C++ pointer to class method

Question

I want to do something like this:

struct CLI_Command{
    CLI_Command(char* s, void (*h)(void)){
        command_string = s;
        handler = h;
    }

    char* command_string;
    void (*handler)(void);
};

class CLI  {
    public:
        CLI();

    private:
        CLI_Command cli_table[NO_CLI_COMMANDS] = {
                CLI_Command("Command1",   handler1),
                CLI_Command("Command2",   handler2)
        };

        void handler1(){};
        void handler2(){};
};

I know that I need something similar to CLI::*handler, but I can't get the syntax right. I keep running into errors like this:

"error: no matching function for call to 'CLI_Command::CLI_Command(const char [4], <unresolved overloaded function type>)"

The problem is that you are trying to take the address of a non-static member function, which is disallowed. I have [answered](http://stackoverflow.com/a/25953204/1392132) a similar question recently. Maybe it will help you. Note that modern C++ provides powerful tools to write your program much cleaner and safer. — 5gon12eder, Oct 10 '14 at 18:50
@5gon12eder: It's perfectly legal to take the address of a non-static member function, but the result is a pointer-to-member, not a function pointer. — Ben Voigt, Oct 10 '14 at 18:52
@5gon12eder OK, can you elaborate on these tools? Or point me in some direction? :) . I am making a command line interface through an UART on an embedded device. — Jolle, Oct 10 '14 at 18:56
@BenVoigt Yes, that's of course correct. So the disallowed thing is “taking the address of a non-static member function an use it as a function pointer”. — 5gon12eder, Oct 10 '14 at 18:57
@Jolle For example, you could make your table a `std::map` where `Command` could be an appropriate `typedef` or – if you want more OO and can live with the overhead, create a `CommandInterface` and put polymorphic command objects in your map that you invoke virtual functions on. — 5gon12eder, Oct 10 '14 at 19:00

Ben Voigt · Accepted Answer · 2014-10-10T19:14:08.843

This illustrates the correct syntax:

class CLI;

struct CLI_Command
{
    CLI_Command(char* s, void (CLI::*h)(void))
    {
        command_string = s;
        handler = h;
    }

    char* command_string;
    void (CLI::*handler)(void);

    void raise( CLI* the_cli ) { return (the_cli->*handler)(); }
};

class CLI
{
    public:
        CLI();

    private:
        static CLI_Command cli_table[NO_CLI_COMMANDS];

        void handler1(){};
        void handler2(){};
};

CLI::CLI_Command cli_table[NO_CLI_COMMANDS] = {
  { "Command1",   &CLI::handler1 },
  { "Command2",   &CLI::handler2 }
};

Names of member functions do not decay to pointer-to-member. You must use & explicitly, and a qualified name, when creating a pointer-to-member.

score 2 · Answer 2 · answered Oct 10 '14 at 19:00

2

In addition to other answers, another option is to use std::function together with std::bind():

struct CLI_Command{
    ...
    std::function<void> handler;
};

class CLI {
    ...
    CLI_Command cli_table[NO_CLI_COMMANDS] = {
      { "Command1",   std::bind(&CLI::handler1, this) },
      { "Command2",   std::bind(&CLI::handler2, this) }
    };

    void handler1(){};
    void handler2(){};
};

answered Oct 10 '14 at 19:00

Anton Savin

40,838
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Oh cool. Do you have an idea of how much overhead this will produce? My target is a little embedded device, so I can't afford to much overhead :) . – Jolle Oct 10 '14 at 19:07
@Jolle no problem, this is for completeness so the question might be helpful to other people. – Anton Savin Oct 10 '14 at 19:09
actually it was a question, because I have no idea if this will produce any overhead :) – Jolle Oct 10 '14 at 19:15
Heh :) Actually it may bring some overhead depending on implementation, but on other hand it brings more flexibility. – Anton Savin Oct 10 '14 at 19:42

score 1 · Answer 3 · answered Oct 10 '14 at 18:53

void handler1(){}
void handler2(){}

are member functions of CLI. The correct way to "address to" them is &CLI::handler1 and not handler1. However then, they won't be accepted by void (*h)(void), which would need to be changed to void (CLI::*h)(void). But that is probably not what you want.

Maybe consider reading about std::function for type erasure, or make your handler1/handler2 static.

score 1 · Answer 4 · answered Oct 10 '14 at 18:55

You should use the syntax for a pointer to class member instead of the syntax for a loose function pointer.

class CLI;

struct CLI_Command{
    CLI_Command(char* s, void (CLI::*h)(void)){
        command_string = s;
        handler = h;
    }

    char* command_string;
    void (CLI::*handler)(void);
};

In addition, make sure you call the function through the pointer of the current CLI class;

void CLI::process(char *cmd) {
    CLI_Command command* = /* lookup the command */
    this->(command->handle)();
}

You need `->*` (or `.*`) when dealing with pointer-to-member. — Ben Voigt, Oct 10 '14 at 18:55

score 0 · Answer 5 · edited May 23 '17 at 10:25

0

To get it working, make your methods static

static void handler1(){};
static void handler2(){};

Whatever consequences (read here please, for more detailed info) this will have :-( .

edited May 23 '17 at 10:25

Community

1
1

answered Oct 10 '14 at 18:48

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