MFC Programmer's SourceBook : Thinking in C++

Reducing clutter

In a book like this, the simplicity and terseness of putting inline definitions inside classes is very useful because more fits on a page or screen (in a seminar). However, Dan Saks[27] has pointed out that in a real project this has the effect of needlessly cluttering the class interface and thereby making the class harder to use. He refers to member functions defined within classes using the Latin in situ (in place) and maintains that all definitions should be placed outside the class to keep the interface clean. Optimization, he argues, is a separate issue. If you want to optimize, use the inline keyword. Using this approach, the earlier Rectangle.cpp example becomes

//: C09:Noinsitu.cpp
// Removing in situ functions

class Rectangle {
  int width, height;
public:
  Rectangle(int w = 0, int h = 0);
  int getWidth() const;
  void setWidth(int w);
  int getHeight() const;
  void setHeight(int h);
};

inline Rectangle::Rectangle(int w, int h)
  : width(w), height(h) {
}

inline int Rectangle::getWidth() const {
  return width;
}

inline void Rectangle::setWidth(int w) {
  width = w;
}

inline int Rectangle::getHeight() const {
  return height;
}

inline void Rectangle::setHeight(int h) {
  height = h;
}

int main() {
  Rectangle r(19, 47);
  // Transpose width & height:
  int iHeight = r.getHeight();
  r.setHeight(r.getWidth());
  r.setWidth(iHeight);
} ///:~

Now if you want to compare the effect of inlining with out-of-line functions, you can simply remove the inline keyword. (Inline functions should normally be put in header files, however, while non-inline functions must reside in their own translation unit.) If you want to put the functions into documentation, it’s a simple cut-and-paste operation. In situ functions require more work and have greater potential for errors. Another argument for this approach is that you can always produce a consistent formatting style for function definitions, something that doesn’t always occur with in situ functions.

[27] Co-author with Tom Plum of C++ Programming Guidelines , Plum Hall, 1991.

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