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Thinking in C++
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Access functions

One of the most important uses of inlines inside classes is the access function. This is a small function that allows you to read or change part of the state of an object – that is, an internal variable or variables. The reason inlines are so important for access functions can be seen in the following example:

//: C09:Access.cpp
// Inline access functions

class Access {
  int i;
  int read() const { return i; }
  void set(int ii) { i = ii; }

int main() {
  Access A;
  int x =;
} ///:~

Here, the class user never has direct contact with the state variables inside the class, and they can be kept private, under the control of the class designer. All the access to the private data members can be controlled through the member function interface. In addition, access is remarkably efficient. Consider the read( ), for example. Without inlines, the code generated for the call to read( ) would typically include pushing this on the stack and making an assembly language CALL. With most machines, the size of this code would be larger than the code created by the inline, and the execution time would certainly be longer.

Without inline functions, an efficiency-conscious class designer will be tempted to simply make i a public member, eliminating the overhead by allowing the user to directly access i. From a design standpoint, this is disastrous because i then becomes part of the public interface, which means the class designer can never change it. You’re stuck with an int called i. This is a problem because you may learn sometime later that it would be much more useful to represent the state information as a float rather than an int, but because int i is part of the public interface, you can’t change it. Or you may want to perform some additional calculation as part of reading or setting i, which you can’t do if it’s public. If, on the other hand, you’ve always used member functions to read and change the state information of an object, you can modify the underlying representation of the object to your heart’s content.

In addition, the use of member functions to control data members allows you to add code to the member function to detect when that data is being changed, which can be very useful during debugging. If a data member is public, anyone can change it anytime without you knowing about it.

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