How to Convert MQL5 Code to MQL4: A Comprehensive Guide

Overview of MQL5: Features and Capabilities

MQL5 represents a significant advancement over MQL4, offering enhanced capabilities for algorithmic trading. Key features include object-oriented programming (OOP) support, improved event handling, and a more comprehensive standard library. MQL5 allows for more complex and modular code structures, facilitating the development of sophisticated trading strategies.

• OOP Support: Enables the creation of classes, inheritance, and polymorphism.
• Enhanced Event Handling: Provides more precise control over trading events.
• Comprehensive Standard Library: Offers a wider range of built-in functions and classes.
• Strategy Tester: The multi-threaded strategy tester allows optimizing trading strategies much faster, compared to MQL4.

Overview of MQL4: Features and Limitations

MQL4, while powerful, has limitations compared to MQL5. It lacks native OOP support and has a more restricted standard library. MQL4 is procedural language.

• Procedural Programming: Relies on sequential execution of code.
• Limited Standard Library: Requires more manual implementation of common functions.
• Simpler Event Handling: Offers basic event handling capabilities.

Key Differences Between MQL5 and MQL4

The core differences between MQL5 and MQL4 are:

1. Syntax: MQL5 has a more modern and structured syntax, resembling C++. MQL4’s syntax is more basic.
2. OOP: MQL5 supports OOP, while MQL4 does not.
3. Data Types: MQL5 offers a richer set of data types, including enum and struct.
4. Event Handling: MQL5’s event handling is more sophisticated, allowing for more granular control.
5. Standard Library: MQL5 boasts a more extensive standard library, providing numerous built-in functions and classes.
6. Execution Model: MQL5 programs are compiled into native code, potentially offering performance advantages.

Why Convert from MQL5 to MQL4?

Despite MQL5’s advantages, there are reasons to convert code to MQL4:

• Platform Compatibility: Some brokers or traders may still rely on MetaTrader 4.
• Legacy Code: Existing MQL4 codebases might need to be integrated with newer strategies.
• Resource Constraints: Simpler strategies might not require MQL5’s advanced features, making MQL4 a viable option.
• Marketplace requirements: Some strategies can be sold only in MQL4.

Understanding the Conversion Process

Identifying Incompatible Features

The first step is to identify MQL5 features that have no direct equivalents in MQL4. These include:

• Classes and Objects
• Templates
• Enumerations (enum)
• Structures with methods
• Some advanced event handling functions

Planning the Conversion Strategy

The conversion strategy involves:

1. Feature Decomposition: Breaking down MQL5 features into simpler MQL4 equivalents.
2. Library Substitution: Replacing MQL5 standard library functions with custom implementations or available MQL4 libraries.
3. Code Restructuring: Reorganizing code to fit MQL4’s procedural paradigm.
4. Testing: Writing thorough test cases to ensure the converted code functions correctly.

Setting Up the Development Environment

Ensure you have the MetaTrader 4 platform installed and the MetaEditor opened. This will be your primary environment for writing and testing the converted MQL4 code.

Step-by-Step Guide to Converting MQL5 Code to MQL4

Data Types and Variable Declarations

MQL5:

enum TradeType {BUY, SELL};
TradeType orderType = BUY;

MQL4:

#define BUY 0
#define SELL 1
int orderType = BUY;

• Explanation: MQL4 does not have enum. We can use #define to declare named constants. bool type also should be replaced to int.

Function Definitions and Calls

MQL5:

int CalculateSum(int a, int b) {
 return a + b;
}

int result = CalculateSum(5, 3);

MQL4:

int CalculateSum(int a, int b) {
 return (a + b);
}

int result = CalculateSum(5, 3);

• Explanation: Function definitions and calls are mostly similar, but pay attention to potential changes in data types and parameter passing conventions.

Object-Oriented Programming (OOP) Considerations

Since MQL4 lacks OOP, classes need to be converted into structures and functions that operate on those structures.

MQL5:

class MyClass {
 public:
 int value;
 void SetValue(int newValue) {
 value = newValue;
 }
};

MyClass obj;
obj.SetValue(10);

MQL4:

struct MyClass {
 int value;
};

void SetValue(MyClass &obj, int newValue) {
 obj.value = newValue;
}

MyClass obj;
SetValue(obj, 10);

• Explanation: The class MyClass is converted into a struct. Methods like SetValue become standalone functions that accept a pointer to the struct as an argument. Passing by reference is crucial to modify the original struct.

Event Handling and Built-in Functions

MQL5:

void OnTradeTransaction(const MqlTradeTransaction& trans, const MqlTradeRequest& request, const MqlTradeResult& result) {
 // Handle trade transaction
}

MQL4:

MQL4 has OnTick() event. For more complex event handling, it’s generally needed to create custom logic. MQL4 does not have such a variety of event handling functions, and often requires a more manual handling of events. Consider using OrderSend() and OrderClose() functions.

void OnTick() {
 // Check for new ticks and handle events
}

• Explanation: MQL5’s OnTradeTransaction event needs to be adapted to MQL4’s simpler event model, typically using the OnTick function or custom logic.

Handling Complex Code Structures

Converting Classes and Structures

When converting classes, focus on replicating their functionality using structures and functions. Encapsulation and inheritance are not directly possible, so carefully manage data access and code organization.

Dealing with MQL5 Standard Library Functions

Many MQL5 standard library functions have no direct equivalents in MQL4. You’ll need to find or create custom implementations. For example, string manipulation functions might require using basic string handling techniques in MQL4.

Implementing MQL5 Features in MQL4 (Workarounds)

• Templates: MQL4 does not support templates. Create specialized versions of functions for each required data type.
• Enums: Use #define or const int to simulate enumerated types.
• Dynamic Arrays: MQL4’s dynamic arrays are less flexible than MQL5’s. Consider using fixed-size arrays or custom memory management.

Testing and Debugging the Converted Code

Creating a Testing Plan

Develop a comprehensive testing plan that covers all critical functionalities of the converted code. Include unit tests for individual functions and integration tests for the entire system.

Debugging Techniques for MQL4

Use the MetaEditor’s debugger to step through the code, inspect variables, and identify errors. The Print() function is also invaluable for logging information and tracing the execution flow.

Optimizing Performance in MQL4

MQL4 can be less performant than MQL5, so pay attention to optimization:

• Minimize Calculations: Reduce unnecessary calculations and function calls.
• Efficient Data Structures: Use appropriate data structures to minimize memory usage and access times.
• Avoid Loops: Optimize loops for efficiency, especially in the OnTick function.

Common Errors and Troubleshooting

• Data Type Mismatches: Ensure that data types are correctly converted and used.
• Memory Management Issues: Be careful with memory allocation and deallocation, especially when using dynamic arrays.
• Logic Errors: Thoroughly test the code to identify and fix any logic errors introduced during the conversion process.