C++ using string_view in constexpr if

Question

I want to compare string views at compile time like this:

constexpr bool isMatch(const string_view str) {
  if constexpr (str == "firstMatch"sv) {
      return true;
  } else if constexpr (str == "secondMatch"sv) {
     return true;
  }
  // ....
}

And the function is called like this:

isMatch("something"sv);

Which is obviously constant because it's right in the code.

But the compiler says that the expression inside the if constexpr 'did not evaluate to a constant'. But operator== of a string_view is constexpr. A solution would be to declare the parameter constexpr but it's not possible.

So how can I have a constexpr function that checks for these kind of matches at compile time?

Answer 1

how can I have a constexpr function that checks for these kind of matches

Just remove the constexpr from ifs.

constexpr bool isMatch(const string_view str) {
  if (str == "firstMatch"sv) {
      return true;
  } else if (str == "secondMatch"sv) {
     return true;
  }
  return false;
}

std::string_view::operator== is constexpr so all is fine.

at compile time?

Call your function in compile time context. Like:

#include <array>
#include <cassert>
#include <string_view>
using namespace std::literals;

constexpr bool isMatch(const std::string_view str) {
  if (str == "firstMatch"sv) {
      return true;
  } else if (str == "secondMatch"sv) {
     return true;
  }
  return false;
}

int main() {
    std::array<int, isMatch("secondMatch"sv)> arr;
    static_assert(isMatch("firstMatch"sv));
    constexpr bool var = isMatch("No match for you!"sv);
}

In C++20 you can make sure that your function is called only at compile time with consteval.