STL source code analysis: functor

• Functor is a function object

• Function object:

  • Overloaded operator () class objects

  • Normal function and use the same, so called function object or functor

• For the parameters for the STL functor special handling, we define two special classes, class type definition there is only

• Unary functions class, unary_function

template <class Arg, class Result>
struct unary_function
{
    typedef Arg argument_type;
    typedef Result result_type;
};

• • Binary function class, binary_function

template <class Arg1, class Arg2, class Result>
struct binary_function
{
    typedef Arg1 first_argument_type;
    typedef Arg2 second_argument_type;
    typedef Result result_type;
}

• • example

template <class T>
struct plus : public binary_function<T, T, T>
{
    T operator()(const T& x, const T& y)
    {
        return x + y;
    }
}

• Card with

• After the value of permits anyone to use the same function, get his or her own

template <class T>
struct identity ：public : public unary_function<T, T>
{
    T& operator()(const T& x) const
    {
        return x;
    }
}

• Select function

• Receiving one pair, first or second return element

template <class Pair>
struct select1st : public unary_function<Pair, typedef pair::first_type>
{
    const typename Pair::first_type& operator()(const Pair& x) const
    {
        return x.first;
    }
}

template <class Pair>
struct select2nd : public unary_function<Pair, typedef pair::second_type>
{
    const typename Pair::second_type& operator()(const Pair& x) const
    {
        return x.second;
    }
};

• Projection function

  • Returns to the first or second parameter

template <class Arg1, class Arg2>
struct project1st : public binary_function<Arg1, Arg2, Arg1>
{
    Arg1 operator()(const Arg1& x, const Arg2&) const
    {
        return x;
    }
};

template <class Arg1, class Arg2>
struct project2nd : public binary_function<Arg1, Arg2, Arg2>
{
    Arg2 operator()(const Arg1&, const Arg2& y) const
    {
        return y;
    }
};