4.6 Modules

A common problem in large systems is pollution of the global namespace: over time, as isolated systems are integrated, name clashes become quite likely. Slice provides the module construct to alleviate this problem:

module ZeroC {
    module Client {
        // Definitions here...
    };
    module Server {
        // Definitions here...
    };
};

A module can contain any legal Slice construct, including other module defini­tions. Using modules to group related definitions together avoids polluting the global namespace and makes accidental name clashes quite unlikely. (You can use a well-known name, such as a company or product name, as the name of the outer­most module.)

Slice requires all definitions to be nested inside a module, that is, you cannot define anything other than a module at global scope. For example, the following is illegal:

interface I {   // Error: only modules can appear at global scope
    // ...
};

Definitions at global scope are prohibited because they cause problems with some implementation languages (such as Python, which does not have a true global scope).

Note: Throughout the remainder of this book, you will occasionally see Slice definitions that are not nested inside a module. This is to keep the examples short and free of clutter. Whenever you see such a definition, assume that it is nested in a module.

Modules can be reopened:

module ZeroC {
    // Definitions here...
};

// Possibly in a different source file:

module ZeroC {  // OK, reopened module
    // More definitions here...
};

Reopened modules are useful for larger projects: they allow you to split the contents of a module over several different source files. The advantage of doing this is that, when a developer makes a change to one part of the module, only files dependent on the changed part need be recompiled (instead of having to recompile all files that use the module).

Modules map to a corresponding scoping construct in each programming language. (For example, for C++ and C#, Slice modules map to namespaces whereas, for Java, they map to packages.) This allows you to use an appropriate C++ using or Java import declaration to avoid excessively long identifiers in the source code.