Errors, bugs, questions - page 1991

[fxsaber]

Alexey Kozitsyn:
Add a default constructor to the structure.

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Bugs, bugs, questions

fxsaber, 2017.09.06 09:38

// Нужен только для того, чтобы показать необходимость решения без конструктора
union UNION
{
  STRUCT Struct; // конструктора не должно быть
  int i;
};

[[Deleted]]

fxsaber:

Inattentive me:) Can you explain why this is needed? Why can't the constructor be used?

Or is it to show an error of structure initialization with a closed field?

[fxsaber]

Alexey Kozitsyn:

Inattentive me:) Can you explain why this is needed? Why can't the constructor be used?

Because when writing my own code I came across a situation where my structure is involved in a union. The solution with an empty constructor came immediately to mind but it was a bummer that my code had a union. That's why I was asking this question not for the sake of boring me but for practical use.

I had to write this twist to avoid this "bug" (is it a bug or not?).

// Зануляем структуру без конструктора со скрытыми полями
template <typename T>
void StructToNull( T& Struct )
{    
  union TMP_UNION
  {
    T Struct;
    
    struct TMP_STRUCT
    {
      uchar Array[sizeof(T)];
    } Tmp;
    
    TMP_UNION( T& Value )
    {
      ::ArrayInitialize(this.Tmp.Array, 0);
      
      Value = this.Struct;
    }
  } TmpUnion(Struct);    
}

I haven't seen any other solution yet.

[[Deleted]]

fxsaber:

Run it, saw the problem. It seems that only structures with public fields are allowed with unions now. Then it would be possible to write struct = {0};

[fxsaber]

Alexey Kozitsyn:
Run it, saw the problem. It seems that only structures with public fields are allowed with unions now. Then it would be possible to write struct = {0};

This is not the case.

[[Deleted]]

fxsaber:

This is not the case.

Maybe I misunderstood you again, but this code works without any warnings:

struct STRUCT
{
//private:
  int i;

};

// Нужен только для того, чтобы показать необходимость решения без конструктора
union UNION
{
  STRUCT Struct; // конструктора не должно быть
  int i;
};

// Обходим Warning "possible use of uninitialized variable"
template <typename T>
void StructInit( T& ) {}

STRUCT Func()
{
  STRUCT Res = {0}; // ок!
  
//  StructInit(Res); // с этой строкой Warning не появляется, но это какой-то абсурд!
  
  return(Res);  
}

void OnStart()
{
  Func();
}

[fxsaber]

Alexey Kozitsyn:

Maybe I misunderstood you again, but this code works without any warnings:

That's right, the public structures are initialized. But union has nothing to do with it.

Interestingly, if a structure with a hidden field is made a public field of a new structure, then the new one will not initialize either, even if all fields are open.

[giros]

Hi, could you tell me if it's possible to declare an array by specifying the number of elements in it with a variable? If not, how can this be done?

extern int Period = 10;
int Massiv[Period]; //Выдаёт ошибку: "invalid index value"

[fxsaber]

giros:

Hi, could you tell me if it's possible to declare an array by specifying the number of elements in it with a variable? If not, how can this be done?

https://www.mql5.com/ru/docs/array/arrayresize

[Yury Kirillov]

giros:

Hi, could you tell me if it's possible to declare an array by specifying the number of elements in it with a variable? If not, how can this be done?

ArrayResize()