c++ - Taking sink parameters by rvalue reference instead of by value to enforce performant usage of interfaces

Question

In a code review, my co-worker and I were discussing the interface for a function I was writing. Our code base uses C++17 and we do not use exceptions (video games).

I claimed that the idiomatic C++ way of taking sink parameters would be performant while also keeping the interface flexible, allowing the caller to pass a copy or move from an owned value as desired. By idiomatic way, I mean either one function taking the parameter by value, or an overoad set for const lvalue and rvalue references (which requires one less move in the lvalue case, at the cost of some code duplication).

struct A {};

class ByValue
{
public:
    ByValue(std::vector<A> v)
        : m_v(std::move(v))
    {}

private:
    std::vector<A> m_v;
};

class RefOverloads
{
public:
    RefOverloads(std::vector<A> const& v)
        : m_v(v)
    {}

    RefOverloads(std::vector<A>&& v)
        : m_v(std::move(v))
    {}

private:
    std::vector<A> m_v;
};

int main()
{
    std::vector<A> v0;
    ByValue value0(v0);
    ByValue value1(std::move(v0));

    std::vector<A> v1;
    RefOverloads ref0(v1);
    RefOverloads ref1(std::move(v1));
}

My coworker on the other hand does not like that it is easy to implicitly make expensive copies. He would prefer that these sink arguments are always by rvalue reference (with no const lvalue ref overload), and that if the caller wishes to pass a copy they must make a local copy and move it into the function.

class RvalueRefOnly
{
public:
    RvalueRefOnly(std::vector<A>&& v)
        : m_v(std::move(v))
    {}

private:
    std::vector<A> m_v;
};

int main()
{
    std::vector<A> v;
    //RvalueRefOnly failedCopy(v); // Fails purposefully.

    std::vector<A> vCopy = v;               // Explicit copy of v.
    RvalueRefOnly okCopy(std::move(vCopy)); // Move into okCopy.
}

I've never even thought of an interface like this. A counter-argument I had was that taking by value expresses intent better, i.e, with signature

void f(T x);

the caller knows that f has taken ownership of x. With

void g(T&& x);

g may have ownership or may not, depending on the implementation of f.

Is there a best way? Am I missing some argument one way or the other?

Answer 1

You basically have those constructor options:

class myclass {
public:
    // #1 myclass(const std::string& s) : s(s) {}
    // #2 myclass(std::string&& s) : s(std::move(s)) {}

    // #3 myclass(std::string s) : s(std::move(s)) {}

    // #4 template <typename T> myclass(T&& t) : s(std::forward<T>(t)) {}

    std::string s;
};

#3 cannot be present with #1 or #2 -> ambiguous call

and call

std::string s;
myclass A(s);
myclass B(std::string(s));
myclass C(std::move(s));
myclass D("temporary");
myclass E({5, '*'});

Following is count of copy/move constructor.

                        | A | B | C | D | E |
------------------------|---|---|---|---|---|
1                  Copy |<1>| 2 | 1 | 1 | 1 | <.> Denotes best result (by column)
const l-value ref  Move |<0>| 1 | 1 | 0 | 0 |
                  Other |<0>| 0 | 0 | 1 | 1 |
------------------------|---|-v-|-v-|-v-|-v-| B/C/D/E would prefer overload 2
2                  Copy | X |<1>|<0>| 0 |<0>|
r-value ref        Move | X |<1>|<1>| 1 |<1>| X denotes invalid case
                  Other | X |<0>|<0>| 1 |<1>|
------------------------|---|---|---|---|---|
3                  Copy | 1 | 1 | 0 | 0 |<0>|
by value           Move | 1 | 2 | 2 | 1 |<1>|
                  Other | 0 | 0 | 0 | 1 |<1>|
------------------------|---|---|---|---|---|
4                  Copy |<1>|<1>|<0>|<0>| X |
Forwarding ref     Move |<0>|<1>|<1>|<0>| X |
                  Other |<0>|<0>|<0>|<1>| X |
--------------------------------------------/

Possible configurations:

• #1 only: handle all cases, but does copy for temporary
• #1/#2: (B/C/D/E would use #2), so best results except for in-place construct
• #3 only: Handle all cases, but does extra move
• #4 only: Handle most regular cases, best results
• #1/#2/#4: best results (Notice that #4) has exact match over non-const l-value)
• #2/#4: Best results
• #2 only: Forbid copy, but explicit copy (B) does 1 extra move than #1/#2

As you can see:

• Forwarding reference (#4) has best results.
• By const ref (#1) has best performance for copy, but worse performance for other.
• Then By Value (#3) is the second "worst", but only one extra move from the best.

Other points:

• only #1 alone was available pre-C++11 (so was the default in most interface)
• only #1 might mean no ownership transfers.
• Move only (#2) forbids implicit copy
• Purpose of By Value #3 is to write only one overload as good compromise.

Now compare #1/#2, #2 and #3:

• For move only type:

• #1/#2 is irrelevant

• #3 sinks immediately.

• #2 has the opportunity to handle exception guarantee: if it throws, object is not necessary consumed.

Unless you want guaranteed and/or immediate sink, I would use pass-by-rvalue (#2).

For existing code base, I would keep consistency.

For copyable types:

• #1/#2 is the most efficient, but allows unwanted copy.
• #3 is convenient (and only one extra move), but allows unwanted copy, guaranteed sink.
• #2 avoid unwanted copy.

Now it is mostly what you want to guaranty and allow:

• best performance -> #1/#2
• no (implicit) copies -> #2
• immediate/guaranteed sink -> #3
• consistency with movable only types

For existing code base, I would keep consistency.