Names and Scoping

Slice has a number of rules regarding identifiers. You will typically not have to concern yourself with these. However, occasionally, it is good to know how Slice uses naming scopes and resolves identifiers.

On this page:

Naming Scope

The following Slice constructs establish a naming scope:

  • the global (file) scope
  • modules
  • interfaces
  • classes
  • structures
  • exceptions
  • parameter lists

Within a naming scope, identifiers must be unique, that is, you cannot use the same identifier for different purposes. For example:

Slice
interface Bad {
    void op(int p, string p);   // Error!
};

Because a parameter list forms a naming scope, it is illegal to use the same identifier p for different parameters. Similarly, data members, operation names, interface and class names, etc. must be unique within their enclosing scope.

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Case Sensitivity

Identifiers that differ only in case are considered identical, so you must use identifiers that differ not only in capitalization within a naming scope. For example:

Slice
struct Bad {
    int    m;
    string M;   // Error!
};

The Slice compiler also enforces consistent capitalization for identifiers. Once you have defined an identifier, you must use the same capitalization for that identifier thereafter. For example, the following is in error:

Slice
sequence<string> StringSeq;

interface Bad {
    stringSeq op();     // Error!
};

Note that identifiers must not differ from a Slice keyword in case only. For example, the following is in error:

Slice
interface Module {      // Error, "module" is a keyword
    // ...
};

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Qualified Names

The scope-qualification operator :: allows you to refer to a type in a non-local scope. For example:

Slice
module Types {
    sequence<long> LongSeq;
};

module MyApp {
    sequence<Types::LongSeq> NumberTree;
};

Here, the qualified name Types::LongSeq refers to LongSeq defined in module Types. The global scope is denoted by a leading ::, so we could also refer to LongSeq as ::Types::LongSeq.

The scope-qualification operator also allows you to create mutually dependent interfaces that are defined in different modules. The obvious attempt to do this fails:

Slice
module Parents {
    interface Children::Child;  // Syntax error!
    interface Mother {
        Children::Child* getChild();
    };
    interface Father {
        Children::Child* getChild();
    };
};

module Children {
    interface Child {
        Parents::Mother* getMother();
        Parents::Father* getFather();
    };
};

This fails because it is syntactically illegal to forward-declare an interface in a different module. To make it work, we must use a reopened module:

Slice
module Children {
    interface Child;                    // Forward declaration
};

module Parents {
    interface Mother {
        Children::Child* getChild();    // OK
    };
    interface Father {
        Children::Child* getChild();    // OK
    };
};

module Children {                       // Reopen module
    interface Child {                   // Define Child
        Parents::Mother* getMother();
        Parents::Father* getFather();
    };
};

While this technique works, it is probably of dubious value: mutually dependent interfaces are, by definition, tightly coupled. On the other hand, modules are meant to be used to place related definitions into the same module, and unrelated definitions into different modules. Of course, this begs the question: if the interfaces are so closely related that they depend on each other, why are they defined in different modules? In the interest of clarity, you probably should avoid this construct, even though it is legal.

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Names in Nested Scopes

Names defined in an enclosing scope can be redefined in an inner scope. For example, the following is legal:

Slice
module Outer {
    sequence<string> Seq;

    module Inner {
        sequence<short> Seq;
    };
};

Within module Inner, the name Seq refers to a sequence of short values and hides the definition of Outer::Seq. You can still refer to the other definition by using explicit scope qualification, for example:

Slice
module Outer {
    sequence<string> Seq;

    module Inner {
        sequence<short> Seq;

        struct Confusing {
            Seq          a;     // Sequence of short
            ::Outer::Seq b;     // Sequence of string
        };
    };
};

Needless to say, you should try to avoid such redefinitions — they make it harder for the reader to follow the meaning of a specification.

Same-named constructs cannot be nested inside each other in certain situations. For example, a module named M cannot (recursively) contain any construct also named M. The same is true for interfaces and classes, which cannot define an operation with the same name as the enclosing interface or class. For example, the following examples are all in error:

Slice
module M {
    interface M { /* ... */ };  // Error!

    interface I {
        void I();               // Error!
    };
};

module Outer {
    module Inner {
        interface outer {       // Error, even if case differs!
            // ...
        };
    };
};

The reason for this restriction is that nested types that have the same name are difficult to map into some languages. For example, C++ and Java reserve the name of a class as the name of the constructor, so an interface I could not contain an operation named I without artificial rules to avoid the name clash.

Similarly, some languages (such as C# prior to version 2.0) do not permit a qualified name to be anchored at the global scope. If a nested module or type is permitted to have the same name as the name of an enclosing module, it can become impossible to generate legal code in some cases.

In the interest of simplicity, Slice prohibits the name of a nested module or type from being the same as the name of one of its enclosing modules.

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Introduced Identifiers

Within a naming scope, an identifier is introduced at the point of first use; thereafter, within that naming scope, the identifier cannot change meaning.

