Call to non-static member function without an object argument error

Question

Can anyone explain why I am getting this error?

I am working on an Interface class that gets keyboard input and checks to see whether it is correct through looping through an array of structs which contains strings to compare to and strings to output depending if it is equal to the compare string or not. If the input is correct, it will print the string within the struct and a function within the structure is called and does some action.

interface.hpp

#include <string>
class Input_Interface {
public:
    struct command_output {
    std::string command;
    std::string success_string;
    std::string failure_string;
    void output_function();
    }

    bool stop_loop = false;
    void Clear();
    void Quit_loop();

private:
    std::string input_str;
};

interface.cpp

#include <iostream>
void Input_Interface::Quit_loop() {
   stop_loop = true;
   // ends loop and closes program
}

void Input_Interface::clear() {
   // does some action
}

Input_Interface::command_output clear_output{"CLEAR", "CLEARED", "", Input_Interface::Clear()};
Input_Interface::command_output quit_output{"QUIT", "GOODBYE", "", Input_Interface::Quit_loop()};
Input_Interface::command_output output_arr[]{clear_output, quit_output};

void Input_Interface::begin() {
while (stop_loop == false) {
    Input_Interface::get_input(); //stores input into var called input_str shown later
    this->compare_input();
    }
}

void Input_Interface::compare_input() {
for (unsigned int i=0; i<2; i++) {
    if (this->input_str == output_arr[i].command) {
        std::cout << output_arr[i].success_string << std::endl;
        output_arr[i].output_function();
        break;
        }
    }
    // ... goes through else for failure printing invalid if it did not match any of the command string in the command_output struct array

My issue is with these lines

Input_Interface::command_output clear_output{"CLEAR", "CLEARED", "", Input_Interface::Clear()};
//error: call to non-static function without an object argument

Input_Interface::command_output quit_output{"QUIT", "GOODBYE", "", Input_Interface::Quit_loop()};
//error: call to non-static function without an object argument

I know this is passed through member functions of the class but I don't know how to go about fixing this problem. I'm not really sure if the problem is the scope resolution operator inside the struct object that is causing the error or not because I can use it outside of the struct just fine. Clear() and Quit() cannot be static because it must have access to the the objects in the class.

Answer 1

If I'm not mistaken, the output_function in the struct command_output is supposed to store a member function of class Input_Interface.

You'll need to change this:

struct command_output {
    std::string command;
    std::string success_string;
    std::string failure_string;
    void output_function();
}

Into this:

struct command_output {
    std::string command;
    std::string success_string;
    std::string failure_string;
    void (Input_Interface::*output_function)();
}

Then, change the way you initialize the instances from:

Input_Interface::command_output clear_output{"CLEAR", "CLEARED", "", Input_Interface::Clear()};
Input_Interface::command_output quit_output{"QUIT", "GOODBYE", "", Input_Interface::Quit_loop()};

To:

Input_Interface::command_output clear_output{"CLEAR", "CLEARED", "", &Input_Interface::Clear};
Input_Interface::command_output quit_output{"QUIT", "GOODBYE", "", &Input_Interface::Quit_loop};

And finally call them correctly, changing this:

output_arr[i].output_function();

Into this:

(this->*output_arr[i].output_function)();

---

The error was coming from the fact that you were writing Input_Interface::Clear() or Input_Interface::Quit_loop().

This notation implies two things: the parenthesis indicates that the functions are to be evaluated, and the result passed to the structure initializer, which is not possible since they return void and these functions are called as if they were static member functions, which is not the case.