Add operator bool to modint

[szdytom]

It can be useful if you want something like:

mint x;
// ...
if (x)
  do_sth(x);

[mizar]

https://github.com/atcoder/ac-library/pull/169/commits/95a6273bb67a1440dffe06687d62b13e12937abd Strange to convert zero value to true.

For conversion rules between bool and int types in the C++ language,

https://timsong-cpp.github.io/cppwp/n4659/conv.prom#6

A prvalue of type bool can be converted to a prvalue of type int, with false becoming zero and true becoming one.

https://timsong-cpp.github.io/cppwp/n4659/conv.bool#1

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. For direct-initialization, a prvalue of type std​::​nullptr_­t can be converted to a prvalue of type bool; the resulting value is false.

[szdytom]

it is preferred to make conversion function explicit unless any reason not to do so.

This is the place where we need implicit conversion. no one wants to write if ((bool)x) { ... } anyway.

[yaito3014]

it is preferred to make conversion function explicit unless any reason not to do so.

This is the place where we need implicit conversion. no one wants to write if ((bool)x) { ... } anyway.

You can write if (x) { ... } even if the conversion operator to bool is marked as explicit.

cf. https://en.cppreference.com/w/cpp/language/implicit_conversion#Contextual_conversions

[TumoiYorozu]

This repository does not accept requests for new features.

For now, we are not planning to add new features

In this case, I don't think it's necessary because you can add a custom bool cast as shown below:

#include <atcoder/modint>
#include <bits/stdc++.h>

struct mint : public modint998244353 {
    using static_modint::static_modint;
    mint(modint998244353  m) : modint998244353 (m) {}
    operator bool() const { return val() != 0; }
};

int main() {
    mint m(123);

    if (m) {
        puts("not zero");
    } else {
        puts("zero");
    }
}