Source files and header files

A typical C++ code consists of

• source files, with names ending with .cpp

• header files, with names ending with .hpp

Header files can be included in source files, using the #include command. Say we have two files, my_header.hpp and main.cpp:

my_header.hpp:

#include <iostream>

void print_message()
{
  std::cout << "Hello, World!\n";
}

main.cpp:

#include "my_header.hpp"

int main()
{
  print_message();

  return 0;
}

The line #include "my_header.hpp" in main.cpp simply tells the compiler to insert the content of my_header.hpp here before compiling. So the code that ends up being compiled is completely equivalent to this single .cpp file:

#include <iostream>

void print_message()
{
  std::cout << "Hello, World!\n";
}

int main()
{
  print_message();

  return 0;
}

(And this code is of course equivalent to the code you would get if you replaced #include <iostream> with the full content of iostream.hpp from the standard C++ library. But let’s not do that…)

Note

When including our own header files, we use the syntax #include "someheader.hpp", rather than #include <someheader>.

In in larger C++ projects it is common to put all source files (.cpp) in a folder called src/, and all header files (.hpp) in a folder called (include/). More on this in the next section.

(When you should not be) using namespace

Towards the top of .cpp files you will often see a line like

using namespace std;

and similar for other namespaces, e.g.

using namespace arma;

if the code is using parts of the Armadillo library. What e.g. using namespace std; means is effectively let me access everything from the std namespace without first writing std::. (It’s analogous to doing e.g. from numpy import * in a Python program that is using the numpy library.)

Here’s an example program that uses cout rather than std::cout to print a message to screen:

#include <iostream>

using namespace std;

int main()
{
  cout << "Hello, World!\n";

  return 0;
}

Using this using namespace trick in your source files (.cpp) is usually fine. Just remember that in programs that involve many different libraries/namespaces, it may be important for code readability to actually write out the namespaces, since they tell us where you are getting different functions/variables/classes from.

However, you should not put using namespace lines in your header files (.hpp). Doing this is generally regarded very bad practice. The reason is that if you do e.g.

using namespace some_namespace;

in a header file, this line will also affect all .cpp files (all compilation units) that include your header file at the beginning. So you end up “polluting” the main namespace of that .cpp file with everything from some_namespace, even though the person who wrote that .cpp file probably just included your header file to get access to some specific variables/functions/classes from your code. And if some_namespace e.g. contained a variable some_variable, this means that the person writing the .cpp file would get a surprising error message if they happened to also have a variable named some_variable in their code.