Is it okay to pass projection as predicate argument to std::ranges::any_of/all_of/none_of?

Question

This works, but is it legal/safe?

(i.e.: Is it okay to pass projection as predicate argument to std::ranges::any_of/all_of/none_of?)

#include <ranges>
#include <iostream>
#include <algorithm>
#include <vector>

int main()
{
    struct boo{
        bool b;
        bool get(){ return b; }
    };
    std::vector<boo> bools{ {0}, {0}, {0}, {1}, {0} };
    if(std::ranges::any_of(bools, &boo::b )){
        std::cout << "&boo::b\n";
    }
    if(std::ranges::any_of(bools, &boo::get )){
        std::cout << "&boo::get\n";
    }
    // check for non zero ptr
    const char *ptrs[6]{0, 0, 0, "0", 0, 0};
    if(std::ranges::any_of(ptrs, std::identity{} )){
        std::cout << "std::identity{}\n";
    }
}

Godbolt

Answer 1

A predicate is roughly speaking a callable that returns a bool when called.

It's OK to pass your projections here for the predicate (pred) parameter because they return a bool (it could also be something convertible to bool).

If your projection was returning something which is not convertible to bool it will no longer compile.

For example:

struct NoBool {};

struct boo {
    NoBool b;
    NoBool get() { return b; }
};

std::vector<boo> bools{ {}, {}, {}, {}, {} };
if (std::ranges::any_of(bools, &boo::b)) {
    std::cout << "&boo::b\n";
}

if (std::ranges::any_of(bools, &boo::get)) {
    std::cout << "&boo::get\n";
}

Will cause compiler errors for both above calls of std::ranges::any_of:

<source>:17:31: error: no match for call to '(const std::ranges::__any_of_fn) (std::vector<main()::boo>&, main()::NoBool main()::boo::*)'
   17 |         if(std::ranges::any_of(bools, &boo::b )){
      |            ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~

...

Live demo - Godbolt