For example:

Slice
module M {
    sequence<string> Seq;

    interface Bad {
        Seq op1();      // Seq and op1 introduced here
        int Seq();      // Error, Seq has changed meaning
    };
};

The declaration of op1 uses Seq as its return type, thereby introducing Seq into the scope of interface Bad. Thereafter, Seq can only be used as a type name that denotes a sequence of strings, so the compiler flags the declaration of the second operation as an error.

Note that fully-qualified identifiers are not introduced into the current scope:

Slice
module M {
    sequence<string> Seq;

    interface Bad {
        ::M::Seq op1(); // Only op1 introduced here
        int Seq();      // OK
    };
};

In general, a fully-qualified name (one that is anchored at the global scope and, therefore, begins with a :: scope resolution operator) does not introduce any name into the current scope. On the other hand, a qualified name that is not anchored at the global scope introduces only the first component of the name:

Slice
module M {
    sequence<string> Seq;

    interface Bad {
        M::Seq op1();   // M and op1 introduced here, but not Seq
        int Seq();      // OK
    };
};

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Name Lookup Rules

When searching for the definition of a name that is not anchored at the global scope, the compiler first searches backward in the current scope of a definition of the name. If it can find the name in the current scope, it uses that definition. Otherwise, the compiler successively searches enclosing scopes for the name until it reaches the global scope. Here is an example to illustrate this:

Slice
module M1 {
    sequence<double> Seq;

    module M2 {
        sequence<string> Seq;   // OK, hides ::M1::Seq

        interface Base {
            Seq op1();          // Returns sequence of string
        };
    };

    module M3 {
        interface Derived extends M2::Base {
            Seq op2();          // Returns sequence of double
        };

        sequence<bool> Seq;     // OK, hides ::M1::Seq

        interface I {
            Seq op();           // Returns sequence of bool
        };
    };

    interface I {
        Seq op();               // Returns sequence of double
    };
};

Note that M2::Derived::op2 returns a sequence of double, even though M1::Base::op1 returns a sequence of string. That is, the meaning of a type in a base interface is irrelevant to determining its meaning in a derived interface — the compiler always searches for a definition only in the current scope and enclosing scopes, and never takes the meaning of a name from a base interface or class.

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Scoping Rules for Parameters and Data Members

A Slice operation creates a new naming scope in which all parameter names must be unique:

Slice
interface I {
    void op1(string p, int P);    // Error, differs only in case
    void op2(int n, out int n);   // Error, duplicate
    void op3(string s, int i);    // OK
};

It's legal for parameters to reuse the names of symbols in enclosing scopes, including the name of the operation, class, interface or module:

Slice
module M {
    sequence<string> Seq;
 
    interface I {
        string query(string query);   // OK to reuse operation name
        void op1(int I);              // OK to reuse name of enclosing type
        void op2(Seq Seq);            // OK to reuse type name
        void op3(int M);              // OK to reuse module name
    };
};

The rules for data members are similar to those of parameters:

  • Structures
    Member names must be unique within the structure.
  • Exceptions
    Member names must be unique within the exception, including any members inherited from base exceptions.
  • Classes
    Member names must be unique within the class, including any members inherited from base classes. Members must not duplicate the names of operations defined by the class or inherited by any base classes or interfaces.

As for parameters, data members can reuse the names of symbols in enclosing scopes. The examples below illustrate these rules:

Slice
module M {
    struct S {
        int i;
        string s;    // OK to reuse name of enclosing type
        long I;      // Error, differs only in case
        bool M;      // OK to reuse module name
    };
 
    interface I {
        void op();
    };
 
    class Base {
        string name;
    };
 
    class C extends Base implements I {
        S S;         // OK to reuse type name
        byte c;      // OK to reuse name of enclosing type
        string op;   // Error, duplicates inherited I::op
        string Name; // Error, differs only in case from Base::name
    };
 
    exception ErrorBase {
        string reason;
    };
 
    exception Error extends ErrorBase {
        long error;  // OK to reuse name of enclosing type
        int reason;  // Error, duplicates inherited ErrorBase::reason
    };
};

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Scoping Rules in Prior Ice Releases

The scoping rules for parameters and data members were more restrictive in Ice 3.5 and earlier releases:

  • A data member cannot have the same name as its enclosing type

    Slice
    class C {
    int c; // Error
};

  • A data member cannot have the same name as its type

    Slice
    module M {
    sequence<string> Seq;
    struct S {
        Seq Seq; // Error, use ::M::Seq as the type instead
    };
};

    You can work around this limitation by using the fully-qualified type name.

  • A parameter cannot have the same name as its operation

    Slice
    void op(int op); // Error

  • A parameter cannot have the same name as its type

    Slice
    module M {
    sequence<string> Seq;
    interface I {
        void op(Seq Seq); // Error, use ::M::Seq as the type instead
    };
};

    You can work around this limitation by using the fully-qualified type name.

You must adhere to these more-restrictive rules if your Slice definitions need to maintain backward compatibility with Ice 3.5 or earlier.

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See Also