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Thinking in C++ Vol 2 - Practical Programming
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Classification of function objects

The Standard C++ library classifies a function object based on the number of arguments its operator( ) takes and the kind of value it returns. This classification is based on whether a function object s operator( ) takes zero, one, or two arguments:

Generator: A type of function object that takes no arguments and returns a value of an arbitrary type. A random number generator is an example of a generator. The standard library provides one generator, the function rand( ) declared in <cstdlib>, and has some algorithms, such as generate_n( ), which apply generators to a sequence.

Unary Function: A type of function object that takes a single argument of any type and returns a value that may be of a different type (which may be void).

Binary Function: A type of function object that takes two arguments of any two (possibly distinct) types and returns a value of any type (including void).

Unary Predicate: A Unary Function that returns a bool.

Binary Predicate: A Binary Function that returns a bool.

Strict Weak Ordering: A binary predicate that allows for a more general interpretation of equality. Some of the standard containers consider two elements equivalent if neither is less than the other (using operator<( )). This is important when comparing floating-point values, and objects of other types where operator==( ) is unreliable or unavailable. This notion also applies if you want to sort a sequence of data records (structs) on a subset of the struct s fields. That comparison scheme is considered a strict weak ordering because two records with equal keys are not really equal as total objects, but they are equal as far as the comparison you re using is concerned. The importance of this concept will become clearer in the next chapter.

In addition, certain algorithms make assumptions about the operations available for the types of objects they process. We will use the following terms to indicate these assumptions:

LessThanComparable: A class that has a less-than operator<.

Assignable: A class that has a copy-assignment operator= for its own type.

EqualityComparable: A class that has an equivalence operator== for its own type.

We will use these terms later in this chapter to describe the generic algorithms in the standard library.

Thinking in C++ Vol 2 - Practical Programming
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire