Modules (C++20)

// Old: Headers - slow, fragile
// math.h
#ifndef MATH_H  // Include guards needed
#define MATH_H

namespace math {
    double square(double x);
    double cube(double x);
}

#endif

// C++20: Modules - fast, clean
// math.ixx (module interface unit)
export module math;

export namespace math {
    double square(double x) { return x * x; }
    double cube(double x) { return x * x * x; }
}

// main.cpp
import math;  // Fast, no macros leaked

int main() {
    auto result = math::square(5.0);
}

// Module interface unit (math.ixx)
export module math;

// Import other modules
import std.core;

// Module implementation (can be in same file or separate)
module math;

// Private (not exported) - internal to module
namespace {
    double internal_helper(double x) {
        return x * 2;
    }
}

// Exported interface
export double calculate(double x) {
    return internal_helper(x) + 1;
}

// Export block
export {
    struct Point { double x, y; };
    double distance(Point a, Point b);
}