Related
What is the difference between std::initializer_list and std::span? Both are contiguous sequences of values of some type. Both are non-owning.
So when do we use the first and when do we use the latter?
The short answer is that std::initializer_list<T> is used to create a new range, for the purposes of initialization. While std::span<T> is used to refer to existing ranges, for better APIs.
std::initializer_list<T> is a language feature that actually constructs a new array and owns it. It solves the problem of how to conveniently initialize containers:
template <typename T>
struct vector {
vector(std::initializer_list<T>);
};
vector<int> v = {1, 2, 3, 4};
That creates a std::initializer_list<int> on the fly containing the four integers there, and passes it into vector so that it can construct itself properly.
This is really the only place std::initializer_list<T> should be used: either constructors or function parameters to pass a range in on the fly, for convenience (unit tests are a common place that desires such convenience).
std::span<T> on the other hand is used to refer to an existing range. Its job is to replace functions of the form:
void do_something(int*, size_t);
with
void do_something(std::span<int>);
Which makes such functions generally easier to use and safer. std::span is constructible from any appropriate contiguous range, so taking our example from earlier:
std::vector<int> v = {1, 2, 3, 4};
do_something(v); // ok
It can also be used to replace functions of the form:
void do_something_else(std::vector<unsigned char> const&);
Which can only be called with, specifically, a vector, with the more general:
void do_something_else(std::span<unsigned char const>);
Which can be called with any backing contiguous storage over unsigned char.
With span you have to be careful, since it's basically a reference that just doesn't get spelled with a &, but it is an extremely useful type.
The principle differences between the two are how the language treats them. Or more specifically, how it doesn't in the case of a span.
You cannot create an initializer_list from an existing container object or array. They can only be created by copying from other initializer_lists or from a braced-init-list (ie: { stuff }). That is, the compiler governs the creation of an initializer_list and the array it points into.
Similarly, a constructor that takes only an initializer_list has special meaning to the compiler. When performing list initialization on that type, such constructors are given priority in overload resolution. span is given no special meaning by the compiler.
initializer_list also has lifetime rules that are different from span. The lifetime of the array pointed to by an initializer_list is the lifetime of the initializer_list object that was created from a braced-init-list. This is not true of span; the lifetime of a span created from a container is whatever you say it is based on how you use it in code.
Broadly speaking, you should only use initializer_list when you're initializing an object.
One difference is that std::span can be used dynamically, unlike std::initializer_list.
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?
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.
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.
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:
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.
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.)
For any parameter of type T, if a copy of each argument is needed for all operations, use unifying.
If both copy and move construction of T have ignorable cost, use unifying.
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.
Otherwise, avoid unifying.
Here are some examples need to avoid unifying:
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.
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.
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.)
For the general case where the value will be stored, the pass-by-value only is a good compromise-
For the case where you know that only lvalues will be passed (some tightly coupled code) it's unreasonable, unsmart.
For the case where one suspects a speed improvement by providing both, first THINK TWICE, and if that didn't help, MEASURE.
Where the value will not be stored I prefer the pass by reference, because that prevents umpteen needless copy operations.
Finally, if programming could be reduced to unthinking application of rules, we could leave it to robots. So IMHO it's not a good idea to focus so much on rules. Better to focus on what the advantages and costs are, for different situations. Costs include not only speed, but also e.g. code size and clarity. Rules can't generally handle such conflicts of interest.
Pass by value, then move is actually a good idiom for objects that you know are movable.
As you mentioned, if an rvalue is passed, it'll either elide the copy, or be moved, then within the constructor it will be moved.
You could overload the copy constructor and move constructor explicitly, however it gets more complicated if you have more than one parameter.
Consider the example,
class Obj {
public:
Obj(std::vector<int> x, std::vector<int> y)
: X(std::move(x)), Y(std::move(y)) {}
private:
/* Our internal data. */
std::vector<int> X, Y;
}; // Obj
Suppose if you wanted to provide explicit versions, you end up with 4 constructors like so:
class Obj {
public:
Obj(std::vector<int> &&x, std::vector<int> &&y)
: X(std::move(x)), Y(std::move(y)) {}
Obj(std::vector<int> &&x, const std::vector<int> &y)
: X(std::move(x)), Y(y) {}
Obj(const std::vector<int> &x, std::vector<int> &&y)
: X(x), Y(std::move(y)) {}
Obj(const std::vector<int> &x, const std::vector<int> &y)
: X(x), Y(y) {}
private:
/* Our internal data. */
std::vector<int> X, Y;
}; // Obj
As you can see, as you increase the number of parameters, the number of necessary constructors grow in permutations.
