How do generics (Vector) work inside the AVM?

Question

Support for generics (currently only Vector.<*>, and called 'postfix type parameters' by Adobe) was added in Flash Player 10, but the only AVM2 documentation does not describe how these objects are accessed.

Specifically, I noticed a new opcode (0x53) and a new multiname kind (0x1D) that seem relevant, but their usage is not documented.

NB: This question was created with the answer already known as it is more easily found here than on my blog or the Adobe Bug DB.

Answer 1

The reverse engineering work I did on this did not include declaring your own generic types, though it's very likely possible.

References to the declaring (parameterless) generic type (Vector) are made through a regular qualified name (though any multiname should do).

References to a typed generic type (Vector.<int> as opposed to Vector.<>) are made by a new multiname kind (0x1D), which I call GenericName. GenericName has a format like so:

[Kind] [TypeDefinition] [ParamCount] [Param1] [Param2] [ParamN]

Where:

• [TypeDefinition] is a U30 into the multiname table
• [ParamCount] is a U8 (U30?) of how many type parameters there are
• [ParamX] is a U30 into the multiname table.

Obviously generics are not generally supported yet, so ParamCount will always be 1 (for Vector.<*>).

The other interesting thing is how instances of the class are created. A new opcode was added in Flash 10 (0x53), which I will call MakeGenericType. MakeGenericType is declared with the following stack:

TypeDefinition, ParameterType1, ParameterTypeN -> GenericType

It also has one parameter, a U8 (U30?) specifying how many parameters are on the stack. You will generally see MakeGenericType being used like this:

GetLex [TypeDefinitionMultiname]
GetLex [ParameterTypeMultiname]
MakeGeneric [ParamCount]
Coerce [GenericNameMultiname]
Construct [ConstructorParamCount]

So if you had the following...

GetLex __AS3__.vec::Vector
GetLex int
MakeGeneric 1
Coerce __AS3__.vec::Vector.<int>
Construct 0

You would now have an instance of Vector.<int>