You provide any text document with embedded fragments of Python, and Cog renders it:

using namespace std;

#include <iostream>

int main() {
    cout << "Hello Earth." << endl;


/*[[[cog

print("""\
cout << "Hello Mars." << endl;
printf("Goodbye Pluto.\\n");
""")

]]]*/

//[[[end]]]


  return 0;
}

Notice that newlines have to be escaped.

Translate:

$ cog.py hello_world.cpp.cog > hello_world.cpp

Output:

using namespace std;

#include <iostream>

int main() {
    cout << "Hello Earth." << endl;


/*[[[cog

print("""\
cout << "Hello Mars." << endl;
printf("Goodbye Pluto.\\n");
""")

]]]*/
cout << "Hello Mars." << endl;
printf("Goodbye Pluto.\n");

//[[[end]]]


  return 0;
}

General build and run:

$ g++ -o hello_world hello_world.cpp
$ ./hello_world
Hello Earth.
Hello Mars.
Goodbye Pluto.