Comment by HarHarVeryFunny
2 months ago
C++ supports both pass-by-value and pass-by-reference parameters. Pass-by-value means making a copy (a deep copy if it's a deep type), but you could always choose to optimize by passing large parameters by reference instead, and this is common practice.
The real value of std::move is cases where you to HAVE to (effectively) make a deep copy, but still want to avoid the inefficiency. std::move supports this because moving means "stealing" the value from one variable and giving it to another. A common use case is move constructors for objects where you need to initialize the object's member variables, and can just move values passed by the caller rather than copying them.
Another important use case for std::move is returning values from functions, where the compiler will automatically use move rather than copy if available, allowing you to define functions returning large/complex return types without having to worry about the efficiency.
1990's C practice was to document whether initialization values were copied or adopted. I'm curious why the concept became "move" rather than "adopt", since move gives the parameter/data agency instead of giving agency to the consuming component.
I'm not sure how any notion of ownership and adoption in C would carry over to C++, since one of the problems with C is that it didn't define ownership. If you have a pointer value in C it means nothing about ownership.
Given the limits of C, "adopting" a pointer variable would just mean that the foster-parent now had a copy of the pointer, and was considered owner of it, now responsible for freeing it too presumably. It's basically a move, except the source can now mess things up by still using the adopted value after the adoption.
(More so since c++17) std::move should not be used for returns because this pessimises optimisations.