C++ Programming

Chapter 16: Compilation and Execution

Translation process

Compiling a C++ program consists of three steps:

Step 1: Preprocessor processing

Step 2: Compile from code file to object file

Step 3: Link from object file to executable program

A program can consist of several files, which are first subjected to steps 1 and 2 separately. Finally, the files are linked to a complete program in step 3.

The precompiler

Before the actual compilation of the code, the code is processed by the pre-compiler to which you can give commands that begin with # and are called directives. Common directives include #include, which inserts code from another file, and #define, which defines a macro.

We have used the #include directive in the past to use the standard library, giving the file name in angle brackets (such as <iostream>). When we include code from a file we have written ourselves, we put the file name in quotation marks.

In the following example, the code from the file add.inc will be inserted into the code in the file code.cpp by the precompiler.

// add.inc
    cout << "Hello!\n";

// code.cpp
#include <iostream>

using namespace std;

int main() {
#include "add.inc"
}

This is the same as the following code:

// code.cpp
#include <iostream>

using namespace std;

int main() {
// add.inc
    cout << "Hello!\n";
}

The #define directive defines a macro, which means that a certain string in the code is replaced by another string. For example, the following macro replaces pb with push_back.

#define pb push_back

This macro will mean that the code

v.pb(1);

changes in the precompiler processing to the following:

v.push_back(1);

Many files

The following program consists of two compiled files code.cpp and test.cpp. The test.cpp file has the test function, which is presented in the header file test.h. The idea is that code.cpp knows the header file thanks to the test function, although it does not see the function definition.

// test.h
void test();

// test.cpp
#include <iostream>

using namespace std;

void test() {
    cout << "Hello!\n";
}

// code.cpp
#include <iostream>
#include "test.h"

using namespace std;

int main() {
    test();
}

Compilation of the program takes both filenames code.cpp and test.cpp:

$ g++ code.cpp test.cpp -o code

You can also compile files separately:

$ g++ code.cpp -c
$ g++ test.cpp -c
$ g++ code.o test.o -o code

Here, the first two lines create the compiled object files code.o and test.o and the last line links these files together into an executable program.

Command line parameters

When you start the program from the command line, it is possible to give it parameters after the program name. These parameters can be found in the program using the broader main function definition with argc (number of parameters) and argv (the parameters themselves, in character arrays).

For example, the following code will print everything given to the program as parameters:

int main(int argc, char **argv) {
    for (int i = 0; i < argc; i++) {
        cout << i << " " << argv[i] << "\n";
    }
}

Now we can start the program:

$ ./code monkey banana harpsichord
0 ./code
1 monkey
2 banana
3 harpsichord

The first parameter is always the program name, i.e. argc is always at least 1, and after that come any other parameters, separated by spaces.

The return value

Because the main function type is int, it may return an integer. If the main function does not have a return line, it returns 0 by default, whose usual interpretation is that the program ended successfully. However, if the program returns something else (non-zero), this is interpreted as an error.

For example, the following program returns 13 as an error:

int main() {
    return 13;
}

After running the program, we can check its return value. In Linux, for example, we have access to the return value from the command line via variable $?:

$ ./code
$ echo $?
13

For example, we could use a return value in a script that executes the program and checks after execution whether the program ended successfully or an error occurred.

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