Type Checking - Introduction to Compiler Construction in a Java World - page 132

FIGURE 4.2 The structure of a context.
The pointer surroundingContext points to the context for the surrounding region of
scope; the chain of these pointers is the stack that captures the nested scope in j-- programs.
The pointer compilationUnitContext points to the context for the enclosing compilation
unit, that is, a CompilationUnitContext . In the current definition of j--, there is just one
compilation unit but one might imagine an extension to j-- permitting the compilation of
several files, each of which defines a compilation unit. The pointer classContext points to
the context (a ClassContext ) for the enclosing class. As we shall see below, no names are
declared in a ClassContext but this could change if we were to add nested type declarations
to j--.
A CompilationUnitContext represents the scope of the entire program and contains a
mapping from names to types:
The implicitly declared types, java.lang.Object , and java.lang.String
Imported types
User-defined types, that is, types introduced in class declarations
A ClassContext represents the scope within a class declaration. In the j-- symbol table,
no names are declared here. All members, that is all constructors, methods and fields are
recorded in the Class object that represents the type; we discuss this in the next section.
If we were to add nested type declarations to j--, they might be declared here.
A MethodContext represents the scope within a method declaration. A method's formal
parameters are declared here. A MethodContext is a kind of LocalContext .
A LocalContex t represents the scope within a block, that is, the region between two
curly brackets { and } . This includes the block defining the body to a method. Local
variables are declared here.
Each kind of context derives from (extends) the class Context , which supplies the
mapping from names to definitions ( IDefns ). Because a method defines a local context,
MethodContext extends LocalContext . The inheritance tree for contexts is illustrated in
Figure 4.3