Can I use a lambda function or std::function object in place of a function pointer?

Question

I\'ve got a library that I need to use that defines the following:

typedef void CallbackFunction(const int& i);

and has a function to regis

Answer 1

You can't use lambdas, because your registerCallback function expects pointer to non-member function. The only way to pass additional parameter to free standing function is 1) using global (static) data or 2) making it a template taking compile time constants, e.g.

template<int data> void callback(const int& i)
{ /* can use data */ }

registerCallback(callback<10>);

Answer 2

In general, no you can't use a lambda or function object where a function pointer is required. Function objects are full-fledged objects that have overloaded the application operator (()) so you can use them as functions syntactically.

The standard algorithms are written as templates, which allows you to pass either a function address (pointer) or a function object to them, because they use the same syntax. When you pass a function object, a template is instantiated which accepts that function object; when you pass a function pointer, a different template is instantiated.

So to get lambdas/function objects to work with your code, you would need to modify the library you're using.

Answer 3

Short Answer: No

Long Answer:

Lamda's are just syntactic sugar for functors.
Functors are objects that act like functions.

A function pointer is not the same type of thing. In general you can not use functors where you need a function pointer.

Extra:

On the other hand it is easy to wrap a function to become a functor.

Historically:

Function pointers are generally used by C libraries. As C++ libraries will use an interface to achieve the same affect. Thus you can see why it is hard to pass functors where a function pointer is requried (the C code has no way to understand how to use the functor).

Answer 4

Since you capture nothing, you are supposed to be able to do what you're trying to do, namely assign a lambda expression as a function pointer (though your syntax is wrong there).

Since you're using VS2010 though, you won't be able to. The feature of lambda that you're trying to use did not come to be until after VS2010 was released: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3043.html

So while yes, it's supposed to work it in fact does not.

Of course, while you are not, in fact, capturing anything you do state that you want to. Lambdas that capture data cannot be converted to function pointers even in the post VS2010 state of the C++ draft standard.