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

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.

Problem well spotted. I wouldn't go as far as to say that the pass-by-value-and-then-move construct is a bad idiom but it definitely has its potential pitfalls.

If your type is expensive to move and / or moving it is essentially just a copy, then the pass-by-value approach is suboptimal. Examples of such types would include types with a fixed size array as a member: It may be relatively expensive to move and a move is just a copy. See also

• Small String Optimization and Move Operations and
• "Want speed? Measure." (by Howard Hinnant)

in this context.

The pass-by-value approach has the advantage that you only need to maintain one function but you pay for this with performance. It depends on your application whether this maintenance advantage outweighs the loss in performance.

The pass by lvalue and rvalue reference approach can lead to maintenance headaches quickly if you have multiple arguments. Consider this:

#include <vector>
using namespace std;

struct A { vector<int> v; };
struct B { vector<int> v; };

struct C {
  A a;
  B b;
  C(const A&  a, const B&  b) : a(a), b(b) { }
  C(const A&  a,       B&& b) : a(a), b(move(b)) { }
  C(      A&& a, const B&  b) : a(move(a)), b(b) { }
  C(      A&& a,       B&& b) : a(move(a)), b(move(b)) { }  
};

If you have multiple arguments, you will have a permutation problem. In this very simple example, it is probably still not that bad to maintain these 4 constructors. However, already in this simple case, I would seriously consider using the pass-by-value approach with a single function

C(A a, B b) : a(move(a)), b(move(b)) { }

instead of the above 4 constructors.

So long story short, neither approach is without drawbacks. Make your decisions based on actual profiling information, instead of optimizing prematurely.

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.

The current answers are quite incomplete. Instead, I will try to conclude based on the lists of pros and cons I find.

Short answer

In short, it may be OK, but sometimes bad.

This idiom, namely the unifying interface, has better clarity (both in conceptual design and implementation) compared to forwarding templates or different overloads. It is sometimes used with copy-and-swap (actually, as well as move-and-swap in this case).

Detailed analysis

The pros are:

• It needs only one function for each parameter list.
  • It needs indeed only one, not multiple ordinary overloads (or even 2ⁿ overloads when you have n parameters when each one can be unqualified or const-qualified).
  • Like within a forwarding template, parameters passed by value are compatible with not only const, but volatile, which reduce even more ordinary overloads.
    • Combined with the bullet above, you don't need 4ⁿ overloads to serve to {unqulified, const, const, const volatile} combinations for n parameters.
  • Compared to a forwarding template, it can be a non-templated function as long as the parameters are not needed to be generic (parameterized through template type parameters). This allows out-of-line definitions instead of template definitions needed to be instantiated for each instance in each translation unit, which can make significant improvement to translation-time performance (typically, during both compiling and linking).
  • It also makes other overloads (if any) easier to implement.
    • If you have a forwarding template for a parameter object type T, it may still clash with overloads having a parameter const T& in the same position, because the argument can be a lvalue of type T and the template instantiated with type T& (rather than const T&) for it can be more preferred by the overloading rule when there is no other way to differentiate which is the best overloading candidate. This inconsistency may be quite surprising.
      • In particular, consider you have forwarding template constructor with one parameter of type P&& in a class C. How many time will you forget to excluded the instance of P&& away from possibly cv-qualified C by SFINAE (e.g. by adding typename = enable_if_t<!is_same<C, decay_t<P>> to the template-parameter-list), to ensure it does not clash with copy/move constructors (even when the latter are explicitly user-provided)?
• Since the parameter is passed by value of a non-reference type, it can force the argument be passed as a prvalue. This can make a difference when the argument is of a class literal type. Consider there is such a class with a static constexpr data member declared in some class without an out-of-class definition, when it is used as an argument to a parameter of lvalue reference type, it may eventually fail to link, because it is odr-used and there is no definition of it.
  • Note since ISO C++ 17 the rules of static constexpr data member have changed to introduce a definition implicitly, so the difference is not significant in this case.

The cons are:

• A unifying interface can not replace copy and move constructors where the parameter object type is identical to the class. Otherwise, copy-initialization of the parameter would be infinite recursion, because it will call the unifying constructor, and the constructor then call itself.
• As mentioned by other answers, if the cost of copy is not ignorable (cheap and predictable enough), this means you will almost always have the degeneration of performance in the calls when the copy is not needed, because copy-initialization of a unifying passed-by-value parameter unconditionally introduce a copy (either copied-to or moved-to) of the argument unless elided.
  • Even with mandatory elision since C++17, copy-initialization of a parameter object is still hardly free to be removed away - unless the implementation try very hard to prove the behavior not changed according to as-if rules instead of the dedicated copy elision rules applicable here, which might be sometimes impossible without a whole program analysis.
  • Likewise, the cost of destruction may not be ignorable as well, particularly when non-trivial subobjects are taken into account (e.g. in cases of containers). The difference is that, it does not only apply to the copy-initialization introduced by the copy construction, but also by the move construction. Making move cheaper than copy in constructors can not improve the situation. The more cost of copy-initialization, the more cost of destruction you have to afford.
• A minor shortcoming is that there is no way to tweak the interface in different ways as plural overloads, for example, specifying different noexcept-specifiers for parameters of const& and && qualified types.
  • OTOH, in this example, unifying interface will usually provide you with noexcept(false) copy + noexcept move if you specifies noexcept, or always noexcept(false) when you specify nothing (or explicit noexcept(false)). (Note in the former case, noexcept does not prevent throwing during copy because that will only occur during evaluation of arguments, which is out of the function body.) There is no further chance to tune them separately.
  • This is considered minor because it is not frequently needed in reality.
  • Even if such overloads are used, they are probably confusing by nature: different specifiers may hide subtle but important behavioral differences which are difficult to reason about. Why not different names instead of overloads?
  • Note the example of noexcept may be particularly problematic since C++17 because noexcept-specification now affect the function type. (Some unexpected compatibility issues can be diagnosed by Clang++ warning.)

