CODEWITHSUNDEEP
c++callbackfunctional-programming
Offirmo
Offirmo

Reputation: 19860

What is the interest of <functional> function objects in C++03?

I'm writing a wrapper around an http server in C++. My compiler only support C++03 (gcc 4.1.2) and I can't use boost.

I wanted to implement a generalized callback mechanism to answer the requests, able to register either a function or an object method or a static object method.

A (too) quick glance on <functional> function objects (C++03, http://www.cplusplus.com/reference/functional/) make me think that it was the answer.

However, it seems that <functional> function objects are not meant to provide a generalized callback mechanism.

So I wonder : what is the use of <functional> function objects in C++03 ? What are they meant for ? What true benefits are they supposed to provide over simple functions pointers ? Or is the C++03 version flawed and only the C++11 version is actually useful ?

[edit] For what I had understood at first, it seemed to me that a C++03 function object was just a useless wrapping over a function pointer. I'd rather use the function pointer directly, then. Correcting this wrong analysis is the point of this question !

Upvotes: 3

Views: 899

Answers (1)

Benjamin Lindley
Benjamin Lindley

Reputation: 103741

I don't really understand your question, <functional> is a header, not a mechanism. The header provides utilities for making function objects conveniently. As a simple example, let's say you wanted to call transform, multiplying the elements of one range by another, and storing the result in a third. You could define a function like this:

double mult(double lhs, double rhs) { return lhs * rhs; };

Then call it like this:

std::transform(lhs.begin(), lhs.end(), rhs.begin(), out.begin(), mult);

Or you could use std::multiplies, which is already defined for you:

std::transform(lhs.begin(), lhs.end(), rhs.begin(), out.begin(), std::multiplies<double>());

There are many other examples. You just need to be creative. C++11 lambdas have made many of the facilities in <functional> obsolete, or at least a lot less useful, because you can just define your functions inline with your algorithm calls.

I wanted to implement a generalized callback mechanism to answer the request

The C++03 version of <functional> does not help you with this. The C++11 version does, with std::function, adapted from boost::function.

Upvotes: 2

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