C++ binary predicate implementation requirements for std::search

Question

On this site it states that one of the constraints for a binary predicate passed into std::search is that:

The types Type1 and Type2 must be such that objects of types ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly converted to both Type1 and Type2.

However, in the C++ International Standard documentation I see no reference to such a constraint. I am wondering if this constraint is explicitly stated in the C++ International Standard documentation which section it is under. In addition if it is not then why is this constraint placed upon the std::search function as isn't it possible to have two distinct types that while not implicitly convertible have an equality operator overload defined?

Answer 1

It's a mistake on the site.

That parameter description is populated from this template. It doesn't distinguish between binary predicates that are only called as pred(*first1, *first2) from those that are called as both pred(*first1, *first2) and pred(*first2, *first1)

Further digging: It appears that this edit added a named parameter to the parent template's use of cast req2, which controls whether it says "both" or "respectively". The eq variant does not have a passthrough of this new parameter. I don't have edit privileges to add it in to the template and the entry for std::search

Answer 2

The requirement of a BinaryPredicate is stated in [algorithms.requirements]/7:

The BinaryPredicate parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type T when T is part of the signature returns a value testable as true. In other words, if an algorithm takes BinaryPredicate binary_­pred as its argument and first1 and first2 as its iterator arguments, it should work correctly in the construct binary_­pred(*first1, *first2) contextually converted to bool ([conv]). BinaryPredicate always takes the first iterator's value_­type as its first argument, that is, in those cases when T value is part of the signature, it should work correctly in the construct binary_­pred(*first1, value) contextually converted to bool ([conv]). binary_­pred shall not apply any non-constant function through the dereferenced iterators.

There is no requirement for constructing Type1 from dereferencing ForwardIt2. The word 'both' in OP's quotation from cppreference is inaccurate.