What are relative costs of the 3 performance hits associated with allocating member objects from the freestore?, C++ FAQ

C++ FAQ / Section 31 / FAQ 31.5

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?

[31.5] What are relative costs of the 3 performance hits associated with allocating member objects from the freestore?

The three performance hits are enumerated in the previous FAQ:

By itself, an extra layer of indirection is small potatoes
Freestore allocations can be a performance issue (the performance of the typical implementation of malloc() degrades when there are many allocations; OO software can easily become "freestore bound" unless you're careful)
The extra dynamic binding comes from having a pointer rather than an object. Whenever the C++ compiler can know an object's exact class, virtual function calls can be statically bound, which allows inlining. Inlining allows zillions (would you believe half a dozen :-) optimization opportunities such as procedural integration, register lifetime issues, etc. The C++ compiler can know an object's exact class in three circumstances: local variables, global/static variables, and fully-contained member objects

Thus fully-contained member objects allow significant optimizations that wouldn't be possible under the "member objects-by-pointer" approach. This is the main reason that languages which enforce reference-semantics have "inherent" performance challenges.

Note: Please read the next three FAQs to get a balanced perspective!