How to define a static pointer to a class in MQL?

Question

I've got the following MQL code:

class Collection {
  public: void *Get(void *_object) { return NULL; }
};

class Timer      {
  protected:
    string name;
    uint start, end;
  public:
    void Timer(string _name = "") : name(_name) { };
    void TimerStart() { start = GetTickCount(); }
    void TimerStop()  { end = GetTickCount();   }
};

class Profiler {
  public:
    static Collection *timers;
    static ulong min_time;
    void Profiler()  { };
    void ~Profiler() { Deinit(); };
    static void Deinit() { delete Profiler::timers; };
};

// Initialize static global variables.
Collection *Profiler::timers = new Collection();
ulong Profiler::min_time = 1;

void main() {
  // Define local variable.
  static Timer *_timer = new Timer(__FUNCTION__); // This line doesn't.
  //Timer *_timer = new Timer(__FUNCTION__); // This line works.

  // Start a timer.
  ((Timer *) Profiler::timers.Get(_timer)).TimerStart();
  /* Some code here. */
  // Stop a timer.
  ((Timer *) Profiler::timers.Get(_timer)).TimerStop();
}

which defines a Timer class which is used as a timer to profile the functions how long it took. The original version uses a list of timers to store time separately on each call, however, the code has been simplified to provide a minimum working example and focus on the actual compilation problem.

The problem is when I'm using the following line in order to initialize a static variable:

static Timer *_timer = new Timer(__FUNCTION__); // Line 30.

the compilation fails with:

'Timer' - local variables cannot be used TestProfiler.mqh 30 30

When I drop static word, the code compiles fine.

But it doesn't help me, as I want to define this variable as a static pointer to the class, as I don't want to destroy my object each time when the same function is called over and over again, so the timers can be added to the list which can be read later on. I don't really see why the MQL compiler would prevent from compiling the above code. I also believe this syntax worked fine in the previous builds.

I'm using MetaEditor 5.00 build 1601 (May 2017).

What is wrong with my static variable declaration and how can I correct it, so it can point to a Timer class?

Daniel Kniaz Daniel Kniaz · Accepted Answer · 2018-01-15T10:00:43

Keyword static has two different meanings in MQL4/5: it indicates that a member of a class is static (which is obvious), and it also says that a variable is static... for instance, if you have a variable that is used only in one function, you probably do not need to declare it globally but as a static. You can find an example of isNewBar() function that has static datetime lastBar=0; in the articles about new bar at mql5.com. This keyword in such a function says that the variable is not deleted after function is finished, but remains in memory and is used with the next call. And if you need a variable in OnTick() function - it does not make sence to have it static, declare it globally.

How to define a static pointer to a class in MQL?

1 Answers