__if_not_exists Statement
The __if_not_exists statement tests whether the specified identifier exists. If the identifier does not exist, the specified statement block is executed.
Syntax
__if_not_exists ( identifier ) {
statements
};
Parameters
identifier
The identifier whose existence you want to test.
statements
One or more statements to execute if identifier does not exist.
Remarks
Caution
To achieve the most reliable results, use the __if_not_exists statement under the following constraints.
Apply the
__if_not_existsstatement to only simple types, not templates.Apply the
__if_not_existsstatement to identifiers both inside or outside a class. Do not apply the__if_not_existsstatement to local variables.Use the
__if_not_existsstatement only in the body of a function. Outside of the body of a function, the__if_not_existsstatement can test only fully defined types.When you test for overloaded functions, you cannot test for a specific form of the overload.
The complement to the __if_not_exists statement is the __if_exists statement.
Example
For an example about how to use __if_not_exists, see __if_exists Statement.