Type Variance
Part 1
Parameter type Contravariance
Return type Covariance
Warm-up concepts
Inheritance
Variance
Inheritance
- parent is broad type
- child is narrow / specialized type
- Parent can be substituted by Parent
- Parent can be substituted by Child
- Child cannot be substituted by Parent
- the >= symbol suggests the direction in which the type is getting narrower
- the >= symbol suggest the direction in which substitution can happen
Variance
- can Child override the function with the same signature or can it replace the Parent parameter with Child?
- can Child override the function with and substitute the return type?
- the term variance answers the questions above and refers to the direction of substitution that is allowed in the overridden method
class Parent {
function someFunction(Parent $parent): Parent;
}
class Child extends Parent {
}