c++ - Is the pass-by-value-and-then-move construct a bad idiom?

Question

Since we have move semantics in C++, nowadays it is usual to do

void set_a(A a) { _a = std::move(a); }

The reasoning is that if a is an rvalue, the copy will be elided and there will be just one move.

But what happens if a is an lvalue? It seems there will be a copy construction and then a move assignment (assuming A has a proper move assignment operator). Move assignments can be costly if the object has too many member variables.

On the other hand, if we do

void set_a(const A& a) { _a = a; }

There will be just one copy assignment. Can we say this way is preferred over the pass-by-value idiom if we will pass lvalues?

Answer 1

Expensive-to-move types are rare in modern C++ usage. If you are concerned about the cost of the move, write both overloads:

void set_a(const A& a) { _a = a; }
void set_a(A&& a) { _a = std::move(a); }

or a perfect-forwarding setter:

template <typename T>
void set_a(T&& a) { _a = std::forward<T>(a); }

that will accept lvalues, rvalues, and anything else implicitly convertible to decltype(_a) without requiring extra copies or moves.

Despite requiring an extra move when setting from an lvalue, the idiom is not bad since (a) the vast majority of types provide constant-time moves and (b) copy-and-swap provides exception safety and near-optimal performance in a single line of code.