If you don't have a concrete type but have a templatized constructor, you can use perfect-forwarding like so:
class Obj {
public:
template <typename T, typename U>
Obj(T &&x, U &&y)
: X(std::forward<T>(x)), Y(std::forward<U>(y)) {}
private:
std::vector<int> X, Y;
}; // Obj
References:
Want Speed? Pass by Value
C++ Seasoning
I am answering myself because I will try to summarize some of the answers. How many moves/copies do we have in each case?
(A) Pass by value and move assignment construct, passing a X parameter. If X is a...
Temporary: 1 move (the copy is elided)
Lvalue: 1 copy 1 move
std::move(lvalue): 2 moves
(B) Pass by reference and copy assignment usual (pre C++11) construct. If X is a...
Temporary: 1 copy
Lvalue: 1 copy
std::move(lvalue): 1 copy
We can assume the three kinds of parameters are equally probable. So every 3 calls we have (A) 4 moves and 1 copy, or (B) 3 copies. I.e., in average, (A) 1.33 moves and 0.33 copies per call or (B) 1 copy per call.
If we come to a situation when our classes consist mostly of PODs, moves are as expensive as copies. So we would have 1.66 copies (or moves) per call to the setter in case (A) and 1 copies in case (B).
We can say that in some circumstances (PODs based types), the pass-by-value-and-then-move construct is a very bad idea. It is 66% slower and it depends on a C++11 feature.
On the other hand, if our classes include containers (which make use of dynamic memory), (A) should be much faster (except if we mostly pass lvalues).
Please, correct me if I'm wrong.
Readability in the declaration:
void foo1( A a ); // easy to read, but unless you see the implementation
// you don't know for sure if a std::move() is used.
void foo2( const A & a ); // longer declaration, but the interface shows
// that no copy is required on calling foo().
Performance:
A a;
foo1( a ); // copy + move
foo2( a ); // pass by reference + copy
Responsibilities:
A a;
foo1( a ); // caller copies, foo1 moves
foo2( a ); // foo2 copies
For typical inline code there is usually no difference when optimized.
But foo2() might do the copy only on certain conditions (e.g. insert into map if key does not exist), whereas for foo1() the copy will always be done.
initializer list constructors in C++ often cause trouble; for example
using std::vector;
using std::string;
vector<string> v{3}; // vector of three empty strings
vector<int> u{3}; // vector of one element with value 3
(Just to clarify, I mean <int> constructor is an initializer list constructor, while the <string> one is not.)
The int case matches the initializer list constructor, while the string case doesn't. This is somewhat ugly, and often causes trouble. It was also noted in an early chapter (item 7) in Scott Meyers' Effective Modern C++, and he describes it as a somewhat unpleasant part of the standard, that whenever an initializer list constructor is available, compilers will jump through hoops to try to match it, giving it priority over every single other constructor.
Of course, the int case can be easily fixed by changing u{3} to u(3), but that's not the point.
Is this desirable behavior? Has there been any discussion or plans by the C++ standard committee to fix this type of ambiguity / unpleasantness? One example would be requiring initializer list constructors to be called like this: vector<int> u({3}), which is already currently legal.
Has there been any discussion or plans by the C++ standard committee to fix this type of ambiguity / unpleasantness?
There have been many fixes to initialization since C++11. For instance, you initially couldn't copy construct aggregates using list-initialization (CWG 1467). This really minor fix broke some code in an undesirable way that lead to a new issue to refine that previous fix to undo it if there's an initializer_list constructor (CWG 2137). It's hard to touch anything in these clauses without lots of unexpected consequences and breaking code, even in small cases. I doubt there will be a push to make any kind of large change to initialization in the future. At this point, the amount of code breakage would be tremendous.
The best solution is just to be aware about the pitfalls of initialization and be careful about what you're doing. My rule of thumb is to only use {}s when I deliberately need the behavior that {} provides, and () otherwise.
Note that this isn't really any different from the more well-known pitfall of:
vector<int> a{10}; // vector of 1 element
vector<int> b(10); // vector of 10 elements
One example would be requiring initializer list constructors to be called like this: vector<int> u({3}), which is already currently legal.
