Solved: C++ Member Function to Function Pointer Argument

[Octacone]

Right now I am working on my shell implementation, and I developed an easy system for adding new commands.

Basically I have a command structure that contains some information about that specific command, including a function pointer:

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void (*function_pointer)();

Also I have a function that does all the "adding":

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void Command_List_Add_Command(char* name, char* description, void (*function_pointer)());
//Assignment 
command_list[command_count].function_pointer = function_pointer;

There is a function that contains all the add calls. //This is the problem

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Command_List_Add_Command("help", "Displays all available commands.", Commands.Help);

So, I keep getting an error saying: "error: invalid use of non-static member function", even if I make it static.
Side note: all the commands are located in a separate C++ file. "Commands" is an instance/object of "Commands_Class" class.

I tried everything I remembered of, nothing seemed to be okay. I even trying declaring an external "C" virtual function, that did compile but I was getting opcode exceptions, general protection fault exceptions, etc...

I would appreciate any help.

[eryjus]

There's a lot of details missing here, but I'm going to bet you are either using the class name to qualify the method call rather than the instance name or calling the function from within a static method. In short, the error is telling you that there is no instance to operate on.

[Octacone]

There's a lot of details missing here, but I'm going to bet you are either using the class name to qualify the method call rather than the instance name or calling the function from within a static method. In short, the error is telling you that there is no instance to operate on.

It is neither of those. I tried almost everything. Static and non-static, with the instance and without. I actually don't even know if this is possible. What details am I missing? Only thing I didn't include was the commands file.

[eryjus]

There's a lot of details missing here

What details am I missing?

How about the Commands_Class class definition and the function responsible for calling the member in question? And anything else that will help us. We do not have the benefit of seeing your code in context and what you have posted is not enough to determine the problem.

[Octacone]

There's a lot of details missing here

What details am I missing?

How about the Commands_Class class definition and the function responsible for calling the member in question? And anything else that will help us. We do not have the benefit of seeing your code in context and what you have posted is not enough to determine the problem.

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#ifndef COMMANDS_H
#define COMMANDS_H
...includes...snip...

namespace Basic_OS
{
	class Commands_Class
	{
		public:
			void Help();
	};
}

#endif

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#include "Headers/Kernel/Commands.h"

using namespace Basic_OS;

extern Text_User_Interface TUI;
extern Shell_Class Shell;

void Commands_Class::Help() //tried defining it as static but didn't help :|
{
	//Irrelevant code
}

the function responsible for calling the member in question?

I guess you wanted this part?

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 void (*function)();
function = command_list[i].function_pointer;
function();

[max]

Your "Help" method is not static and you must use the :: operator for static methods of a class. You can not pass a pointer to the method of a class instance. That would be a lambda.

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class Commands {
public:
	static void Help() {
		printf("HELP\n");
	}
};


void callThatStaticMethod(void(*method)()) {
	method();
}

int main() {
	callThatStaticMethod(Commands::Help);
}

[Octacone]

Your "Help" method is not static and you must use the :: operator for static methods of a class. You can not pass a pointer to the method of a class instance. That would be a lambda.

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class Commands {
public:
	static void Help() {
		printf("HELP\n");
	}
};


void callThatStaticMethod(void(*method)()) {
	method();
}

int main() {
	callThatStaticMethod(Commands::Help);
}

Thank you so so much for helping me!!!
I figured it out, it was such a shameful mistake. Instead of declaring functions as static inside the header I was doing it inside the source file. I thought it was the other way around this entire time. I don't use static functions quite often.

[dozniak]

If the functions inside Commands class are all static there's no point in keeping it a class, just make it a namespace.

There are things like mem_fn if you want to address non-static members of a class.

Function template std::mem_fn generates wrapper objects for pointers to members, which can store, copy, and invoke a pointer to member. Both references and pointers (including smart pointers) to an object can be used when invoking a std::mem_fn.