Does return statement copy values

Question

I am wondering about this because of scope issues. For example, consider the code

typedef struct {     int x1;/*top*/     int x2;/*bottom*/     int id; } subline_t;    subline_t subline(int x1, int x2, int id) {     subline_t t = { x1, x2, id };     return t; }  int main(){     subline_t line = subline(0,0,0); //is line garbage or isn't it? the reference     //to subline_t t goes out of scope, so the only way this wouldn't be garbage     //is if return copies } 

So my question is, will the return statement always copy? In this case it seems to work, so I am led to believe that return does copy. If it does copy, will it copy in every case?

Answer 1

Yes, in that case there will be a copy made. If you change the function declaration like this:

subline_t &subline(int x1, int x2, int id) { 

then no copy will be made. However, in your specific case it would not be valid to return a reference to an object allocated on the stack. The problem is that the object would be destructed and invalidated before the caller had a chance to use it.

This is related to the common Return Value Optimization for C++ that can avoid doing an actual copy operation in the case you have described. The end result is (or should be) the same as if a copy were done, but you should be aware of the optimization. The presence of this optimization can, in some cases, change the observable behaviour of the program.