The Ruby Programming Language

Constant Lookup

When a constant is referenced without any qualifying namespace, the Ruby interpreter must find the appropriate definition of the constant. To do so, it uses a name resolution algorithm, just as it does to find method definitions. However, constants are resolved much differently than methods.

Ruby first attempts to resolve a constant reference in the lexical scope of the reference. This means that it first checks the class or module that encloses the constant reference to see if that class or module defines the constant. If not, it checks the next enclosing class or module. This continues until there are no more enclosing classes or modules. Note that top-level or “global” constants are not considered part of the lexical scope and are not considered during this part of constant lookup. The class method Module.nesting returns the list of classes and modules that are searched in this step, in the order they are searched.

If no constant definition is found in the lexically enclosing scope, Ruby next tries to resolve the constant in the inheritance hierarchy by checking the ancestors of the class or module that referred to the constant. The ancestors method of the containing class or module returns the list of classes and modules searched in this step.

If no constant definition is found in the inheritance hierarchy, then top-level constant definitions are checked.

If no definition can be found for the desired constant, then the const_missing method—if there is one—of the ...