Operator Next (Iteration)
Determines if a `For...Next` loop should be terminated
Syntax
`
{ Type | Class | Union } typename
Declare Operator Next ( [ ByRef | ByVal ] cond As typename ) As Integer
Declare Operator Next ( [ ByRef | ByVal ] cond As typename, [ ByRef | ByVal ] stp As typename ) As Integer
...
End { Type | Class | Union }
`
Usage
`
For iterator [ As typename ] = start_value To end_value [ Step step_value ]
[ ...statements... ]
Next
`
Parameters
`(including arguments)`
`typename`
name of the `Type`, `Class`, or `Union`
`cond`, `end_value`
a `typename` object used as a loop-terminating value
`stp`, `step_value`
a `typename` object used as an incremental value
`iterator`
a `typename` object used as an iterator
`start_value`
a `typename` object used to copy construct or assign to the iterator initially
Description
`Operator For`, `Operator Next` and `Operator Step` can be overloaded in user-defined type definitions to allow objects of that type to be used as iterators and step values in `For...Next` loops.
As all non-static member procedures, they have passed a hidden `This` parameter that allows to access by reference to the iterator object in the code body of the 3 operators.
`Operator Next` is called every time the iterator object needs to be checked against the end value. This happens immediately after the call to its `Operator For`, and immediately after any calls to its `Operator Step`. `Operator Next` should return zero (0) if the loop should be terminated, or non-zero if the loop should continue iterating. The first time `Operator Next` is called, no statements in the `For...Next` body, if any, have been executed yet.
The first version of `Operator Next` is used if no step value is given in the `For...Next` statement. If a step value is given, the second version is used and is passed the step value because testing for iterating end may depend on it.
Advanced usage
The above description seems to imply that the 3 arguments `start_value`, `end_value`, and `step_value` must be of the same type as the `iterator` (this is the more obvious use), but it is not quite true:
- The `start_value`, `end_value`, and `step_value` arguments can be of any type (of different types among themselves and also of different types from the one of the `iterator`).
- The only constraint is that the `iterator` could be constructed (in case of local `iterator`) or assigned (in case of global `iterator`) from the `start_value` argument (because the `iterator` is implicitly constructed or assigned under the hood).
- Similarly the other parameters `end_value`, and `step_value` must be able to be converted into objects of the same type as the `iterator`.
Example
See the Operator Step examples.
Dialect Differences

- Only available in the -lang fb dialect.

See also

- `Operator For`
- `Operator Step`
- `For...Next`