Passing child class objects to std::function parameter












0















I have the following message class psuedo-code (third-party, cannot be changed).



messages.h:



class AbstractMessage {
// pure virtual
};

class MessageA : public AbstractMessage {
// implements everything
const static std::string messageName = "MessageA";
};

class MessageB : public AbstractMessage {
// implements everything
const static std::string messageName = "MessageB";
};


In my application code, I am attempting to write a function to register an external function to handle any message:



application.cpp:



bool register_handler(std::string messageType, std::function<void(AbstractMessage&)>messageHandler) {
// add the string and function to a map to be called
}


However, I need to be able to pass in MessageA or MessageB functions. For example:



main.cpp:



void processMessageA(MessageA &msg) {
// do something
}

Application myApp;


This becomes problematic when I attempt to do the following:



std::function<void(MessageA&)> f(processMessageA);
myApp.register_handler("MessageA", f);


MessageA should be a valid AbstractMessage via inheritance, but I get the following error:



'Error C2664: 'bool Application::register_handler(std::string,const std::function<void (AbstractMessage &)> &)': cannot convert argument 2 from 'std::function<void(MessageA &)>' to 'const std::function<void (AbstractMessage &)> &



Is this possible in C++? In Java I would use a functional interface, and pass any valid method via this::. Inheritance would validate the method parameters.










share|improve this question























  • Let's say you manage to cast your void(MessageA&) to a void(AbstractMessage&). What happens when you pass in a MessageB to that function?

    – super
    Nov 26 '18 at 23:23











  • @super I‘m following. What would be the alternative or best practice for generic message handler mapping? I tried using a template, but had issues using it as a map key to determine which handler to call.

    – Alan
    Nov 26 '18 at 23:37











  • A better design would not need to know the exact type, the interface (AbstractMessage&) should be enough. But if you third party code is preventing that, casting is your only option AFAIK.

    – super
    Nov 26 '18 at 23:46











  • @super Now I’m not following, my current scenario is I have the interface AbstractMessage&.

    – Alan
    Nov 27 '18 at 0:10
















0















I have the following message class psuedo-code (third-party, cannot be changed).



messages.h:



class AbstractMessage {
// pure virtual
};

class MessageA : public AbstractMessage {
// implements everything
const static std::string messageName = "MessageA";
};

class MessageB : public AbstractMessage {
// implements everything
const static std::string messageName = "MessageB";
};


In my application code, I am attempting to write a function to register an external function to handle any message:



application.cpp:



bool register_handler(std::string messageType, std::function<void(AbstractMessage&)>messageHandler) {
// add the string and function to a map to be called
}


However, I need to be able to pass in MessageA or MessageB functions. For example:



main.cpp:



void processMessageA(MessageA &msg) {
// do something
}

Application myApp;


This becomes problematic when I attempt to do the following:



std::function<void(MessageA&)> f(processMessageA);
myApp.register_handler("MessageA", f);


MessageA should be a valid AbstractMessage via inheritance, but I get the following error:



'Error C2664: 'bool Application::register_handler(std::string,const std::function<void (AbstractMessage &)> &)': cannot convert argument 2 from 'std::function<void(MessageA &)>' to 'const std::function<void (AbstractMessage &)> &



Is this possible in C++? In Java I would use a functional interface, and pass any valid method via this::. Inheritance would validate the method parameters.










share|improve this question























  • Let's say you manage to cast your void(MessageA&) to a void(AbstractMessage&). What happens when you pass in a MessageB to that function?

    – super
    Nov 26 '18 at 23:23











  • @super I‘m following. What would be the alternative or best practice for generic message handler mapping? I tried using a template, but had issues using it as a map key to determine which handler to call.

    – Alan
    Nov 26 '18 at 23:37











  • A better design would not need to know the exact type, the interface (AbstractMessage&) should be enough. But if you third party code is preventing that, casting is your only option AFAIK.

    – super
    Nov 26 '18 at 23:46











  • @super Now I’m not following, my current scenario is I have the interface AbstractMessage&.

    – Alan
    Nov 27 '18 at 0:10














0












0








0








I have the following message class psuedo-code (third-party, cannot be changed).



messages.h:



class AbstractMessage {
// pure virtual
};

class MessageA : public AbstractMessage {
// implements everything
const static std::string messageName = "MessageA";
};

class MessageB : public AbstractMessage {
// implements everything
const static std::string messageName = "MessageB";
};


In my application code, I am attempting to write a function to register an external function to handle any message:



application.cpp:



bool register_handler(std::string messageType, std::function<void(AbstractMessage&)>messageHandler) {
// add the string and function to a map to be called
}


However, I need to be able to pass in MessageA or MessageB functions. For example:



main.cpp:



void processMessageA(MessageA &msg) {
// do something
}

Application myApp;


