CODEWITHSUNDEEP
c++function-pointers
Jérémy Pouyet
Jérémy Pouyet

Reputation: 2019

Function pointer on object received by parameter

I have a function which receives an object instance and a function pointer. How can I execute my function pointer on my object instance?

I have tried:

void    Myclass::my_fn(Object obj, bool (*fn)(std::string str))
{
   obj.fn("test"); // and (obj.fn("test"))
}

But nothing works. It tells me "unused parameters my_fn and fn"

Upvotes: 0

Views: 78

Answers (2)

Some programmer dude
Some programmer dude

Reputation: 409482

It's because fn is not a member function pointer of Object. For that you have to do e.g.

bool (Object::*fn)(std::string)

Then to call it you have to do

(obj.*fn)(...);

However, if your compiler and library can support C++11, I suggest you to use std::function and std::bind:

void MyclasS::my_fn(std::function<bool(std::string)> fn)
{
    fn("string");
}

Then call the my_fn function as

using namespace std::placeholders;  // For `_1` below

Object obj;
myclassobject.my_fn(std::bind(&Object::aFunction, &obj, _1));

With std::function you can also use other function pointers or lambdas:

myclassobject.my_fn([](std::string s){ std::cout << s << '\n'; return true; });

Upvotes: 4

tumdum
tumdum

Reputation: 2031

Function pointers are something completely different form member function pointers (pointers to methods).

In case of function pointers, you don't need any object to call them:

void    Myclass::my_fn(bool (*fn)(std::string str))
{
   fn("test");
}

In case of member function pointers correct syntax would be:

void    Myclass::my_fn(Object obj, bool (Object::*fn)(std::string str))
{
   obj.*fn("test");
}

Upvotes: 0

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