About pointers in mql5

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Hello,

I came across the function "get pointer", and I didn't understand why do we need it in mql5.

I know that if I make an object like that:

obj* foo=new obj();

foo is pointer, so if I print/use/return foo I get the pointer to the object of "obj".

Then why do I need the function get pointer?


Thank you!

 

Because there's no other way to dereference an object. In languages like C or C++ dereferencing is done with the address operator & like so: 

void *something = (void *)&obj; // & used as address operator in C/C++, in MQL this results in an illegal operation use error

DoSomethingWith(something);

But in MQL the usage of the & operator is strictly associated with call by reference operations like so:

void SetVal(int &key, int val) // & used as reference operator
{
   key = val;
}

In MQL it is not allowed to use & as address operator. And so GetPointer is filling this gap.

void *something = GetPointer(obj); // how MQL wants it

DoSomethingWith(something);
 
lippmaje:

Because there's no other way to dereference an object. In languages like C or C++ dereferencing is done with the address operator & like so:

But in MQL the usage of the & operator is strictly associated with call by reference operations like so:

In MQL it is not allowed to use & as address operator. And so GetPointer is filling this gap.

Actually you can use it, but only with statically allocated class instances.

#include <trade/trade.mqh>
#include <arrays/list.mqh>


void OnStart() {
   CList list;
   CSymbolInfo eurusd, gbpusd;
   eurusd.Name("EURUSD");
   gbpusd.Name("GBPUSD");
   list.Add(&eurusd);
   list.Add(&gbpusd);
   CSymbolInfo *ptr = &eurusd;
}

The purpose of GetPointer is that it will always return a pointer to an object regardless of whether or not it is statically or dynamically allocated. Think of it like this:

// overloaded
template <typename T>
T* getPointer(T &obj) {
   return &obj;
}

template <typename T>
T* getPointer(T *obj) {
   return obj;
}



 

 
nicholi shen:

Actually you can use it, but only with statically allocated class instances.

...

That's great to know, especially that you can use a list for this.

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