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Thinking in C++
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Reducing recompilation

The project manager in your programming environment will cause a recompilation of a file if that file is touched (that is, modified) or if another file it’s dependent upon – that is, an included header file – is touched. This means that any time you make a change to a class, whether it’s to the public interface or to the private member declarations, you’ll force a recompilation of anything that includes that header file. This is often referred to as the fragile base-class problem. For a large project in its early stages this can be very unwieldy because the underlying implementation may change often; if the project is very big, the time for compiles can prohibit rapid turnaround.

The technique to solve this is sometimes called handle classes or the “Cheshire cat”[37] – everything about the implementation disappears except for a single pointer, the “smile.” The pointer refers to a structure whose definition is in the implementation file along with all the member function definitions. Thus, as long as the interface is unchanged, the header file is untouched. The implementation can change at will, and only the implementation file needs to be recompiled and relinked with the project.

Here’s a simple example demonstrating the technique. The header file contains only the public interface and a single pointer of an incompletely specified class:

//: C05:Handle.h
// Handle classes
#ifndef HANDLE_H
#define HANDLE_H

class Handle {
  struct Cheshire; // Class declaration only
  Cheshire* smile;
  void initialize();
  void cleanup();
  int read();
  void change(int);
#endif // HANDLE_H ///:~

This is all the client programmer is able to see. The line

struct Cheshire;

is an incomplete type specification or a class declaration (A class definition includes the body of the class.) It tells the compiler that Cheshire is a structure name, but it doesn’t give any details about the struct. This is only enough information to create a pointer to the struct; you can’t create an object until the structure body has been provided. In this technique, that structure body is hidden away in the implementation file:

//: C05:Handle.cpp {O}
// Handle implementation
#include "Handle.h"
#include "../require.h"

// Define Handle's implementation:
struct Handle::Cheshire {
  int i;

void Handle::initialize() {
  smile = new Cheshire;
  smile->i = 0;

void Handle::cleanup() {
  delete smile;

int Handle::read() {
  return smile->i;

void Handle::change(int x) {
  smile->i = x;
} ///:~

Cheshire is a nested structure, so it must be defined with scope resolution:

struct Handle::Cheshire {

In Handle::initialize( ), storage is allocated for a Cheshire structure, and in Handle::cleanup( ) this storage is released. This storage is used in lieu of all the data elements you’d normally put into the private section of the class. When you compile Handle.cpp, this structure definition is hidden away in the object file where no one can see it. If you change the elements of Cheshire, the only file that must be recompiled is Handle.cpp because the header file is untouched.

The use of Handle is like the use of any class: include the header, create objects, and send messages.

//: C05:UseHandle.cpp
//{L} Handle
// Use the Handle class
#include "Handle.h"

int main() {
  Handle u;
} ///:~

The only thing the client programmer can access is the public interface, so as long as the implementation is the only thing that changes, the file above never needs recompilation. Thus, although this isn’t perfect implementation hiding, it’s a big improvement.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire