Errors, bugs, questions - page 2206

[Andrey Dik]

Alexander Nikolaev:
Who knows why I can't access recently created demo accounts, which were created directly in MT4 terminal, on Metaquotes Demo server? I created one a week ago. The first 2 days it works, but after some time the account disappears, as if the password becomes incorrect (although it was copied, I could not enter it incorrectly) and I have to reopen the account to test the Expert Advisor. Is it really necessary to create a new account every week?

it's been like this for a long time, if there is no activity on the account, it gets closed. create a new one and don't worry about it.

[Andrii Djola]

Denis Sartakov:

I've recently written a function like this, so try and figure it out for yourself,

If it doesn't work, I'll explain.

thanks

[Andrii Djola]

another question:

the description ofCHART_IS_MAXIMIZED andCHART_IS_MINIMIZED does not say that these properties are read-only, but ChartSetInteger(ChartID(),CHART_IS_MAXIMIZED,true); does not work, what could be the problem???

[pavlick_]

Why doesn't it compile?

template <typename T>
void f(T &t)
{
   t.f();
}

void OnStart()
{
   class Q
   {
   public:
      void f() {}
   }q;
   
   f(q);
}

Older versions of the compiler ate such constructs fine, took to recompile valid earlier code and error. Is it an accident or has it broken off intentionally?

[Konstantin]

pavlick_:

Why doesn't it compile?

Older versions of the compiler were fine with such constructs, I recompiled previously valid code and there's an error. Is it an accident or has it broken off intentionally?

but can we create classes inside functions according to the language specification?

[pavlick_]

Konstantin:

but can you create classes within functions according to the language specification?

What language specification? Mcl is not well documented at all, while in c++ it is normal practice, from the reference:

Local classes

A class declaration can appear in namespace scope (in which case it defines an ordinary class), inside another class definition (in which case it defines a nested class), and inside the body of a function, in which case it defines a local class. The name of such a class only exists within the function scope, and is not accessible outside.

http://en.cppreference.com/w/cpp/language/class

[pavlick_]

The answer is silence, as usual. Actually it's easy to work around the problem - instead of elegant placement inside the function, I put it outside with intimidating name internal__Chart_bar_shift_Comp. But the point is different - they just cut such a construct. I.e. there's no guarantee that my code will compile tomorrow. You can somehow with keys to select the dialect -std=mql18, for example. But no, don't care about backward compatibility. I find it hard to imagine that this is possible on some gcc.

I guess a simple scripting mql4 was quite sufficient for a trading platform. Or better yet, import plugins (in dynamic libraries) with scripts/experts. But no, you need your own, as they often say here, "infrastructure", where you are big and important.

[Konstantin]

pavlick_:

According to which language specification? Mcl is not well documented at all, but in c++ it is normal practice, from the reference book:

Local classes

A class declaration can appear in namespace scope (in which case it defines an ordinary class), inside another class definition (in which case it defines a nested class), and inside the body of a function, in which case it defines a local class. The name of such a class only exists within the function scope, and is not accessible outside.

http://en.cppreference.com/w/cpp/language/class

i met a discussion somewhere that it's not foreseen in mql5 specification, check on forum, there was also a discussion about lambas, that they are not foreseen

[Konstantin]

pavlick_:

The answer is silence, as usual. Actually it's easy to work around the problem - instead of elegant placement inside the function, I put it outside with intimidating name internal__Chart_bar_shift_Comp. But the point is in something else - they just cut such a construct. I.e. there's no guarantee that my code will compile tomorrow. You can somehow with keys to select the dialect -std=mql18, for example. But no, don't care about backward compatibility. I can hardly imagine that it is possible in some gcc.

Probably, a simple mql4 script was quite sufficient for a trading platform. Or better - import plugins (in dynamic libraries) with scripts/experts. But no, you need your own, as they often say here, "infrastructure", where you are big and important.

To be honest, I don't understand why they do that. It's easier to put all the auxiliary entities in a separate listing of the source code, for example util.mqh

[pavlick_]

Konstantin:

To be honest, I don't understand why, it's easier to put all the auxiliary entities in a separate listing of the source code, for example util.mqh

This is a predicate for binary search, which is not universal and is used only once in a function, why should I put it somewhere? For example, we have an array of structures (no comparison operator, they can't be compared clumsily using <, or the comparison condition is very tricky), and we want to find an element through binary search. In the plus library, algorithm functions take a functor (conveniently passed as a lambda), inside which we compare array elements. A very elegant solution in my opinion.

int main()
{
    typedef pair<int, double> myPair; // typedef to shorten the type name
    vector <myPair> vec(5);

    myPair low_val; // reference value (set this up as you want)
    auto it = lower_bound(vec.begin(), vec.end(), low_val, 
        [](myPair lhs, myPair rhs) -> bool { return lhs.second < rhs.second; });
}

But due to the limitations of µl, created an instance of the predicate class.