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CS253 Separate Compilation

Separation, by Edvard Munch

Separate Compilation

It’s often useful to separate the class interface (*.h) from the class implementation (*.cc).

Example

#ifndef FOO_H_INCLUDED
#define FOO_H_INCLUDED

#include <iostream>

class Foo {
  public:
    Foo();
    int get_data() const;
  private:
    double data;
};

std::ostream & operator<<(std::ostream &,
			  const Foo &);

#endif /* FOO_H_INCLUDED */

#include "Foo.h"

using namespace std;

Foo::Foo() : data(42.0) {
}

int Foo::get_data() const {
    return data;
}

ostream &operator<<(ostream &os,
		    const Foo &f) {
    return os << f.get_data();
}

Contents

The *.h file only contains the method declarations (signatures), function declarations (for non-member functions & operators) and declarations of data members. The *.cc file contains the actual code for the methods and functions. Neither one contains main().

Why?

• Consider the *.h file as containing the interface, the contract between you, the developer, and the end user. We can develop the interface long before we have to actually write the code. It’s easier to consider the merits of the interface unencumbered by the details of the implementation.
• The users of your class are only interested in the interface, not the implementation. They don’t need the *.cc.
• This enables the class to be compiled separately. For example, if the class is part of a product that you sell, you can deliver only Foo.h and Foo.o to your customers. This keeps your implementation source code out of your customer’s hands.

Typing

“But it’s more typing!”

If you’re afraid of typing, then you have certainly chosen the wrong profession, and the wrong century to live in.

Compilation

Compile the code like this:

    g++ -Wall main.cc Foo.cc

If you do this with a Makefile, make sure to have main.cc also depend upon Foo.h. After all, if you change Foo.h (say, by adding a member variable) then you must recompile main.cc, because the size of Foo has changed.

Beware

Do not compile the *.h file, unless you really know what you’re doing. That will result in a *.gch (compiled header) file, which will cause no end of trouble. Remove it if you ever find it.

Example

Consider this simple program. It gets π from a header file:

    % cat main.cc
    #include "pi.h"
    #include <iostream>
    int main() {
        std::cout << pi << '\n';
        return 0;
    }
    % cat pi.h
    constexpr auto pi = 3.14;
    % g++ -Wall main.cc
    % ./a.out
    3.14
    % ls
    a.out  main.cc  pi.h

Note that pi.h is not mentioned in the compile command.

What if we try to compile pi.h?

    % g++ -Wall main.cc pi.h
    % ls
    a.out  main.cc  pi.h  pi.h.gch
    % ./a.out
    3.14

    % echo "constexpr auto pi = 3.14159;" >pi.h
    % g++ -Wall main.cc 
    % ./a.out
    3.14

    % rm pi.h.gch 
    % g++ -Wall main.cc 
    % ./a.out
    3.14159

Moral of the story

• When the compiler sees #include "pi.h", it first looks for pi.h.gch, and uses that precompiled header file if it’s present.
• This means that, if you change pi.h, it doesn’t matter, because the compiler uses pi.h.gch if it’s there. Ack!
• Same principle as having foo.cc and foo.o.
• This can be a powerful tool, if used properly with a well-written Makefile.
• My attitude: Compiled headers ain’t worth it. Stay away from them. Write a Makefile so you won’t screw up your compile comand.