You have the same problem you had before, for the same reasons:
vector<int> u({3}); // vector of one element: 3
vector<string> v({3}); // vector of three elements: "", "", and ""
Even if you were to require the former (which would be impossible), you couldn't make the latter ill-formed.
First is an uniform initializer, and it was introduced to solve ambiguity of language. The problem was called Most Vexing Parse in relation to declaring variables, intialized by "round" () parenthesis. MVP is a kind of ambiguity resolution in code similar to following:
class SomeInitClass;
void bleh()
{
int foo(SomeInitClass());
}
foo here actually is a prototype of a function that takes as its parameter a function that returns a Bar, and the foo function's return value is an int. Essentially if something looks like prototype, C++ treats it as such.
int foo{SomeInitClass{}};
SomeInitClass{} would always creates a temporary. int foo{...} always would create a variable.
While those two lines work differently:
vector<string> v{3}; // vector of three empty strings
vector<int> u{3}; // vector of one element with value 3
they do bear same semantics, they are declaration of variables. That way of how it works (and fact that you can declare constructor that takes initializer list as parameter) does bear much significance on C++ language, which adopts the concept of concealing true value and amount of actions behind its syntax.
It's not inconsistency, not a major one at least. vector<string> and vector<int> are NOT same class and do NOT have same constructors, because they are not identical instantiations of template std::vector. To avoid confusionone may use aliases and use slightly different syntax.
StringCollecton v{3}; //three strings;
IntCollection u = {3}; // or {{3}}
Of course, StringCollecton test = {3}; will not work in this case, because 3 is not literal that can be casted into proper stored type.
Because you can have only one initializer in declaration, setting values of created string container would look so:
std::vector<std::string> test{3, {"string"}}; // all values initialized by "string"
std::vector<std::string> test{{"string1","",""}}; // constructor introduced in C++11
While I can be lazy and left out innermost braces, the are the syntax sugar allows me show that it IS the initializer_list in there.
In C++ when using initializer_list syntax to initialize an object, the regular constructors of the object also participate in overload resolution, when no other list initialization rule applies.
As far as I understand it, the following code calls X::X(int)
class X { int a_; X(int a):a_(a) {} );
void foo() {
X bar{3};
}
But I don't understand, why regular constructors also are considered in context of initializer_lists. I feel that a lot of programmers now write X{3} to call a constructor instead of X(3) to call the construcor.
I don't like this style at all, as it makes me think the object does not have a regular constructor.
What is the reason why the initializer_list syntax can also be used to call regular constructor? Is there a reason to now prefer this syntax over regular constructor calls?
Essentially it is a mess up. For C++11 it was attempted to create one uniform way to initialize objects instead of multiple approaches otherwise necessary:
T v(args...); for the usual case
T d = T(); when using the default constructor for stack-based objects
T m((iterator(x)), iterator()); to fight the Most Vexing Parse (note the extra parenthesis around the first parameter)
T a = { /* some structured values */ }; for aggregate initialization
Instead the Uniform Initialization Syntax was invented:
T u{ /* whatever */ };
The intention was that uniform initialization syntax would be used everywhere and the old stule would go out of fashion. Everything was fine except that proponents of initialization from std::initializer_list<S> realized that the syntax would be something like that:
std::vector<int> vt({ 1, 2, 3 });
std::vector<int> vu{{ 1, 2, 3 }};
That was considered unacceptable and uniform initialization syntax was irreparably compromised to allow the so much better
std::vector<int> vx{ 1, 2, 3 };
The problem with this mixture is that it now is sometimes unclear what is actually meant and uniform initialization syntax isn’t uniform any more. It is still necessary in some contexts (especially to value initialize stack-based objects in generic code) but it isn’t the correct choice in all cases. For example, the following two notations were meant to mean the same thing but they don’t:
std::vector<int> v0(1, 2); // one element with value 2
std::vector<int> v1{1, 2}; // two elements: 1 and 2
tl;dr: initializer list and uniform initialization syntax are the two separate notations. Sadly, they conflict.
I don't like this style at all, as it makes me think the object does not have a regular constructor.
If it's an aggregate type, aggregate initialization is performed. Otherwise, it considers the constructors. If it makes you think that the class is an aggregate, that is not a language problem.
Is there a reason to now prefer this syntax over regular constructor calls?
If you're a proponent of uniform initialization yes. If you're not, you can stick with the old style. Note that the other answer talks about std::initializer_list but this doesn't apply to your question whatsoever, since you have no constructor which takes a std::initializer_list. Braced-init-lists and std::initializer_list are separate concepts. It's best not to get them confused this early on.
