Multipe Source Files

Overview

Teaching: 15 min
Exercises: 5 min

Questions

• How do I compile programs consiting of multiple source files?

Objectives

• Learn how to use the compile command for multiple source files.

• Learn how to use header files.

Like Python, C++ programs can be comprised of multiple modules. Modules in C++ are not imported the same way they are for Python, however. Instead, the modules must be combined together into a single executable in order for the functionality to be accessible.

Let’s try it. First, create a new source file called func.cpp and put the following code in it:

// A function to increment its argument
int func(int a) {
	return a+1;
}

Next we need to modify the hello_world.cpp program to use the function. Edit hello_world.cpp and change it to:

#include <iostream>

extern int func(int); // Declare the interface we want to use

int
main (int argc, char *argv[]) 
{
	int a = 1;
	std::cout << "Hello World!\n";
	std::cout << "func is " << func(a) << "\n";
	exit(0);
}

Now lets try the same command to compile hello_world.cpp we tried before.

$ c++ -o hello_world hello_world.cpp

This time, however, we get an error similar to this:

Undefined symbols for architecture x86_64:
  "func(int)", referenced from:
      _main in hello_world-76e383.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

The reason for this error is that we’re trying to call func(a) in the program, but the symbol (or name) func can’t be found. In order to resolve this, we need to add the func.cpp file to the compile command:

$ c++ -o hello_world hello_world.cpp func.cpp

This time the program compiles without error and we can again run the program:

$ ./hello_world
Hello World!
func is 2
$

Challenge

In C++, you must first declare the interface of the function you want to use. We did this by adding a line to hello_world.cpp. However the more usual approach is to put these declarations into a header file, which by convention has the same name as the source file, but ends in .h instead of .cpp. These header files are then included using a #include statement in order to use them. The source code for the function itself is called the definition of the function.

Replace the line

extern int func(int);

in hello_world.cpp with the line:

#include "func.h"

Then create a new file called func.h and add the following line:

extern int func(int);

Finally, try compiling the program again using the same command as last time. Did you get it to work?

Key Points

• Multiple source files can be combined into a single executable.

• Header files are used to separate declarations from definitions.