runtime.type_system

<h2>Type System</h2>

1. Primitive Types

In HipHop, booleans, integers (8-, 16-, 32- and 64-bit) and doubles are
represented as is with C++'s built-in primitive type support:

- bool
- char, short, int, long long
- double

2. String and Array classes

Strings are represented as smart pointers of StringData objects, and arrays are
represented as smart pointers of ArrayData objects:

  class String : public SmartPtr<StringData>
  class Array : public SmartPtr<ArrayData>

We need smart pointers for both classes to implement pass-by-value semantics of
PHP without making real copies of internal data (StringData or ArrayData).

Both classes have also utilized smart pointers to implement copy-on-write. They
will only make real copies, when they are being modified AND reference count is
larger than 1.

3. Object classes

In HipHop, all user classes derive from ObjectData. PHP object variables will
become smart pointers of different types of ObjectData objects.

In PHP, objects are passed by reference automatically. Therefore, SmartPtr is
only helping ObjectData to do reference counting for proper deallocation when
reference count reaches zero.

4. Variant class

Variant type is union of all other types. When type inference fails to find
out a symbol's type, it will automatically have Variant type.

Variant class itself serves as both a "smart pointer wrapper" class and a
countable class, so to make it a compact and reference-count-aware data
structure.


<h2>Pass-by-reference for strings and arrays</h2>

Variables passed by reference are always given Variant type.

A variable passed by reference is strongly bound to its argument variable.
Strong binding is implemented by replacing a variable's original value with a
reference cell holding that value. When another variable is strongly bound to
that variable, it acquires a reference to the cell. The reference cell is
implemented with a Variant. This is to easily pass operations from the outer
variant to the inner variant. Because operations only change the inner variant,
every outer variant that has a reference to it will see the change.