Why does std::find_if(first, last, p) not take predicate by reference?

问题

I was looking at the various signatures for std::find_if on cppreference.com, and I noticed that the flavors that take a predicate function appear to accept it by value:

template< class InputIt, class UnaryPredicate >
InputIt find_if( InputIt first, InputIt last,
             UnaryPredicate p );

If I understand them correctly, lambdas with captured variables allocate storage for either references or copies of their data, and so presumably a "pass-by-value" would imply that the copies of captured data are copied for the call.

On the other hand, for function pointers and other directly addressable things, the performance should be better if the function pointer is passed directly, rather than by reference-to-pointer (pointer-to-pointer).

First, is this correct? Is the UnaryPredicate above going to be a by-value parameter?

Second, is my understanding of passing lambdas correct?

Third, is there a reason for passing by value instead of by reference in this situation? And more to the point, is there not some sufficiently ambiguous syntax (hello, universal reference) that would let the compiler do whatever it wants to get the most performance out?

回答1:

Is the UnaryPredicate above going to be a by-value parameter?

Yes, that's what it says in the function parameter list. It accepts a deduced value type.

Beyond that, lambda expressions are prvalues. Meaning, with c++17's guaranteed copy elision, that p is initialized directly from the lambda expression. No extra copies of the closure or the captured objects are being made when passing it into the function (the function may make more copies internally however, though that's not common).

If the predicate was passed by reference, a temporary object would need to be materialized. So for a lambda expression, nothing is gained by a switch to pass by reference.

If you have other sorts of predicates, which are expansive to copy, then you can pass in std::reference_wrapper to that predicate object, for a cheap "handle" to it. The wrapper's operator() will do the right thing.

The definition is mostly historic, but nowadays it's really a non-issue to do it with pass by value.

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To elaborate on why referential semantics would suck, let's try to take it through the years. A simple lvalue reference won't do, since now we don't support binding to an rvalue. A const lvalue reference won't do either, since now we require the predicate to not modify any internal state, and what for?

So up to c++11, we don't really have an alternative. A pass by value would be better than a reference. With the new standard, we may revise our approach. To support rvalues, we may add an rvalue reference overload. But that is an exercise in redundancy, since it doesn't need to do anything different.

By passing a value, the caller has the choice in how to create it, and for prvalues, in c++17, it's practically free. If the caller so desires, they can provide referential semantics explicitly. So nothing is lost, and I think much is gained in terms of simplicity of usage and API design.

回答2:

There are actually multiple reasons:

1. you can always turn deduced value arguments into using reference semantics but not vice verse: just pass std::ref(x) instead of x. std::reference_wrapper<T> isn't entirely equivalent to passing a reference but especially for function object it does the Right Thing. That is, passing generic arguments by value is the more general approach.

2. Pass by reference (T&) doesn't work for temporary or const objects, T const& doesn't work for non-const&, i.e., the only choice would be T&& (forwarding reference) which didn't exist pre-C++11 and the algorithm interfaces didn't change since they were introduced with C++98.

3. Value parameters can be copy elided unlike any sort of reference parameters, including forwarding references.

来源：https://stackoverflow.com/questions/48803927/why-does-stdfind-iffirst-last-p-not-take-predicate-by-reference