Calling a pointer-to-member function from a struct array

Question

I'm running into what seems like a syntax problem. Looking at the other StackOverflow answer doesn't give me an answer applicable to my problem. At least not one which I can understand.

My scheduler class:

#define MAX_TASKS 10

typedef struct taskProps {
    int interval;
    int elapsedTime;
    int (Controller::*taskFunction)(void);
} taskProps;

class TaskScheduler {
public:
    TaskScheduler();
    int setUpdateInterval(int interval);
    int addTask(int interval, int (Controller::*taskFunction)(void));
    int startTimer();
    void clearTasks();
    int checkTasks();

private:
    int numberOfTasks;
    int updateInterval;
    taskProps scheduledTasks[MAX_TASKS];
};

This all compiles fine, but the problem lays within call the member function pointer in this function:

int TaskScheduler::checkTasks(){
    int tasksExecuted = 0;

    for(int i = 0; i < numberOfTasks; i++){
        if(scheduledTasks[i].elapsedTime >= scheduledTasks[i].interval){
            scheduledTasks[i].taskFunction;
            scheduledTasks[i].elapsedTime = 0;
            tasksExecuted++;
        }

        scheduledTasks[i].elapsedTime += updateInterval;
    }

    return tasksExecuted;
}

Compiling this gives me the error;

../Core/Src/TaskScheduler.cpp:88:22: warning: statement has no effect [-Wunused-value]

Other tries:

scheduledTasks[i].*taskFunction;
../Core/Src/TaskScheduler.cpp:88:23: error: 'taskFunction' was not declared in this scope


scheduledTasks[i].taskFunction();
../Core/Src/TaskScheduler.cpp:88:35: error: must use '.*' or '->*' to call pointer-to-member function in '((TaskScheduler*)this)->TaskScheduler::scheduledTasks[i].taskProps::taskFunction (...)', e.g. '(... ->* ((TaskScheduler*)this)->TaskScheduler::scheduledTasks[i].taskProps::taskFunction) (...)'

Anyone able to help me out and explain what piece of knowledge I'm missing here?

This gives the same error: ../Core/Src/TaskScheduler.cpp:88:24: error: 'taskFunction' was not declared in this scope. That's why I posted this... I hate it got deleted — MrEmper
– MrEmper, Commented Mar 25, 2021 at 13:57
I've reopend the question as your problem is a little more complex then the dupe takes care of. I'll be adding an answer in a moment. — NathanOliver
– NathanOliver, Commented Mar 25, 2021 at 14:13
i can understand your frustration, but note that a question getting closed is something temporarily, there are mechanics in place to reopen the question after a review, or it can be reopened by voting. Deleting a question is different — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Mar 25, 2021 at 14:20
Yes, I didn't know that hence why I opened a new one. Learned something today! — MrEmper
– MrEmper, Commented Mar 25, 2021 at 14:30

Remy Lebeau · Accepted Answer · 2021-03-25 16:33:08Z

3

When you want to call a member function pointer the syntax you use is

(object_of_type_mem_func_pointer_points_to.*function_pointer)(parameters)

or

(pointer_to_object_of_type_mem_func_pointer_points_to->*function_pointer)(parameters)

Unfortunately, (scheduledTasks[i].*taskFunction)() isn't going to work, as taskFunction requires a Controller object to call the taskFunction on. That needs code more like this:

(controller_object.*(scheduledTasks[i].taskFunction))()

or

(pointer_to_controller_object->*(scheduledTasks[i].taskFunction))()

edited Mar 25, 2021 at 16:33

Remy Lebeau

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answered Mar 25, 2021 at 14:20

NathanOliver

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2 Comments

MrEmper Over a year ago

Thanks for reconsidering my question, this fixed my problem AND made me understand the problem! Very clear explanation.

NathanOliver Over a year ago

@MrEmper No problem. Sorry for the early close but we got you there :)

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