proj¶
Header¶
#include <boost/hof/proj.hpp>
Description¶
The proj 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 proj_adaptor<Projection, F> proj(Projection p, F f);
template<class Projection>
constexpr proj_adaptor<Projection> proj(Projection p);
Semantics¶
assert(proj(p, f)(xs...) == f(p(xs)...));
assert(proj(p)(xs...) == p(xs)...);
Requirements¶
Projection must be:
- UnaryInvocable
- MoveConstructible
F must be:
- ConstInvocable
- MoveConstructible
Example¶
#include <boost/hof.hpp>
#include <cassert>
using namespace boost::hof;
struct foo
{
foo(int x_) : x(x_)
{}
int x;
};
int main() {
assert(boost::hof::proj(&foo::x, _ + _)(foo(1), foo(2)) == 3);
}