User defined value types aka structs

Added by Bill Wood

Value types in boo are like value types in C# and include int, double, and other basic types. User-defined value types can be created in boo by defining a struct or inheriting from System.ValueType.

Value types are constructed on the stack rather than the heap as objects are. This makes them faster to create and dispose of, and they don't need to be garbage collected. User-defined value types can have methods and fields, just like classes. Value types cannot inherit from other types (exception - all value types inherit from System.ValueType, which itself inherits from object), and other types cannot inherit from value types. Value types can implement interfaces.

Here's an example showing the implementation of a Point value type by inheriting from System.ValueType.

import System

class Point(ValueType):
    public X as int
    public Y as int

p1 = Point(X: 200, Y: 300)
p2 = p1 # value type semantics means this creates a copy

p1.X = 250
assert 200 == p2.X # copy still unchanged

When p1 is assigned to p2, the contents of p1 are physically copied to p2. If Point were a class (classes are reference types), the assignment would simply copy a pointer to p1 into p2 and the assertion would fail.

Here's the equivalent definition for Point implemented using struct:

import System

struct Point:
    X as int
    Y as int

Note that by default, fields are public in a struct.