6.2 Multi-file Builds and Headers

As programs grow, put declarations in .h header files and implementations in .cpp source files.

cpp

// geometry.h
#ifndef GEOMETRY_H
#define GEOMETRY_H

double circle_area(double radius);

#endif

cpp

// geometry.cpp
#include "geometry.h"

#define PI 3.1415926

double circle_area(double radius) {
    return PI * radius * radius;
}

cpp

// main.cpp
#include <iostream>
#include "geometry.h"

int main() {
    std::cout << circle_area(5) << std::endl;
    return 0;
}

Compile the related source files together:

bash

g++ -Wall geometry.cpp main.cpp -o area

The #ifndef / #define / #endif lines in a header are called an include guard; they stop the same header from being included twice.

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