Question

boost::bind overloads several operators for its placeholders:

For convenience, the function objects produced by bind overload the logical not operator ! and the relational and logical operators ==, !=, <, <=, >, >=, &&, ||.

For example, this allows me to pass _1 == desired_value as a predicate to STL algorithms.

Unfortunately, std::bind does not seem to overload these operators 🙁

1. Why is that?

2. What is a good workaround to simulate _1 == desired_value with std::bind?

Answer 1

IIRC, Boost.Bind only overloads those operators for the placeholders because the original Boost Lambda Library, which Boost.Bind is an improvement of, did (Boost.Bind is obsolete thanks to Boost.Phoenix, btw). std::bind‘s placeholders are only intended for exactly that purpose, as placeholders for arguments to std::bind.

As a workaround, use polymorphic functors:

struct compare_equal{
  template<class LHS, class RHS>
  bool operator()(LHS&& lhs, RHS&& rhs){ // assume bool return
    return std::forward<LHS>(lhs) == std::forward<RHS>(rhs);
  }
};

// ...
auto bound = std::bind(compare_equal(), _1, desired_value);

Live example on Ideone.