Overloading Procedures and Functions

You can declare more than one routine in the same scope with the same name. This is called overloading. Overloaded routines must be declared with the overload directive and must have distinguishing parameter lists. For example, consider the declarations function Divide(X, Y: Real): Real; overload; begin

Result := X/Y; end function Divide(X, Y: Integer): Integer; overload; begin

These declarations create two functions, both called Divide, that take parameters of different types. When you call Divide, the compiler determines which function to invoke by looking at the actual parameters passed in the call. For example, Divide(6.0, 3.0) calls the first Divide function, because its arguments are real-valued.

You can pass to an overloaded routine parameters that are not identical in type with those in any of the routine's declarations, but that are assignment-compatible with the parameters in more than one declaration. This happens most frequently when a routine is overloaded with different integer types or different real types - for example, procedure Store(X: Longint); overload; procedure Store(X: Shortint); overload;

In these cases, when it is possible to do so without ambiguity, the compiler invokes the routine whose parameters are of the type with the smallest range that accommodates the actual parameters in the call. (Remember that real-valued constant expressions are always of type Extended.)

Overloaded routines must be distinguished by the number of parameters they take or the types of their parameters. Hence the following pair of declarations causes a compilation error.

function Cap(S: string): string; overload; procedure Cap(var Str: string); overload;

But the declarations function Func(X: Real; Y: Integer): Real; overload;

function Func(X: Integer; Y: Real): Real; overload;

are legal.

When an overloaded routine is declared in a forward or interface declaration, the defining declaration must repeat the routine's parameter list.

The compiler can distinguish between overloaded functions that contain AnsiString/PChar and WideString/WideChar parameters in the same parameter position. String constants or literals passed into such an overload situation are translated into the native string or character type, which is AnsiString/ PChar.

procedure test(const S: String); overload; procedure test(const W: WideString); overload; var a: string; b: widestring; begin a := 'a'; b := 'b';

test(a); // calls String version test(b); // calls WideString version test('abc'); // calls String version test(WideString('abc')); // calls widestring version end;

Variants can also be used as parameters in overloaded function declarations. Variant is considered more general than any simple type. Preference is always given to exact type matches over variant matches. If a variant is passed into such an overload situation, and an overload that takes a variant exists in that parameter position, it is considered to be an exact match for the Variant type.

This can cause some minor side effects with float types. Float types are matched by size. If there is no exact match for the float variable passed to the overload call but a variant parameter is available, the variant is taken over any smaller float type.

For example:

procedure foo(i: integer); overload; procedure foo(d: double); overload; procedure foo(v: variant); overload; var v: variant; begin foo(1); // integer version foo(v); // variant version foo(1.2); // variant version (float literals -> extended precision)

end;

This example calls the variant version of foo, not the double version, because the 1.2 constant is implicitly an extended type and extended is not an exact match for double. Extended is also not an exact match for Variant, but Variant is considered a more general type (whereas double is a smaller type than extended).

This typecast does not work. You should use typed consts instead.

The above code works correctly, and calls the double version.

The above code also calls the double version of foo. Single is a better fit to double than to variant.

When declaring a set of overloaded routines, the best way to avoid float promotion to variant is to declare a version of your overloaded function for each float type (Single, Double, Extended) along with the variant version.

If you use default parameters in overloaded routines, be careful not to introduce ambiguous parameter signatures.

You can limit the potential effects of overloading by qualifying a routine's name when you call it. For example, Unit1.MyProcedure(x, y) can call only routines declared in Unit1; if no routine in Unit1 matches the name and parameter list in the call, an error results.

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