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 (Click here for a personal note from Marshall Cline.) |
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Section 31:
| 31.1 | What is value and/or reference semantics, and which is best in C++? |
| 31.2 | What is "virtual data," and how-can / why-would I use it in C++? |
| 31.3 | What's the difference between virtual data and dynamic data? |
| 31.4 | Should I normally use pointers to freestore allocated objects for my data members, or should I use "composition"? |
| 31.5 | What are relative costs of the 3 performance hits associated with allocating member objects from the freestore? |
| 31.6 | Are "inline virtual" member functions ever actually "inlined"? |
| 31.7 | Sounds like I should never use reference semantics, right? |
| 31.8 | Does the poor performance of reference semantics mean I should pass-by-value? |
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[31.4] Should I normally use pointers to freestore allocated objects for my data members, or should I use "composition"?
Composition.
Your member objects should normally be "contained" in the composite object (but not always; "wrapper" objects are a good example of where you want a pointer/reference; also the N-to-1-uses-a relationship needs something like a pointer/reference).
There are three reasons why fully contained member objects ("composition") has better performance than pointers to freestore-allocated member objects:
- Extra layer of indirection every time you need to access the member object
- Extra freestore allocations (new in constructor, delete in destructor)
- Extra dynamic binding (reason given below)