Check array contains element at compile time

Question

We have a modifyCoefficient(const char* name, int value) function that updates the value of a coefficient in a container. The names are known at compile time, they are read from an XML file in a pre-build step and stored in an array.
Usage: data.modifyCoefficient("ADAPTIVE", 1);
Compiler: MSVC 2017 v15.8

I would like to get a compile time error when the coefficient name does not exist.
With the following code that happens, but is there a way to do it without a macro?

#include <array> #define COEFF(name) returnName<coefficientExists(name)>(name) constexpr std::array<const char*, 3> COEFFICIENTS = { "VERSION", "CHANNELS", "ADAPTIVE" }; constexpr bool coefficientExists(const char* name) { for (auto coefficientIndex = 0U; coefficientIndex < COEFFICIENTS.size(); ++coefficientIndex) { if (COEFFICIENTS[coefficientIndex] == name) return true; } return false; } template<bool CoefficientTest> constexpr const char* returnName(const char* name) { static_assert(CoefficientTest, "coefficient does not exist"); return name; } int main() { static_assert(coefficientExists("VERSION"), "should exist"); static_assert(coefficientExists("TEST") == false, "should not exist"); static_assert(COEFF("ADAPTIVE") == "ADAPTIVE", "should return name"); COEFF("CHANNELS"); // data.modifyCoefficient(COEFF("ADAPTIVE"), 1); return 0; } 

https://godbolt.org/z/kpGcMS

Answer 1

You have to compare values char by char, or even simpler change const char* to std::string_view make your code work.

Thanks to the deduction guides, you can just write std::array = ... and template parameters will be automatically deduced.

Also, you can simplify your loop using a "range-based for". If we generalize for other types of compile-time array, we get:

#include <array> #include <string_view> using namespace std::string_view_literals; constexpr std::array COEFFICIENTS = { "VERSION"sv, "CHANNELS"sv, "ADAPTIVE"sv }; template <typename T, size_t N> constexpr bool array_has(const std::array<T, N>& array, const T& value) { for (const auto& v : array) { if (value == v) return true; } return false; } constexpr bool coefficientExists(const std::string_view name) { return array_has(COEFFICIENTS, name); } int main() { static_assert(coefficientExists("VERSION"), "should exist"); static_assert(!coefficientExists("TEST"), "should not exist"); } 

With C++20 you can even do simpler:

#include <algorithm> //... template <typename T, size_t N> constexpr bool array_has(const std::array<T, N> array, const T value) { return std::any_of(array.begin(), array.end(), [value](auto current){ return current == value; }); } 

Answer 2

With C++20's consteval this is now possible without static_assert or macro's.
Solution based on C++20 to eliminate runtime bugs and Compile-time format string checks.

#include <algorithm> #include <array> #include <string_view> using namespace std::string_view_literals; constexpr auto COEFFICIENTS = std::array{ "VERSION"sv, "CHANNELS"sv, "POWER"sv }; struct CoefficientName { std::string_view str; consteval CoefficientName(std::string_view name) :str(name) { if(std::ranges::find(COEFFICIENTS, name) == COEFFICIENTS.end()) throw; } }; void modifyCoefficient(CoefficientName name, int value) { } int main() { modifyCoefficient("CHANNELS"sv, 42); modifyCoefficient("CURRENT"sv, 99); // compilation error modifyCoefficient("POWER"sv, 1); return 0; } 

https://godbolt.org/z/M3oE4acxc