Pointers, References and Values by Michael D. Crawford Continued...
When to Use Whole-Object Member Variables
Storing members by value improves performance when they are small or frequently allocated.
While I do recommend that you prefer using pointers for member variables rather than storing them by value, you should not always do so. Here are situations when you might choose to store the member as a whole object, that is, to store it by value:
- The header file for the member's class has to be included anyway, perhaps to satisfy some other part of the same class definition.
- The member is a built-in type, like an integer.
- The member is small.
- To save memory from heap manager overhead, as when many objects of the class that holds this member are likely to be in existence at the same time. Note that typical heaps require at least a dozen bytes in overhead per object allocated, which may actually be larger than the memory actually used as payload in the heap block.
- To improve performance in allocating the object, as when objects of this class will be allocated and destroyed frequently.
- As a special situation, when the object is meant for use when it is unsafe to access the heap and must be allocated purely on the stack or into memory previously allocated by the user using the placement new syntax. An example of this would be C++ objects constructed at interrupt time in the classic Mac OS.
An important case where you cannot store a member by value is when the member is being used polymorphically (especially when the base class contains a pure virtual function - then the class declaration will fail to compile). You will usually want a pointer to the member's base class in this case, at times a reference.
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Copyright © 2000, 2001, 2002, 2005 Michael D. Crawford. All Rights Reserved.
