How can i pass a function as a function parameter in another function?

Question

I am trying to use an Arduino library and to use one of it's functions as a parameter in my own function, but I don't know how can I do that. I tried the code below but I get an error.

Any help will be appreciated.

P.S: I do not have an option to use auto keyword.

using namespace httpsserver;
HTTPServer Http;
typedef void (*Register)(HTTPNode*); // My typedef
Register Node = Http.registerNode;

When I am trying to call Node (...), I get the error below.

Cannot convert 'httpsserver::ResourceResolver::registerNode'
from type 'void (httpsserver::ResourceResolver::)(httpsserver::HTTPNode*)' 
to type 'Register {aka void (*)(httpsserver::HTTPNode*)}'

How can I create a function pointer for the type :

'void (httpsserver::ResourceResolver::)(httpsserver::HTTPNode*)'

I want to use it as a parameter in another function:

// My Declaration
void Get(void(*Register)(httpsserver::HTTPNode*), const std::string& path);

// Usage
Get (Http.registerNode(...), ""); // Like so

How can I do that?

In C++, there is a difference between a pointer to a function and a pointer to a memberfunction. They are incompatible, but there may be workarounds. These keywords should get you started though. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented May 15, 2021 at 18:15

Yakk - Adam Nevraumont · Accepted Answer · 2021-05-15 18:17:55Z

1

A member function pointer is not a function pointer.

typedef void (httpsserver::*Register)(HTTPNode*); // My typedef
Register Node = &httpsserver::registerNode;

usage:

void Get(void(httpsserver::*Register)(httpsserver::HTTPNode*), const std::string& path);
Get (&httpsserver::registerNode, "");

you have to pass the httpsserver::HTTPNode* into Register within Get.

If you want to bind the arguments to the function object and call it later, you want std::function<void()>:

void Get(std::function<void()>, const std::string& path);
Get ([&]{ Http.registerNode(...); }, "");

note, however, that this makes lifetime of the objects refered to within the {} above quite dangerous.

answered May 15, 2021 at 18:17

Yakk - Adam Nevraumont

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1 Comment

swartkatt Over a year ago

typedef void (httpsserver::ResourceResolver::*Register)(httpsserver::HTTPNode*); Worked as expected. Thank you very much!

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