c++11 optimal parameter passing

Question

Consider these classes:

#include 
#include 

class A
{
    std::string test;
public:
    A (std::string t) : test(std::move(t))         


        

Answer 1

The "by-value" approach is an option. It is not as optimal as what you have, but only requires one overload:

class B
{
    A a;
public:   
    B (A _a) : a(move(_a)) {}
};

The cost is 1 extra move construction for both lvalues and xvalues, but this is still optimal for prvalues (1 move). An "xvalue" is an lvalue that has been cast to rvalue using std::move.

You could also try a "perfect forwarding" solution:

class B
{
    A a;
public:   
    template ::value
              >::type>
    B (T&& _a) : a(std::forward(_a)) {}
};

This will get you back to the optimal number of copy/move constructions. But you should constrain the template constructor such that it is not overly generic. You might prefer to use is_convertible instead of is_constructible as I've done above. This is also a single constructor solution, but as you add parameters, your constraint gets increasingly complicated.

Note: The reason the constraint is necessary above is because without, clients of B will get the wrong answer when they query std::is_constructible::value. It will mistakenly answer true without a proper constraint on B.

I would say that none of these solutions is always better than the others. There are engineering tradeoffs to be made here.