How to define a lambda function to be able to capture the 'this' pointer of a class?

Question

I have a time class which tracks the time.

I want my other classes to be able to register a callback function if the day changes.

I have this working snippet:

#define MAX_CB_CHANGE_FUNCTIONS 50
typedef void (*dayChangeFunc)(int prevDay,int nowDay);

class TimeSystem {
    private:
        // array for the function pointers.
        dayChangeFunc dayChangeFunctions[MAX_CB_CHANGE_FUNCTIONS];
        // emit function if the day changes.
        void emitDayChange(int prev, int now);
    public:
        void regDayChange(dayChangeFunc);
};


/*
*   Registering a day change function.
*   Maximum function pointer count is MAX_CB_CHANGE_FUNCTIONS ( see timeSys.h )
*/
void TimeSystem::regDayChange(void (*dayChangeFunc)(int prevDay,int nowDay)){
    if( currDayCbIndex >= MAX_CB_CHANGE_FUNCTIONS ){
        if(isDebugOn){
            debug.print(DEBUG_WARN,"[Time_sys] - Can not register an other day change function.\n");
            return;
        }
    }
    dayChangeFunctions[currDayCbIndex] = dayChangeFunc;
    currDayCbIndex++;
}

/*
*   Emitting day change to every registered function.
*/
void TimeSystem::emitDayChange(int prev, int now){
    // Preventing change on initialization.
    if( prev == 0 ){ return; }
    for (size_t i = 0; i < MAX_CB_CHANGE_FUNCTIONS; i++){
        if(dayChangeFunctions[i]){
            dayChangeFunctions[i](prev,now);
        }
    }
}

This works and other calsses can use this like this:

timeSys.regDayChange([](int prevDay,int nowDay){
    Serial.printf("Day changed from prev: %d to now: %d\n",prevDay,nowDay);
});

Now i want to capture the 'this' pointer in the lambda to be able to call some internal function of a class.

timeSys.regDayChange([this](int prevDay,int nowDay){
    callSomeInternalFunction();
});

Error i get:

no suitable conversion function from "lambda [](int prevDay, int nowDay)->void" to "dayChangeFunc"

How could i define my lambda to be able to capture the 'this' pointer?

******** EDIT: SOLVED ********

Here are the modifications:

I had to define my lambda function type like this:

using dayChangeFunc = std::function<void(int, int)>;

Define the function pointer register function like this:

void regDayChange(dayChangeFunc cb);

And rewrite the declaration like this:

/*
*   Registering a day change function.
*   Maximum function pointer count is MAX_CB_CHANGE_FUNCTIONS ( see timeSys.h )
*/
void TimeSystem::regDayChange( dayChangeFunc cb ){
    if( currDayCbIndex >= MAX_CB_CHANGE_FUNCTIONS ){
        if(isDebugOn){
            debug.print(DEBUG_WARN,"[Time_sys] - Can not register an other day change function.\n");
            return;
        }
    }
    dayChangeFunctions[currDayCbIndex] = cb;
    currDayCbIndex++;
}

And now i can use it in any class like this:

class SampleClass(){
  private:
     int exampleCounter = 0;
  public:
    void init(){
      TimeSystem timeSys;
      timeSys.regDayChange([this](int prevDay,int nowDay){
        Serial.printf("Day changed from prev: %d to now: %d\n",prevDay,nowDay);
        exampleCounter++;
      });
    }
}

I got the following error: declaration does not declare anything [-fpermissive] — Dr.Random, Oct 26 '22 at 10:51
You forgot to name the type. `using dayChangeFunc = std::function;`, or `typedef std::function dayChangeFunc;`. (Hardly anyone uses `typedef` these days.) — molbdnilo, Oct 26 '22 at 10:53
How would it look like as a function parameter? This does not work now: — Dr.Random, Oct 26 '22 at 10:55
void TimeSystem::regDayChange(void (*dayChangeFunc)(int prevDay,int nowDay)) — Dr.Random, Oct 26 '22 at 10:55
declaration is incompatible with "void TimeSystem::regDayChange(dayChangeFunc)" — Dr.Random, Oct 26 '22 at 10:57
It would look the same as any other type; `void TimeSystem::regDayChange(dayChangeFunc func);`. (You confused yourself by using the same name for the parameter as for the type alias; normally one would have written exactly this even with your type alias.) — molbdnilo, Oct 26 '22 at 10:57
My definition looks like this: void regDayChange(dayChangeFunc); and my declaration looks like this: void TimeSystem::regDayChange(void (*dayChangeFunc)(int prevDay,int nowDay)){} but it does not work. The array of these function pointers are okay with that type — Dr.Random, Oct 26 '22 at 10:59
Oh it is compiled. Thank you very much!! Can you post it as a solution? — Dr.Random, Oct 26 '22 at 11:02

score 2 · Accepted Answer · answered Oct 26 '22 at 11:06

2

Change the callback type to std::function<void(int, int)>

using dayChangeFunc = std::function<void(int, int)>;

or

typedef std::function<void(int, int)> dayChangeFunc;

and change the function prototype,

void TimeSystem::regDayChange(dayChangeFunc func).

(If you had used your type alias consistently you wouldn't have needed to change the prototype, only the alias itself. Type aliases are most useful if you use them.)

answered Oct 26 '22 at 11:06

molbdnilo

64,751
3
43
82

Thank you very much for the answer! Do you recommend the using keyword or typedef one? – Dr.Random Oct 26 '22 at 11:15
Use `using`. Almost nobody uses `typedef` these days, and there are even situations when you can't. Being consistent is a Good Thing. – molbdnilo Oct 26 '22 at 11:18
Thank you! I will read about using a little bit more. – Dr.Random Oct 26 '22 at 11:21

score 0 · Answer 2 · answered Oct 26 '22 at 10:21

0

A lambda function that captures anything cannot be converted to a function pointer. You need to pass a pointer manually, or need to have a 'pool' of functions, each storing the current this pointer globally.

answered Oct 26 '22 at 10:21

lorro

10,687
23
36

Can i use an std::function for it somehow? – Dr.Random Oct 26 '22 at 10:48
@Dr.Random No, unless you change it on the callback as well. `std::function` also needs a pointer. What you're asking for simply doesn't work within c++. – lorro Oct 26 '22 at 10:49
@lorro What pointer would a `this`-capturing lambda need? – molbdnilo Oct 26 '22 at 10:52
The entire class which calls it. – Dr.Random Oct 26 '22 at 10:52
I'm registering a callback function inside various classes and want to do class related things inside the callback. – Dr.Random Oct 26 '22 at 10:53
I solved it with the help of @molbdnilo – Dr.Random Oct 26 '22 at 11:03

How to define a lambda function to be able to capture the 'this' pointer of a class?

2 Answers2