#include Directives

The Story So Far

With what we've learned, we can now change our original two source files into three source files, where code that's needed by both the original files is moved to its own file.

#include <iostream> // for std::cin, std::cout
#include <string>   // for std::string

// Declares the printMoos function.
void printMoos(int numMoos);

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 <iostream>

// Declares the printMoos function.
void printMoos(int numMoos);

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;
}

printmoos.cpp:

#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 << ".";
}

We need to run clang++ five times:

1. clang++ -c printmoos.cpp compiles printmoos.cpp into printmoos.o
2. clang++ -c cowchat.cpp compiles cowchat.cpp into cowchat.o
3. clang++ -c cowmath.cpp compiles cowmath.cpp into cowmath.o
4. clang++ -o cowchat cowchat.o printmoos.o links cowchat.o and printmoos.o to form cowchat
5. clang++ -o cowmath cowmath.o printmoos.o links cowmath.o and printmoos.o to form cowmath

#include Directives

As we mentioned last lesson, before the compilation process really gets started, the first step is to preprocess the code.

• The preprocessor's job is to manipulate the text in source files in various ways before they are compiled.
• You give commands to the preprocessor with preprocessor directives in your code files; these directives start with the character #.
• The preprocessor executes one directive at a time as it passes over the file.
• After the preprocessor is finished, the modified code is passed into the compiler.

There are several preprocessor directives, but right now we will focus on a particularly important one: #include.

• #include is essentially a kind of copy-and-paste.
  • It takes the entire contents of a specified file and inserts it into the including file before compiling.
• #include "filename" assumes that filename is a file that we have written, and the preprocessor will look for it in the current directory.
• #include <filename> assumes the file is a file that is part of the C++ system (rather than our code) and will look for it somewhere other than the current directory; typically in some system directory.

You can see that we have #include <iostream> in all three files (cowchat.cpp, cowmath.cpp, and printmoos.cpp), which tells the compiler that whatever is in the iostream file is also “in” all three of our .cppfiles.

The good news is that we didn't have to literally make three copies of that code!

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