---
applyTo: "**/*.mq5,**/*.mqh"
---
# CRITICAL MQL5 Syntax Rules (Mandatory for All MQL5 Code)

## ⚠️ Overview

MQL5 is **NOT** standard C++. It has critical syntax differences that cause compilation errors if ignored. These rules are mandatory for all _Thivyam MQL5 development.

---

## 1. Pointer Member Access ❌ → ✅

### Rule
**ALWAYS use dot operator (`.`) for pointer member access**  
**NEVER use arrow operator (`->`)**

### Why
MQL5 parser treats `->` as two separate operators: `-` (minus) and `>` (greater-than), causing parser errors.

### Examples
```mql5
// ✅ CORRECT
CObject *ptr = GetObject();
ptr.Method();
ptr.SetValue(10);

CZoneState *zone = ctx.Zone();  // Returns pointer
zone.IsActive();                 // Use dot operator

// ❌ WRONG - Causes compilation errors
ptr->Method();       // Error: 'operand expected'
zone->IsActive();    // Error: 'undeclared identifier'
```

### Error Messages You'll See
- `'>' - operand expected`
- `undeclared identifier`
- `')' - expression expected`
- `'Method' - object pointer expected`

---

## 2. Loop Variable Declaration ❌ → ✅

### Rule
**NEVER declare variables inside for-loop initializers**  
**ALWAYS declare loop counters BEFORE the for statement**

### Why
MQL5 does not support C99/C++11 style variable declaration in for-loop initializers.

### Examples
```mql5
// ✅ CORRECT
int i;
for(i = 0; i < 10; i++)
{
    Print(i);
}

// Multiple loops
int i, j;
for(i = 0; i < rows; i++)
{
    for(j = 0; j < cols; j++)
    {
        matrix[i][j] = 0;
    }
}

// ❌ WRONG - Causes compilation errors
for(int i = 0; i < 10; i++)  // Error: undeclared identifier
{
    Print(i);
}
```

### Additional Rules
- Use **post-increment** (`i++`) not pre-increment (`++i`) for better compatibility
- Avoid `const` on loop bound variables: use `int total` not `const int total`

### Error Messages You'll See
- `undeclared identifier`
- `'i' - some operator expected`
- `expression expected`

---

## 3. Array Declarations ❌ → ✅

### Rule
**Use dynamic arrays (`Type[]`) for array manipulation functions**  
**Static arrays (`Type[N]`) cannot be used with `ArraySetAsSeries()` and similar functions**

### Why
MQL5 distinguishes between static (fixed-size) and dynamic (variable-size) arrays, and many built-in functions only work with dynamic arrays.

### Examples
```mql5
// ✅ CORRECT - Dynamic arrays
MqlRates rates[];
ArraySetAsSeries(rates, true);
CopyRates(_Symbol, PERIOD_H1, 0, 100, rates);

double buffer[];
ArrayResize(buffer, 50);

// ❌ WRONG - Static arrays
MqlRates rates[100];
ArraySetAsSeries(rates, true);  // Error: cannot be used for static allocated array
```

### Special Cases
```mql5
// CArrayObj from standard library
CArrayObj m_array;

// ✅ CORRECT - Auto-initializes
CArrayObj() { 
    // No initialization needed
}

// ❌ WRONG - Create() doesn't exist
CArrayObj() { 
    m_array.Create();  // Error: undeclared identifier
}
```

### Error Messages You'll See
- `cannot be used for static allocated array`
- `undeclared identifier` (for `.Create()`)

---

## 4. Function Parameters ❌ → ✅

### Rule
**Avoid `const Type &parameter` when passing temporary values**  
**Use pass-by-value (`const Type parameter`) instead**

### Why
Temporary values (from ternary operators, function returns) cannot bind to reference parameters in MQL5.

### Examples
```mql5
// ✅ CORRECT
void CreateLabel(const string text)
{
    ObjectSetString(0, "label", OBJPROP_TEXT, text);
}

// Called with ternary expression
CreateLabel(minimized ? "" : fullText);  // Works fine

// ❌ WRONG
void CreateLabel(const string &text)  // Reference parameter
{
    ObjectSetString(0, "label", OBJPROP_TEXT, text);
}

CreateLabel(minimized ? "" : fullText);  // Error: parameter passed as reference
```

### When to Use References
- Use references for large structures that you DON'T create on-the-fly
- Use value parameters for strings and small types

### Error Messages You'll See
- `parameter passed as reference, variable expected`

---

## 5. Object Property Constants ❌ → ✅

### Rule
**Property names must match MQL5 documentation exactly**  
**Common mistakes have similar but incorrect names**

### Critical Property Names

| ❌ WRONG | ✅ CORRECT | Object Type |
|----------|------------|-------------|
| `OBJPROP_FONT_SIZE` | `OBJPROP_FONTSIZE` | All text objects |
| `OBJPROP_WIDTH` | `OBJPROP_XSIZE` | Rectangle Label |
| `OBJPROP_HEIGHT` | `OBJPROP_YSIZE` | Rectangle Label |
| `OBJPROP_ZORDER` | _(doesn't exist)_ | All objects |

### Examples
```mql5
// ✅ CORRECT
ObjectSetInteger(0, "label", OBJPROP_FONTSIZE, 10);
ObjectSetInteger(0, "rect", OBJPROP_XSIZE, 200);
ObjectSetInteger(0, "rect", OBJPROP_YSIZE, 100);

// ❌ WRONG
ObjectSetInteger(0, "label", OBJPROP_FONT_SIZE, 10);  // Underscore
ObjectSetInteger(0, "rect", OBJPROP_WIDTH, 200);      // Doesn't exist
ObjectSetInteger(0, "rect", OBJPROP_HEIGHT, 100);     // Doesn't exist
ObjectSetInteger(0, "rect", OBJPROP_ZORDER, 0);       // Doesn't exist
```

