how does this pointer assignment work?

Question

I'm looking at a strcpy example where they increase the value of a pointer, and assign it in 1 line, like this:

*ptrA++ = *ptrB++;

I know that the value where the pointer is pointing to in the char array is increased, and the contents is copied.

does c do something like

*ptrA = *ptrB;
ptrA++;
ptrB++;

in the background ?

Answer 1

You got it. (15 char)

Answer 2

Yes it does, remember that the postfix ++ means return the value before the increment. So *ptrA++ increments ptrA but returns the dereference of of ptrA before the increment.

Answer 3

Yes, it does. Because the code is using postfix operators:

Postfix operators are operators that are suffixed to an expression.

operand++;

This causes the value of the operand to be returned. After the result is obtained, the value of the operand is incremented by 1.

Answer 4

Well, yes and no.

Yes, because the second piece of code you provided does indeed do the same thing as the original code. So, in a way, you understood your original code correctly.

No, because your second piece code is not really equivalent to the original one. Remember, it is incorrect to say that postfix ++ operator returns the original value first and increments the pointer later. In C language temporal relationships (what happens "before" and what happens "after") can only be defined by sequence points. The expression

*ptrA++ = *ptrB++;

has no sequence points inside, so there's absolutely no way to say what happens before and what happens after. At the same time, your second variant

*ptrA = *ptrB;
ptrA++;
ptrB++;

explicitly guarantees that increment happens after the dereference, since there's a sequence point at the end of each statement. There's no such guarantee with regard to the first variant. This is what I see as a problem with your interpretation.

In reality it is quite possible that the increment will happen first and the dereference will happen later. For example, the compiler can translate your original expression into something like

tmp1 = ptrA++;
tmp2 = ptrB++;
*tmp1 = *tmp2;

in which case the increment happens first. Or the compiler can translate it into something like

ptrA++;
ptrB++;
*(ptrA - 1) = *(ptrB - 1);

in which case the increment happens first as well.

Once again, remember your interpretation of the original expression is good, but it is just one of the possible interpretations. Never assume that things will happen in the specific order you used in your interpretation.

P.S. About those sequence points: C FAQ, C++ FAQ, Wikipedia