Features of the mql5 language, subtleties and tricks - page 111

[Artyom Trishkin]

pavlick_:

...

Please use the built-in text formatting - there is a menu above the message input field - everything is there. And hotkeys are described there too.

I've fixed your "quote".

[Alexey Navoykov]

pavlick_:

How is it the same? There's an automatic copy constructor and all manipulation will have a view:

... 
ar.Add(new(q));

As I understand it, I meant new Q(q). Now it's clear that we were talking about the absence of automatic copy constructor in MQL.

[fxsaber]

Problem

Forum on trading, automated trading systems and trading strategy testing

Bugs, bugs, questions

fxsaber, 2018.12.21 10:23

I can't figure it out, please ask for a hint. There is such a time acquisition

int GetHandle() { return(0); }

bool SelectHandle( int ) { return(true); }
  
int NewHandle = 0;  
int PrevHandle = GetHandle();    

datetime time = SelectHandle(NewHandle) ? TimeCurrent() : 0;  
SelectHandle(PrevHandle);

How to write a macro that does the same thing

time = MACROS(NewHandle, TimeCurrent()); // Макрос

The problem is that PrevHandle in the macro cannot be created.

Forum on trading, automated trading systems & strategy testing

Bugs, bugs, questions

fxsaber, 2018.12.21 22:15

Let me clarify that NewHandle - can be a constant. I.e. a valid call like this

time = MACROS(0, TimeCurrent()); // TimeCurrent из 0-хендла.
time = MACROS(1, TimeCurrent()); // TimeCurrent из 1-хендла.

MACROS(0, SymbolInfoTick(_Symbol, Tick)); // SymbolInfoTick из 0-хендла.

#define Bid SymbolInfoDouble(_Symbol, SYMBOL_BID)
Price = MACROS(3, Bid); // Bid из 3-хендла.

Solution

template <typename T>
T Memory( const T NewValue, const bool SetFlag = true )
{
  static T PrevValue;
  
  if (SetFlag)
    PrevValue = NewValue;
 
  return(PrevValue); 
}

template <typename T1, typename T2>
T2 MacrosFunc( const T1, const T2 Value, const T1 )
{
  return(Value);
}

#define  MACROS(A, B) MacrosFunc(SelectHandle(Memory(A, false)), B, SelectHandle(Memory(A)))

[Alexey Navoykov]

fxsaber:

Solution

...
template <typename T1, typename T2>
T2 MacrosFunc( const T1, const T2 Value, const T1 )
{
  return(Value);
}

#define  MACROS(A, B) MacrosFunc(SelectHandle(Memory(A, false)), B, SelectHandle(Memory(A)))

You will always have expression B calculated and returned, regardless of the rest.

[fxsaber]

Alexey Navoykov:

Your B expression will always be calculated and returned, regardless of the rest.

There's a sequential (right-to-left) calculation of the input parameters of the auxiliary functions.

[Alexey Navoykov]

fxsaber:

There is a sequential (right-to-left) calculation of the input parameters of the auxiliary functions.

What does this change? The macro always returns the value B

[fxsaber]

Alexey Navoykov:
What does this change? The macro always returns B

Try to refute the decision. I don't see any error.

First it switches to NewHandle, then it takes B, then it switches to the previous handle.

[Alexey Navoykov]

fxsaber:

First it switches to NewHandle, then it takes B, then it switches to the previous handle.

And the original task was like this:

SelectHandle(NewHandle) ? TimeCurrent() : 0;
SelectHandle(PrevHandle);

I.e. the condition must be fulfilled

[fxsaber]

Alexey Navoykov:

And the original problem was this:

This is the same.

template <typename T1, typename T2>
T2 MacrosFunc( const T1 NewHandle, const T2 Value )
{
  return(Value);
}

#define  MACROS(A, B) MacrosFunc(SelectHandle(Memory(A, false)), SelectHandle(Memory(A)) ? B : 0)

After giving a comment, which summarised the problem somewhat - SymbolInfoTick and SymbolInfoDouble, etc.

[Alexey Navoykov]

By the way, I came to the conclusion that in the general case, if the return value type of the function is absolutely any type, then the problem is not solvable by MQL means. A decltype is required, which is not present here.