### Error Messages You'll See
- `undeclared identifier`
- `cannot convert enum`
- `wrong parameters count`

---

## 6. Type Casting ❌ → ✅

### Rule
**Avoid spaces between cast closing paren and pointer asterisk**  
**Store cast results in intermediate variables to avoid parser confusion**

### Examples
```mql5
// ✅ CORRECT - No space, intermediate variable
CObject *obj = m_array.At(i);
CH4ZoneContext *ctx = (CH4ZoneContext*)obj;
CZoneState *zone = ctx.Zone();

// ❌ WRONG - Space in cast
CH4ZoneContext *ctx = (CH4ZoneContext *)obj;  // Can confuse parser

// ❌ WRONG - Chained operations
CZoneState *zone = ((CH4ZoneContext*)m_array.At(i)).Zone();  // Parser errors
```

---

## 7. Const Correctness ❌ → ✅

### Rule
**MQL5 is stricter about const than C++**  
**Avoid const on simple local variables and loop bounds**

### Examples
```mql5
// ✅ CORRECT
uint total = PositionsTotal();
uint i;
for(i = 0; i < total; i++)

// ⚠️ POTENTIALLY PROBLEMATIC
const uint total = PositionsTotal();  // May cause issues in some contexts

// ✅ CORRECT - Use const for true constants
const double PI = 3.14159;
const int MAX_RETRIES = 3;
```

---

## 8. Position Functions (CRITICAL) ❌ → ✅

### Rule
**`PositionSelectByIndex()` does NOT exist in MQL5**  
**Use `PositionGetTicket(index)` or `PositionGetSymbol(index)` instead**

### Why
This is a common error from MT4-to-MT5 migration or incorrect examples. The function simply doesn't exist in the MQL5 API.

### Examples
```mql5
// ✅ CORRECT - Method 1: Get ticket first
int total = PositionsTotal();
int i;
for(i = 0; i < total; i++)
{
   ulong ticket = PositionGetTicket(i);
   if(ticket == 0)
      continue;
   // Now access position properties
   long magic = PositionGetInteger(POSITION_MAGIC);
   double volume = PositionGetDouble(POSITION_VOLUME);
}

// ✅ CORRECT - Method 2: Get symbol (also selects position)
for(i = 0; i < total; i++)
{
   string symbol = PositionGetSymbol(i);
   if(symbol == "")
      continue;
   // Access properties for currently selected position
}

// ❌ WRONG - Function doesn't exist!
for(i = 0; i < total; i++)
{
   if(!PositionSelectByIndex(i))  // Compilation error!
      continue;
}
```

### Error Messages You'll See
- `undeclared identifier` (at the function name)
- `'i' - some operator expected` (at the parameter)
- Error column numbers pointing inside the function call

### Why Errors Are Cryptic
Because the compiler doesn't recognize `PositionSelectByIndex`, it tries to parse the entire line as an expression, producing confusing errors about operators and variables.

---

## 9. Quick Reference Checklist

Before committing MQL5 code, verify:

- [ ] All pointer member access uses `.` not `->`
- [ ] All loop variables declared before `for` statement
- [ ] NO use of `PositionSelectByIndex()` - use `PositionGetTicket()` instead
- [ ] All loop increments use `i++` not `++i`
- [ ] Dynamic arrays used with `ArraySetAsSeries()`, `CopyRates()`
- [ ] Function parameters avoid `const string &` for temporaries
- [ ] Object properties use correct names (verify in docs)
- [ ] No `CArrayObj.Create()` calls
- [ ] Type casts have no spaces: `(Type*)` not `(Type *)`
- [ ] No `OBJPROP_ZORDER`, `OBJPROP_WIDTH`, `OBJPROP_HEIGHT`
- [ ] Use `OBJPROP_FONTSIZE` not `OBJPROP_FONT_SIZE`

---

## 10. Compilation Verification

Always compile in MetaEditor before considering code complete:

```bash
# Windows PowerShell
& "C:\Program Files\MetaTrader 5\MetaEditor64.exe" /compile:"path\to\file.mq5"
```

**Expected Result:** 0 errors, 0 warnings

---

## 11. Common Error Patterns

If you see these errors, check these causes:

| Error Message | Most Likely Cause | Fix |
|---------------|-------------------|-----|
| `'>' - operand expected` | Using `->` operator | Change to `.` |
| `undeclared identifier` (at loop) | Variable declared in for-loop | Declare before loop |
| `undeclared identifier` + `operator expected` | Non-existent function like `PositionSelectByIndex()` | Use `PositionGetTicket(i)` |
| `cannot be used for static allocated array` | Static array with `ArraySetAsSeries()` | Use dynamic array `[]` |
| `parameter passed as reference` | Passing temporary to `&` param | Use value parameter |
| `undeclared identifier` (property) | Wrong property name | Check MQL5 docs |

---

## 12. Resources

- **MQL5 Documentation:** https://www.mql5.com/en/docs
- **Object Properties:** https://www.mql5.com/en/docs/constants/objectconstants/enum_object_property
- **Array Functions:** https://www.mql5.com/en/docs/array

---

**Last Updated:** 2025-11-03  
**Validated Against:** MQL5 Build 3960+