__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.
__if_not_exists ( identifier ) {
statements
};
Parameters
Parameter |
Description |
|---|---|
identifier |
The identifier whose existence you want to test. |
statements |
One or more statements to execute if identifier does not exist. |
Remarks
警告
To achieve the most reliable results, use the __if_not_exists statement under the following constraints.
Apply the __if_not_exists statement to only simple types, not templates.
Apply the __if_not_exists statement to identifiers both inside or outside a class. Do not apply the __if_not_exists statement to local variables.
Use the __if_not_exists statement only in the body of a function. Outside of the body of a function, the __if_not_exists statement 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.