Modules provide an encapsulation mechanism that replaces textual inclusion of header files with explicit interface declarations. An export declaration specifies the module's public interface; import declarations make these declarations available to consuming translation units. Modules are compiled once, cached as binary metadata, and provide namespace isolation that prevents macro leakage and implementation detail exposure.

Header files require textual re-parsing for each translation unit, impose macro pollution and include-order dependencies, and expose implementation details through textual inclusion. Modules eliminate redundant parsing via precompiled metadata, enforce interface boundaries at the language level, and improve build performance through on-demand symbol loading.

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

export int add(int a, int b) {
    return a + b;
}

export namespace geometry {
    double circle_area(double radius) {
        return 3.14159 * radius * radius;
    }
}

// Internal helper - not exported
int internal_helper() { return 42; }

// main.cpp
import math;
import std;

int main() {
    std::println("{}", add(2, 3));
    std::println("{}", geometry::circle_area(5.0));
    // internal_helper();  // Error: not exported
}