sign-conversion warning in catch_tostring.hpp

[Teskann]

Describe the bug

Hello, the latest devel of Catch2 raises a warning on g++ 12.

conversion to ‘std::size_t’ {aka ‘long unsigned int’} from ‘long int’ may change the sign of the result

This warning was introduced a few days ago by PR #2540

File catch_tostring.hpp line 46:

inline std::size_t catch_strnlen(const char *str, std::size_t n) {
    auto ret = std::char_traits<char>::find(str, n, '\0');
    if (ret != nullptr) {
        return ret - str;  // <-- Here !
    }
    return n;
}

Expected behavior

Catch2 builds without warnings.

Reproduction steps

Build Catch2 with g++ .

Platform information:
g++ version 12.1.0 (Ubuntu 12.1.0-2ubuntu1~22.04)

Possible fixes

• Change the return type of catch_strnlen to std::ptrdiff_t. This would prevent to return n without a cast to std::ptrdiff_t.
• Explicitly cast the result to std::size_t.

return static_cast<std::size_t>(ret - str);

This should be relatively safe as str should never be greater than ret. In my opinion this is the way to go.

@rkaminsk what do you think ?

[rkaminsk]

The code safe. We could add a static cast to silence the warning.

[horenmar]

static cast is the correct option, ptrdiff return type would just force the cast in callers.