gists/cpp/compile-time-settings-check.md

It is sometimes useful to be able to verify that certain settings are valid at compile time. An easy approach to do this is to pass the settings as non-type template arguments and use static_assert().

The code snippet below demonstrates an approach which doesn't require templates and cleanly separates the verification logic from the rest of the code.

namespace i2c {


//
//
// Driver Settings
//
//


struct Settings {
    int         sclPin = 0;
    int         sdaPin = 0;
    std::size_t freqHz = 1'000'000;
};


namespace detail {
struct SettingsChecker {
public:
    consteval SettingsChecker(Settings settings) : mSettings{settings} {
        bool validPinSettings =
            (settings.sclPin == 1 && settings.sdaPin == 2) ||
            (settings.sclPin == 3 && settings.sdaPin == 4);
        bool validFreqSettings =
            (settings.freqHz < 10'000'000) && (settings.freqHz > 100);

        // Exceptions thrown in consteval functions lead to compile time errors

        if (!validPinSettings) throw std::logic_error("Wrong I2C pin settings");
        if (!validFreqSettings)
            throw std::logic_error("Wrong I2C frequency settings");
    }

    constexpr operator Settings() const {
        return mSettings;
    }

private:
    Settings mSettings;
};
} // namespace detail


//
//
// Driver class
//
//


class Driver {
public:
    Driver(detail::SettingsChecker settings) : mSettings(settings) {
    }

private:
    Settings mSettings;
};


} // namespace i2c


//
//
// Usage
//
//


int main() {
    constexpr i2c::Settings settings1 = {
        .sclPin = 1, .sdaPin = 2, .freqHz = 1'000'000};

    i2c::Driver driver1{settings1}; // Compiles

    constexpr i2c::Settings settings2 = {
        .sclPin = 1, .sdaPin = 4, .freqHz = 1'000'000};

    i2c::Driver driver2{settings2}; // Compile time error
}

Compiling the above code leads to the following error:

compile-time-settings-check.cpp: In function ‘int main()’:
compile-time-settings-check.cpp:88:34: error: call to consteval function ‘i2c::detail::SettingsChecker(settings2)’ is not a constant expression
   88 |     i2c::Driver driver2{settings2}; // Compile-time error
      |                                  ^
compile-time-settings-check.cpp:88:34:   in ‘constexpr’ expansion of ‘i2c::detail::SettingsChecker(settings2)’
compile-time-settings-check.cpp:36:32: error: expression ‘<throw-expression>’ is not a constant expression
   36 |         if (!validPinSettings) throw std::logic_error("Wrong I2C pin settings");
      |