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Textcolor' - Extern Variable Initialization Is Not Allowed Mql5

5 min read Oct 06, 2024
Textcolor' - Extern Variable Initialization Is Not Allowed Mql5

Understanding "extern variable initialization is not allowed" in MQL5

When working with MQL5, you might encounter the error message "extern variable initialization is not allowed." This error typically arises during the compilation stage, indicating a fundamental misunderstanding of how extern variables are declared and used within MQL5. Let's delve into the reasons behind this error and how to properly handle extern variables.

What are extern Variables?

extern variables in MQL5 are a mechanism for sharing data between different Expert Advisors (EAs) or indicators. When declared as extern, a variable becomes accessible from all scripts and programs running within the same terminal instance.

Imagine a shared database where different applications can access and modify the same information. That's similar to how extern variables work.

Why Can't You Initialize extern Variables Directly?

The core reason behind the "extern variable initialization is not allowed" error is that extern variables are not directly assigned values within the code where they are declared.

Consider this:

  • extern variables reside in the global scope. Their values are set and retrieved through external means – typically via the MetaTrader 5 terminal settings or input parameters.

  • If you were to initialize an extern variable directly within your code, this would create a conflict. Imagine two different scripts trying to assign different values to the same extern variable! This would lead to inconsistent behavior and unpredictable outcomes.

How to Correctly Use extern Variables

To avoid the "extern variable initialization is not allowed" error and leverage extern variables effectively, follow these steps:

  1. Declare the extern Variable:

    • Use the extern keyword to declare the variable, specifying its data type.
    • Do not assign any initial values to the variable.
    extern int MyExternalVariable;

  2. Define the Variable's Value:

    • Assign the extern variable a value through one of these methods:
      • MetaTrader 5 Terminal Settings: Access the expert advisor's settings within the terminal and input a value for the extern variable.
      • Input Parameters: Define the extern variable as an input parameter in your script, allowing users to set values through the Strategy Tester or directly in the terminal.
    input int MyInputVariable = 10; // Example of defining value through input parameter

  3. Access the Variable:

    • Within your MQL5 code, simply reference the extern variable by its name.
    int MyLocalVariable = MyExternalVariable + 5; // Accessing the value from the `extern` variable

Example: Using extern to Share a Trading Strategy Setting

// EA Script

extern int EntryThreshold; // Declares the `extern` variable
input int StopLoss = 20;    // Defines another variable as an input

void OnInit() 
{
    // Access the `extern` variable 
    if (EntryThreshold < 5) 
    {
        Print("Entry threshold is too low, adjusting...");
        EntryThreshold = 5;
    }
}

In this example, EntryThreshold can be adjusted by the user through the MetaTrader 5 terminal settings, allowing for customization of the entry threshold across multiple EAs using the shared value.

Conclusion

The "extern variable initialization is not allowed" error in MQL5 stems from the fundamental design of extern variables. These variables are designed for sharing information between scripts and rely on external sources like terminal settings or input parameters for their values. By understanding this concept and following the correct usage guidelines, you can effectively leverage extern variables for creating flexible and customizable MQL5 programs.

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