by

Description

The by function adaptor applies a projection onto the parameters of another function. This is useful, for example, to define a function for sorting such that the ordering is based off of the value of one of its member fields.

Also, if just a projection is given, then the projection will be called for each of its arguments.

Note: All projections are always evaluated in order from left-to-right.

Synopsis

template<class Projection, class F>
constexpr by_adaptor<Projection, F> by(Projection p, F f);

template<class Projection>
constexpr by_adaptor<Projection> by(Projection p);

Semantics

assert(by(p, f)(xs...) == f(p(xs)...));
assert(by(p)(xs...) == p(xs)...);

Requirements

Projection must be:

F must be:

Example

#include <fit.hpp>
#include <cassert>
using namespace fit;

struct foo
{
    foo(int x_) : x(x_)
    {}
    int x;
};

int main() {
    assert(fit::by(&foo::x, _ + _)(foo(1), foo(2)) == 3);
}