C++ Coding Standards: 101 Rules, Guidelines, and Best Practices

89. Write function objects correctly

Summary

Be cheap, be adaptable: Design function objects to be values that are cheap to copy. Where possible, make them adaptable by inheriting from unary_- or binary_function.

Discussion

Function objects are modeled on function pointers. Like function pointers, the convention is to pass them by value. All of the standard algorithms pass objects by value, and your algorithms should too. For example:

template<class InputIter, class Func>Function for_each( InputIter first, InputIter last, Function f );

Therefore, function objects must be cheap to copy and monomorphic (immune to slicing, so avoid virtual functions; see Item 54). But large and/or polymorphic objects are useful, and using them is okay; just hide ...