Function overloading

Function overloading (also method overloading) is a programming concept that allows programmers to define two or more functions with the same name and in the same scope.

Each function has a unique signature (or header), which is derived from:

1. function/procedure name
2. number of arguments
3. arguments' type
4. arguments' order
5. arguments' name
6. return type

Please note: Not all above signature options are available in all programming languages.

Warning: Function overloading is often confused with function overriding. In Function overloading, a function with a different signature is created, adding to the pool of available functions. In function overriding, however, a function with the same signature is declared, replacing the old function in the context of the new function.

Since functions' names are in this case the same, we must preserve uniqueness of signatures, by changing something from the parameter list (last three alienees).

If the functions' signatures are sufficiently different, the compiler can distinguish which function was intended to be used at each occurrence. This process of searching for the appropriate function is called function resolution and can be quite an intensive one, especially if there are a lot of equally named functions.

Programming languages supporting implicit type conventions usually use promotion of arguments (i.e. type casting of integer to floating-point) when there is no exact function match. The demotion of arguments is rarely used.

When two or more functions match the criteria in function resolution process, an ambiguity error is reported by compiler. Adding more information for the compiler by editing the source code (using for example type casting), can address such doubts.

The example code shows how function overloading can be used. As functions do practically the same thing, it makes sense to use function overloading.

function Generate_Number (MaxValue : Integer) return Integer is
   subtype Random_Type is Integer range 0 .. MaxValue;
   package Random_Pack is new Ada.Numerics.Discrete_Random (Random_Type);
 
   G : Random_Pack.Generator;
begin
   Random_Pack.Reset (G);
   return Random_Pack.Random (G);
end Generate_Number;


function Generate_Number (MinValue : Integer;
                          MaxValue : Integer) return Integer
is
   subtype Random_Type is Integer range MinValue .. MaxValue;
   package Random_Pack is new Ada.Numerics.Discrete_Random (Random_Type);
 
   G : Random_Pack.Generator;
begin
   Random_Pack.Reset (G);
   return Random_Pack.Random (G);
end Generate_Number;

The first code block will generate numbers from 0 up to specified parameter MaxValue. The appropriate function call is:

 Number_1 : Integer := Generate_Number (10);

The second requires another parameter MinValue. Function will return numbers above or equals MinValue and lower than MaxValue.

 Number_2 : Integer := Generate_Number (6, 10);

Ada supports all six signature options, but if you use the arguments' name as option, you will always have to name the parameter when calling the function. i.e.:

Number_2 : Integer := Generate_Number (MinValue => 6,
                                       MaxValue => 10);

Note that you cannot overload a generic procedure or generic function within the same package. The following example will fail to compile:

 package myPackage
   generic
     type Value_Type is (<>);  
   -- The first declaration of a generic subprogram
   -- with the name "Generic_Subprogram"
   procedure Generic_Subprogram (Value : in out Value_Type);
   ...
   generic
     type Value_Type is (<>); 
   -- This subprogram has the same name, but no
   -- input or output parameters. A non-generic
   -- procedure would be overloaded here.
   -- Since this procedure is generic, overloading
   -- is not allowed and this package will not compile.
   procedure Generic_Subprogram;
   ...
   generic
     type Value_Type is (<>); 
   -- The same situation.
   -- Even though this is a function and not
   -- a procedure, generic overloading of
   -- the name "Generic_Subprogram" is not allowed.
   function Generic_Subprogram (Value : Value_Type) return Value_Type;
 end myPackage;

• Ada Programming
• Ada Programming/Subprograms

• 6.6: Overloading of Operators ^[Annotated]
• 8.6: The Context of Overload Resolution ^[Annotated]

• 6.6: Overloading of Operators ^[Annotated]
• 8.6: The Context of Overload Resolution ^[Annotated]