Miscellaneous Notes

Boost.Function vs. Function Pointers
Performance
Combatting virtual function "bloat"
Acknowledgements

Boost.Function vs. Function Pointers

Boost.Function has several advantages over function pointers, namely:

Boost.Function allows arbitrary compatible function objects to be targets (instead of requiring an exact function signature).
Boost.Function may be used with argument-binding and other function object construction libraries.
Boost.Function has predictible behavior when an empty function object is called.

And, of course, function pointers have several advantages over Boost.Function:

Function pointers are smaller (the size of one pointer instead of three)
Function pointers are faster (Boost.Function may require two calls through function pointers)
Function pointers are backward-compatible with C libraries.
More readable error messages.

Performance

Function object wrapper size
Copying efficiency
Invocation efficiency

Function object wrapper size

Function object wrappers will be the size of two function pointers plus one function pointer or data pointer (whichever is larger). On common 32-bit platforms, this amounts to 12 bytes per wrapper. Additionally, the function object target will be allocated on the heap.

Copying efficiency

Copying function object wrappers may require allocating memory for a copy of the function object target. The default allocator may be replaced with a faster custom allocator or one may choose to allow the function object wrappers to only store function object targets by reference (using ref) if the cost of this cloning becomes prohibitive.

Invocation efficiency

With a properly inlining compiler, an invocation of a function object requires one call through a function pointer. If the call is to a free function pointer, an additional call must be made to that function pointer (unless the compiler has very powerful interprocedural analysis).

Combatting virtual function "bloat"

The use of virtual functions tends to cause 'code bloat' on many compilers. When a class contains a virtual function, it is necessary to emit an additional function that classifies the type of the object. It has been our experience that these auxiliary functions increase the size of the executable significantly when many boost::function objects are used.

In Boost.Function, an alternative but equivalent approach was taken using free functions instead of virtual functions. The Boost.Function object essentially holds two pointers to make a valid target call: a void pointer to the function object it contains and a void pointer to an "invoker" that can call the function object, given the function pointer. This invoker function performs the argument and return value conversions Boost.Function provides. A third pointer points to a free function called the "manager", which handles the cloning and destruction of function objects. The scheme is typesafe because the only functions that actually handle the function object, the invoker and the manager, are instantiated given the type of the function object, so they can safely cast the incoming void pointer (the function object pointer) to the appropriate type.

Acknowledgements

Many people were involved in the construction of this library. William Kempf, Jesse Jones and Karl Nelson were all extremely helpful in isolating an interface and scope for the library. John Maddock managed the formal review, and many reviewers gave excellent comments on interface, implementation, and documentation. Peter Dimov led us to the function declarator-based syntax.