Sometimes the unconditional copy is actually useful. Because composition of operations with strong-exception guarantee does not hold the guarantee in nature, a copy can be used as a transactional state holder when the strong-exception guarantee is required and the operation cannot be broken down as sequence of operations with no less strict (no-exception or strong) exception guarantee. (This includes the copy-and-swap idiom, although assignments are not recommended to be unified for other reasons in general, see below.) However, this does not mean the copy is otherwise unacceptable. If the intention of the interface is always to create some object of type T, and the cost of moving T is ignorable, the copy can be moved to the target without unwanted overhead.

Conclusions

So for some given operations, here are suggestions about whether using a unifying interface to replace them:

0. If not all of the parameter types match the unifying interface, or if there is behavioral difference other than the cost of new copies among operations being unified, there cannot be a unifying interface.
1. If the following conditions are failed to be fit for all parameters, there cannot be a unifying interface. (But it can still be broken down to different named-functions, delegating one call to another.)
2. For any parameter of type T, if a copy of each argument is needed for all operations, use unifying.
3. If both copy and move construction of T have ignorable cost, use unifying.
4. If the intention of the interface is always to create some object of type T, and the cost of the move construction of T is ignorable, use unifying.
5. Otherwise, avoid unifying.

Here are some examples need to avoid unifying:

1. Assignment operations (including assignment to the subobjects thereof, typically with copy-and-swap idiom) for T without ignorable cost in copy and move constructions does not meet the criteria of unifying, because the intention of assignment is not to create (but to replace the content of) the object. The copied object will eventually be destructed, which incurs unnecessary overhead. This is even more obvious for cases of self-assignment.
2. Insertion of values to a container does not meet the criteria, unless both the copy-initialization and destruction have ignorable cost. If the operation fails (due to the allocation failure, duplicate values or so on) after copy-initialization, the parameters have to be destructed, which incurs unnecessary overhead.
3. Conditionally creation of object based on parameters will incur the overhead when it does not actually create the object (e.g. std::map::insert_or_assign-like container insertion even in spite of the failure above).

Note the accurate limit of "ignorable" cost is somewhat subjective because it eventually depends on how much cost can be tolerated by the developers and/or the users, and it may vary case by case.

Practically, I (conservatively) assume any trivially copyable and trivailly destructible type whose size is not more than one machine word (like a pointer) qualifying the criteria of ignorable cost in general - if the resulted code actually cost too much in such case, it suggests either a wrong configuration of the build tool is used, or the toolchain is not ready for production.

Do profile if there is any further doubt on performance.

Additional case study

There are some other well-known types preferred to be passed by value or not, depending on the conventions:

• Types need to preserve reference values by convention should not be passed by value.
  • A canonical example is the argument forwarding call wrapper defined in ISO C++, which requires to forward references. Note in the caller position it may also preserve the reference respecting to the ref-qualifier.
  • An instance of this example is std::bind. See also the resolution of LWG 817.
• Some generic code may directly copy some parameters. It may be even without std::move, because the cost of the copy is assumed to be ignorable and a move does not necessarily make it better.
  • Such parameters include iterators and function objects (except the case of argument forwarding caller wrappers discussed above).
  • Note the constructor template of std::function (but not the assignment operator template) also uses the pass-by-value functor parameter.
• Types presumably having the cost comparable to pass-by-value parameter types with ignorable cost are also preferred to be pass-by-value. (Sometimes they are used as dedicated alternatives.) For example, instances of std::initializer_list and std::basic_string_view are more or less two pointers or a pointer plus a size. This fact makes them cheap enough to be directly passed without using references.
• Some types should be better avoided passed by value unless you do need a copy. There are different reasons.
  • Avoid copy by default, because the copy may be quite expensive, or at least it is not easy to guarantee the copy is cheap without some inspection of the runtime properties of the value being copied. Containers are typical examples in this sort.
    • Without statically knowing how many elements in a container, it is generally not safe (in the sense of a DoS attack, for example) to be copied.
    • A nested container (of other containers) will easily make the performance problem of copying worse.
    • Even empty containers are not guaranteed cheap to be copied. (Strictly speaking, this depends on the concrete implementation of the container, e.g. the existence of the "sentinel" element for some node-based containers... But no, keep it simple, just avoid copying by default.)
  • Avoid copy by default, even when the performance is totally uninterested, because there can be some unexpected side effects.
    • In particular, allocator-awared containers and some other types with similar treatment to allocators ("container semantics", in David Krauss' word), should not be passed by value - allocator propagation is just another big semantic worm can.
• A few other types conventionally depend. For example, see GotW #91 for shared_ptr instances. (However, not all smart pointers are like that; observer_ptr are more like raw pointers.)