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STL Pocket Reference by Ray Lischner

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Function Objects

The <functional> header defines several function objects or functors. A function object is an object that has an operator( ), so it can be called using the same syntax as a function.

The standard function objects are defined for C++ operators; for binding function arguments; and for adapting functions, member functions, etc., as function objects.

Using Functors

Functors are most often used with the standard algorithms. For example, to copy a sequence of numbers, adding a fixed amount (42) to each value, you could use the following expression:

std::transform(src.begin(), src.end(), dst.begin( ),
               std::bind2nd(std::plus<int>( ), 42))

The result of combining bind2nd and plus<int> is a function object that adds the value 42 when it is applied to any integer. The transform algorithm copies all the elements from src to dst, applying the functional argument to each element. For details, see the description of transform in the earlier section, Algorithms, and bind2nd and plus in this section.

The next few examples will use the Employee class, shown in Example 1-2.

Example 1-2. Employee class

class Employee {
public:
  Employee(const std::string& name)
    : name_(name), sales_(0)     {}
  int         sales( )      const { return sales_; }
  std::string name( )       const { return name_; }
  void        make_sale(int n)   { sales_ += n; }
private:
  std::string name_;
  int sales_;
};

Suppose you have a vector of Employee objects and you want to find out which employees meet or exceed a sales target. No existing functor lets you ...

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