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?
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.
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.
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:
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.
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.)
For any parameter of type T, if a copy of each argument is needed for all operations, use unifying.
If both copy and move construction of T have ignorable cost, use unifying.
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.
Otherwise, avoid unifying.
Here are some examples need to avoid unifying:
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.
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.
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.)
For the general case where the value will be stored, the pass-by-value only is a good compromise-
For the case where you know that only lvalues will be passed (some tightly coupled code) it's unreasonable, unsmart.
For the case where one suspects a speed improvement by providing both, first THINK TWICE, and if that didn't help, MEASURE.
Where the value will not be stored I prefer the pass by reference, because that prevents umpteen needless copy operations.
Finally, if programming could be reduced to unthinking application of rules, we could leave it to robots. So IMHO it's not a good idea to focus so much on rules. Better to focus on what the advantages and costs are, for different situations. Costs include not only speed, but also e.g. code size and clarity. Rules can't generally handle such conflicts of interest.
Pass by value, then move is actually a good idiom for objects that you know are movable.
As you mentioned, if an rvalue is passed, it'll either elide the copy, or be moved, then within the constructor it will be moved.
You could overload the copy constructor and move constructor explicitly, however it gets more complicated if you have more than one parameter.
Consider the example,
class Obj {
public:
Obj(std::vector<int> x, std::vector<int> y)
: X(std::move(x)), Y(std::move(y)) {}
private:
/* Our internal data. */
std::vector<int> X, Y;
}; // Obj
Suppose if you wanted to provide explicit versions, you end up with 4 constructors like so:
class Obj {
public:
Obj(std::vector<int> &&x, std::vector<int> &&y)
: X(std::move(x)), Y(std::move(y)) {}
Obj(std::vector<int> &&x, const std::vector<int> &y)
: X(std::move(x)), Y(y) {}
Obj(const std::vector<int> &x, std::vector<int> &&y)
: X(x), Y(std::move(y)) {}
Obj(const std::vector<int> &x, const std::vector<int> &y)
: X(x), Y(y) {}
private:
/* Our internal data. */
std::vector<int> X, Y;
}; // Obj
As you can see, as you increase the number of parameters, the number of necessary constructors grow in permutations.
If you don't have a concrete type but have a templatized constructor, you can use perfect-forwarding like so:
class Obj {
public:
template <typename T, typename U>
Obj(T &&x, U &&y)
: X(std::forward<T>(x)), Y(std::forward<U>(y)) {}
private:
std::vector<int> X, Y;
}; // Obj
References:
Want Speed? Pass by Value
C++ Seasoning
I am answering myself because I will try to summarize some of the answers. How many moves/copies do we have in each case?
(A) Pass by value and move assignment construct, passing a X parameter. If X is a...
Temporary: 1 move (the copy is elided)
Lvalue: 1 copy 1 move
std::move(lvalue): 2 moves
(B) Pass by reference and copy assignment usual (pre C++11) construct. If X is a...
Temporary: 1 copy
Lvalue: 1 copy
std::move(lvalue): 1 copy
We can assume the three kinds of parameters are equally probable. So every 3 calls we have (A) 4 moves and 1 copy, or (B) 3 copies. I.e., in average, (A) 1.33 moves and 0.33 copies per call or (B) 1 copy per call.
If we come to a situation when our classes consist mostly of PODs, moves are as expensive as copies. So we would have 1.66 copies (or moves) per call to the setter in case (A) and 1 copies in case (B).
We can say that in some circumstances (PODs based types), the pass-by-value-and-then-move construct is a very bad idea. It is 66% slower and it depends on a C++11 feature.
On the other hand, if our classes include containers (which make use of dynamic memory), (A) should be much faster (except if we mostly pass lvalues).
Please, correct me if I'm wrong.
Readability in the declaration:
void foo1( A a ); // easy to read, but unless you see the implementation
// you don't know for sure if a std::move() is used.
void foo2( const A & a ); // longer declaration, but the interface shows
// that no copy is required on calling foo().
Performance:
A a;
foo1( a ); // copy + move
foo2( a ); // pass by reference + copy
Responsibilities:
A a;
foo1( a ); // caller copies, foo1 moves
foo2( a ); // foo2 copies
For typical inline code there is usually no difference when optimized.
But foo2() might do the copy only on certain conditions (e.g. insert into map if key does not exist), whereas for foo1() the copy will always be done.