This becomes problematic when I attempt to do the following:



std::function<void(MessageA&)> f(processMessageA);
myApp.register_handler("MessageA", f);


MessageA should be a valid AbstractMessage via inheritance, but I get the following error:



'Error C2664: 'bool Application::register_handler(std::string,const std::function<void (AbstractMessage &)> &)': cannot convert argument 2 from 'std::function<void(MessageA &)>' to 'const std::function<void (AbstractMessage &)> &



Is this possible in C++? In Java I would use a functional interface, and pass any valid method via this::. Inheritance would validate the method parameters.










share|improve this question














I have the following message class psuedo-code (third-party, cannot be changed).



messages.h:



class AbstractMessage {
// pure virtual
};

class MessageA : public AbstractMessage {
// implements everything
const static std::string messageName = "MessageA";
};

class MessageB : public AbstractMessage {
// implements everything
const static std::string messageName = "MessageB";
};


In my application code, I am attempting to write a function to register an external function to handle any message:



application.cpp:



bool register_handler(std::string messageType, std::function<void(AbstractMessage&)>messageHandler) {
// add the string and function to a map to be called
}


However, I need to be able to pass in MessageA or MessageB functions. For example:



main.cpp:



void processMessageA(MessageA &msg) {
// do something
}

Application myApp;


This becomes problematic when I attempt to do the following:



std::function<void(MessageA&)> f(processMessageA);
myApp.register_handler("MessageA", f);


MessageA should be a valid AbstractMessage via inheritance, but I get the following error:



'Error C2664: 'bool Application::register_handler(std::string,const std::function<void (AbstractMessage &)> &)': cannot convert argument 2 from 'std::function<void(MessageA &)>' to 'const std::function<void (AbstractMessage &)> &



Is this possible in C++? In Java I would use a functional interface, and pass any valid method via this::. Inheritance would validate the method parameters.







c++ function inheritance stl






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked Nov 26 '18 at 22:39









AlanAlan

269520




269520













  • Let's say you manage to cast your void(MessageA&) to a void(AbstractMessage&). What happens when you pass in a MessageB to that function?

    – super
    Nov 26 '18 at 23:23











  • @super I‘m following. What would be the alternative or best practice for generic message handler mapping? I tried using a template, but had issues using it as a map key to determine which handler to call.

    – Alan
    Nov 26 '18 at 23:37











  • A better design would not need to know the exact type, the interface (AbstractMessage&) should be enough. But if you third party code is preventing that, casting is your only option AFAIK.

    – super
    Nov 26 '18 at 23:46











  • @super Now I’m not following, my current scenario is I have the interface AbstractMessage&.

    – Alan
    Nov 27 '18 at 0:10



















  • Let's say you manage to cast your void(MessageA&) to a void(AbstractMessage&). What happens when you pass in a MessageB to that function?

    – super
    Nov 26 '18 at 23:23











  • @super I‘m following. What would be the alternative or best practice for generic message handler mapping? I tried using a template, but had issues using it as a map key to determine which handler to call.

    – Alan
    Nov 26 '18 at 23:37











  • A better design would not need to know the exact type, the interface (AbstractMessage&) should be enough. But if you third party code is preventing that, casting is your only option AFAIK.

    – super
    Nov 26 '18 at 23:46











  • @super Now I’m not following, my current scenario is I have the interface AbstractMessage&.

    – Alan
    Nov 27 '18 at 0:10

















Let's say you manage to cast your void(MessageA&) to a void(AbstractMessage&). What happens when you pass in a MessageB to that function?

– super
Nov 26 '18 at 23:23





Let's say you manage to cast your void(MessageA&) to a void(AbstractMessage&). What happens when you pass in a MessageB to that function?

– super
Nov 26 '18 at 23:23













@super I‘m following. What would be the alternative or best practice for generic message handler mapping? I tried using a template, but had issues using it as a map key to determine which handler to call.

– Alan
Nov 26 '18 at 23:37





@super I‘m following. What would be the alternative or best practice for generic message handler mapping? I tried using a template, but had issues using it as a map key to determine which handler to call.

– Alan
Nov 26 '18 at 23:37













A better design would not need to know the exact type, the interface (AbstractMessage&) should be enough. But if you third party code is preventing that, casting is your only option AFAIK.

– super
Nov 26 '18 at 23:46





A better design would not need to know the exact type, the interface (AbstractMessage&) should be enough. But if you third party code is preventing that, casting is your only option AFAIK.

– super
Nov 26 '18 at 23:46













@super Now I’m not following, my current scenario is I have the interface AbstractMessage&.

– Alan
Nov 27 '18 at 0:10





@super Now I’m not following, my current scenario is I have the interface AbstractMessage&.

– Alan
Nov 27 '18 at 0:10












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