Mandatory Build-Time Timebase Selection

#include <cstdint>
#include <cstdio>

// Timebase A: SysTick (1 kHz)
struct SysTickTimer {
    static std::uint32_t ticks() {
        return 100;
    }
    static std::uint32_t freq_hz() {
        return 1000;
    }
};

// Timebase B: RTC (32 Hz)
struct RtcTimer {
    static std::uint32_t ticks() {
        return 4;
    }
    static std::uint32_t freq_hz() {
        return 32;
    }
};

/**
 * Scheduler Class
 * Uses a Template parameter to define the hardware timebase.
 * This ensures that the math is resolved for the specific type provided.
 */
template<typename Timer>
class Scheduler {
public:
    std::uint32_t elapsed_ms() const {
        // Calculation: (ticks * 1000) / frequency
        return (Timer::ticks() * 1000U) / Timer::freq_hz();
    }
};

/**
 * Build-Time Selection
 * In a production environment, this alias would be swapped 
 * based on the target hardware build configuration.
 */
using ActiveTimer = SysTickTimer; 

int main() {
    // By passing 'ActiveTimer' (SysTickTimer), we satisfy the template requirement.
    // If you were to write 'Scheduler<> scheduler', it would fail to compile.
    // If you omit the type entirely 'Scheduler scheduler', it fails to compile.
    Scheduler<ActiveTimer> scheduler;

    std::printf("elapsed_ms=%u\n",
                static_cast<unsigned>(scheduler.elapsed_ms()));
    
    return 0;
}