MQL4 Reference 
Standard constants Special constants
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Special constants used to indicate parameters and variables states.
It can be one of the following values:
| Constant | Value | Description |
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| NULL | 0 | Indicates empty state of the string. |
| EMPTY | -1 | Indicates empty state of the parameter. |
| EMPTY_VALUE | 0x7FFFFFFF | Default custom indicator empty value. |
| CLR_NONE | 0xFFFFFFFF | Indicates empty state of colors. |
| WHOLE_ARRAY | 0 | Used with array functions. Indicates that all array elements will be processed. |
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