Header Files and Recompilation

#ifndef PRINTMOOS_HPP_INCLUDED
#define PRINTMOOS_HPP_INCLUDED

void printMoos(int numMoos);

#endif

#include "printmoos.hpp"
#include <iostream> // for std::cin, std::cout

// Prints the given number of "moos"
void printMoos(int numMoos) {
    std::cout << "Moo";
    for (int i = 1; i < numMoos; ++i) {
        std::cout << " moo";
    }
    std::cout << ".";
}

#include "printmoos.hpp"
#include <iostream> // for std::cin, std::cout
#include <string>   // for std::string

int main() {
    std::cout << "Chat with a cow!";
    std::string userIn;
    do {
        std::cout << std::endl  // New line
                  << "> ";      // The prompt
        std::getline(std::cin, userIn);
        int nMoos = 1 + userIn.size() % 2;
                     // ^-- Either 1 or 2 moos;
        printMoos(nMoos);
    } while (std::cin.good() && userIn != "");
    std::cout << "\n\nBye!\n";

    return 0;
}

#include "printmoos.hpp"
#include <iostream> // for std::cin, std::cout

int main() {
    std::cout << "Cow math!\n";

    std::cout << "Enter the first number: ";
    int x = 0;
    std::cin >> x;

    std::cout << "Enter the second number: ";
    int y = 0;
    std::cin >> y;

    if (std::cin.good()) {
        std::cout << "The cow goes: ";
        printMoos(x + y);
        std::cout << std::endl;
    } else {
        std::cout << "\n\nSomething went wrong!\n";
    }

    return 0;
}

We can compile the whole project by running

clang++ -c printmoos.cpp
clang++ -c cowchat.cpp
clang++ -c cowmath.cpp
clang++ -o cowchat cowchat.o printmoos.o
clang++ -o cowmath cowmath.o printmoos.o

Recompiling After Changes

Say we make a change to cowchat.cpp.

Since we only changed cowchat.cpp, we only need to recompile cowchat.o and relink our object files to get our executable cowchat:

clang++ -c cowchat.cpp
clang++ -o cowchat cowchat.o printmoos.o

Okay, how about a trickier one? Now say we make a change to printmoos.hpp.

Remember that we don't compile header files!

But printmoos.hpp is included by all our source files. So whatever is in printmoos.hpp is also in all the files after the preprocessor runs, so changes to printmoos.hpp are, effectively, changes to all our .cpp files!

Therefore, if printmoos.hpp changes, we need to recompile all of our .o files (and then relink as necessary):

clang++ -c printmoos.cpp
clang++ -c cowchat.cpp
clang++ -o cowchat cowchat.o printmoos.o
clang++ -c cowmath.cpp
clang++ -o cowmath cowmath.o printmoos.o

The diagram below shows the dependencies between the files in our project. (A solid arrow from a node to another node means “if the first node is updated, we need to rebuild the second node”; dotted lines show the #includes.)

(When logged in, completion status appears here.)