 内部的指针递增问题

Question

As a part of a fairly complex function, I wrote this bit of code:

if( (((int)*(pointer-11 ....  )   //It is too long and complex
{
    *(pointer++)=*(pointer-12)+1;
}
else
{
    *(pointer++)=*(pointer-12);
}

The if part works well, but the else parts works like:

*(pointer++)=*(pointer-11);

When I put the incrementing code separate from the actual line,

if( .... )
{
    *(pointer)=*(pointer-12)+1;
}
else
{
    *(pointer)=*(pointer-12);
}
pointer++;

It works perfectly. But code on top should work too, right? Any thoughts?

Answer 1

*(pointer++)=*(pointer-12); is undefined behavior. There is no sequence point here, so you don't know if the left or right part of the = will get evaluated first.

Since it's undefined behavior, the compiler might do anything, but these are pretty sensible interpretations of it:

// Left gets evaluated first    
*(pointer)=*((pointer+1)-12);
pointer++;

// Right gets evaluated first
*(pointer)=*(pointer-12);
pointer++;

Note that this has nothing to do with pointers. The expression x++ <op> x causes undefined behavior irregardless of what type x has and what operator <op> is. (That's maybe not 100% true, but you better avoid these expressions anyway)