0

I'm fairly new to C++ and trying to use an existing library ("WebServer").

I guess the language supports what I'm trying to do. I'm wrestling with getting the syntax right.

I'm struggling with correctly setting up a parameter which has type std::function<void(void)>.

In the snippets below, WebServer is the library class. I'm trying to use its "on" method, which requires a parameter of type THandlerFunction.

The compiler throws errors when I try to setup the THandlerFunction parameter and assign it to fhandleRootPageRequest.

Some similar-sounding problems suggested using std::mem_fn(). std::mem_fn looks promising. However, I'm getting some confusing compiler error messages back.

How do I correctly setup fhandleRootPageRequest, so I can call WebServer::on with its THandlerFunction parameter pointing to method WebServerInterface::handleRootPageRequest?

class WebServer {
public:

    typedef std::function<void(void)> THandlerFunction;
    void on(const Uri& uri, THandlerFunction fn);
    ...
};

...

class WebServerInterface {
public:
    WebServerInterface() {};

    void begin() {
        WebServer::THandlerFunction fhandleRootPageRequest = &WebServerInterface::handleRootPageRequest; // Does not compile
        webServerCPP.on("/", fhandleRootPageRequest);
    }

    void handleRootPageRequest() {
        webServerCPP.send(200, "text/html", "<!doctype html> <html> <head> <body> <div> <h1>Hello World</h1> </div> </body> </html>");

private:
    WebServer webServerCPP;

    }
};



// Examples of what I've tried. Variations on the fhandleRootPageRequest theme. Do not compile.
WebServer::THandlerFunction fhandleRootPageRequest = &WebServerInterface::handleRootPageRequest; 
    //  error: conversion from 'void (WebServerInterface::*)()' to non-scalar type 'WebServer::THandlerFunction' {aka 'std::function<void()>'} requested

WebServer::THandlerFunction fhandleRootPageRequest = std::mem_fn(&WebServerInterface::handleRootPageRequest);
    // error: conversion from 'std::_Mem_fn<void (WebServerInterface::*)()>' to non-scalar type 'WebServer::THandlerFunction' {aka 'std::function<void()>'} requested

WebServer::THandlerFunction fhandleRootPageRequest = (WebServer::THandlerFunction) std::mem_fn(&WebServerInterface::handleRootPageRequest);
    // error: no matching function for call to 'std::function<void()>::function(std::_Mem_fn<void (WebServerInterface::*)()>)'
james-h
  • 26
  • 1
  • Use a lambda function to call your member function. – john Aug 20 '23 at 17:15
  • Thanks John - I tweaked with: static void handleRootPageRequest()... which in turn required a similar tweak: static WebServer webServerCPP. File now compiles - useful progress. However, there's a new linker error: "undefined reference to `WebServerInterface::webServerCPP'" . Any hints on what this might mean? – james-h Aug 20 '23 at 17:18
  • ...and just after I posted this, I see your comment re. lambda function. I'll take a look into this next – james-h Aug 20 '23 at 17:19
  • I don't think static is the correct solution, You should use a lambda function as the duplicate suggests. – john Aug 20 '23 at 17:19
  • But if you want to use static then as with any global variable you need to define `webServerCPP`, at the moment you only have a declaration. – john Aug 20 '23 at 17:20

0 Answers0