What optimization does move semantics provide if we already have RVO?

Question

As far as I understand one of the purposes of adding move semantics is to optimize code by calling special constructor for copying "temporary" objects. For example, in this answer we see that it can be used to optimize such string a = x + y stuff. Because x+y is an rvalue expression, instead of deep copying we can copy only the pointer to the string and the size of the string. But as we know, modern compilers support return value optimization, so without using move semantics our code will not call the copy constructor at all.

To prove it I write this code:

#include <iostream>  struct stuff {         int x;         stuff(int x_):x(x_){}         stuff(const stuff & g):x(g.x)         {                 std::cout<<"copy"<<std::endl;         } };    stuff operator+(const stuff& lhs,const stuff& rhs) {         stuff g(lhs.x+rhs.x);         return g; } int main() {         stuff a(5),b(7);         stuff c = a+b; } 

And after executing it in VC++2010 and g++ in optimize mode I'm getting empty output.

What kind of optimization is it, if without it my code still works faster? Could you explain what I'm understanding wrong?

Answer 1

Move semantics should not be thought as an optimization device, even if they can be used as such.

If you are going to want copies of objects (either function parameters or return values), then RVO and copy elision will do the job when they can. Move semantics can help, but are more powerful than that.

Move semantics are handy when you want to do something different whether the passed object is a temporary (it then binds to a rvalue reference) or a "standard" object with a name (a so called const lvalue). If you want for instance to steal the resources of a temporary object, then you want move semantics (example: you can steal the contents a std::unique_ptr points to).

Move semantics allow you to return non copyable objects from functions, which is not possible with the current standard. Also, non copyable objects can be put inside other objects, and those objects will automatically be movable if the contained objects are.

Non copyable objects are great, since they don't force you to implement an error-prone copy constructor. A lot of the time, copy semantics do not really make sense, but move semantics do (think about it).

This also enables you to use movable std::vector<T> classes even if T is non copyable. The std::unique_ptr class template is also a great tool when dealing with non copyable objects (eg. polymorphic objects).

Answer 2

After some digging I find this excellent example of optimization with rvalue references inStroustrup's FAQ .

Yes, swap function:

    template<class T>  void swap(T& a, T& b)   // "perfect swap" (almost) {     T tmp = move(a);    // could invalidate a     a = move(b);        // could invalidate b     b = move(tmp);      // could invalidate tmp } 

This will generate optimized code for any kind of types (assuming, that it have move constructor).

Edit: Also RVO can't optimize something like this(at least on my compiler):

stuff func(const stuff& st) {     if(st.x>0)     {         stuff ret(2*st.x);         return ret;     }     else     {         stuff ret2(-2*st.x);         return ret2;     } } 

This function always calls copy constructor (checked with VC++). And if our class can be moved faster, than with move constructor we will have optimization.