Overloading operators in VHDL

Introduction

Overloading operators lets you use normal symbols like + or = with your own data types. It makes your code easier to read and write.

When you create a new data type and want to add two values of that type using +

When you want to compare two custom objects using = or /=

When you want to define how operators work for records or arrays you made

When you want to make your code look clean and natural with your own types

Syntax

VHDL

function "operator_symbol" (left_operand : operand_type; right_operand : operand_type) return result_type is
begin
    -- your code to define operation
end function;

Use double quotes around the operator symbol, like "+" or "=".

The function must be declared in a package or architecture where the types are visible.

Examples

This example shows overloading + for integers (usually built-in, but for illustration).

VHDL

function "+" (a, b : integer) return integer is
begin
    return a + b;
end function;

This example adds two arrays element-wise using + operator.

VHDL

type my_vector is array (0 to 3) of integer;

function "+" (v1, v2 : my_vector) return my_vector is
    variable result : my_vector;
begin
    for i in v1'range loop
        result(i) := v1(i) + v2(i);
    end loop;
    return result;
end function;

Sample Program

This program defines a custom vector type and overloads the + operator to add two vectors element-wise. It then adds two vectors and reports the results.

VHDL

library ieee;
use ieee.std_logic_1164.all;

package my_pkg is
    type my_vector is array (0 to 1) of integer;
    function "+" (v1, v2 : my_vector) return my_vector;
end package my_pkg;

package body my_pkg is
    function "+" (v1, v2 : my_vector) return my_vector is
        variable result : my_vector;
    begin
        for i in v1'range loop
            result(i) := v1(i) + v2(i);
        end loop;
        return result;
    end function;
end package body my_pkg;

entity test_overload is
end entity test_overload;

architecture behavior of test_overload is
    use work.my_pkg.all;
    signal a : my_vector := (0 => 1, 1 => 2);
    signal b : my_vector := (0 => 3, 1 => 4);
    signal c : my_vector := (0 => 0, 1 => 0);
begin
    process
    begin
        c <= a + b;
        wait for 10 ns;
        report "c(0) = " & integer'image(c(0));
        report "c(1) = " & integer'image(c(1));
        wait;
    end process;
end behavior;

Important Notes

Operator overloading must be done carefully to keep code clear.

Not all operators can be overloaded in VHDL; check the language rules.

Overloaded operators help when working with custom data types like records or arrays.

Summary

Overloading operators lets you define how symbols like + work with your own types.

You write a function with the operator name in quotes to do this.

This makes your code easier to read and use with custom data.

Practice

1. What does overloading an operator in VHDL allow you to do?

easy

A. Create new operators not in VHDL

B. Change the syntax of VHDL keywords

C. Automatically optimize your code for speed

D. Define how operators like + work with your own data types

Solution

1. Step 1: Understand operator overloading concept

   Operator overloading lets you specify how existing operators behave with custom types.

2. Step 2: Identify correct purpose

   It does not create new operators or change syntax, but defines behavior for existing ones.

3. Final Answer:

   Define how operators like + work with your own data types -> Option D

4. Quick Check:

   Operator overloading = custom operator behavior [OK]

Hint: Overloading means customizing operator behavior for your types [OK]

Common Mistakes:

• Thinking overloading creates new operators
• Confusing overloading with syntax changes
• Assuming it optimizes code automatically

2. Which of the following is the correct way to declare an overloaded operator function for + in VHDL?

easy

A. operator + (a, b : my_type) return my_type is

B. function "+" (a, b : my_type) return my_type is

C. procedure "+" (a, b : my_type) return my_type is

D. function add (a, b : my_type) return my_type is

Solution

1. Step 1: Recall VHDL operator overloading syntax

   In VHDL, overloaded operators are declared as functions with the operator symbol in quotes.

2. Step 2: Match correct syntax

   function "+" (a, b : my_type) return my_type is uses function with operator in quotes, correct for overloading.

3. Final Answer:

   function "+" (a, b : my_type) return my_type is -> Option B

4. Quick Check:

   Operator function syntax = function "+" (...) [OK]

Hint: Use function with operator symbol in quotes for overloading [OK]

Common Mistakes:

• Using 'operator' keyword instead of function
• Using procedure instead of function
• Not quoting the operator symbol

3. Given the following VHDL code snippet, what is the output of result?

type my_int is range 0 to 255;
function "+" (a, b : my_int) return my_int is
begin
  return a + b;
end function;

signal x, y : my_int := 10;
signal result : my_int;

-- In a process:
result <= x + y;

medium

A. 20

B. 10

C. Signal assignment error

D. Compilation error due to function

Solution

1. Step 1: Understand overloaded + function

   The function adds two my_int values and returns their sum.

2. Step 2: Calculate result of x + y

   x and y are both 10, so 10 + 10 = 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   10 + 10 = 20 [OK]

Hint: Overloaded + returns sum, so 10 + 10 = 20 [OK]

Common Mistakes:

• Confusing signal assignment delay with immediate value
• Assuming function causes error
• Ignoring overloaded operator effect

4. Identify the error in this overloaded operator function declaration:

function "+" (a : my_type, b : my_type) return my_type is
begin
  return a + b;
end function;

medium

A. Parameters should be separated by semicolons, not commas

B. Return type missing

C. Operator symbol should not be in quotes

D. Function must be declared as a procedure

Solution

1. Step 1: Check parameter list syntax

   In VHDL, parameters in function declarations are separated by semicolons, not commas.

2. Step 2: Identify correct error

   The code uses commas between parameters, which is invalid syntax.

3. Final Answer:

   Parameters should be separated by semicolons, not commas -> Option A

4. Quick Check:

   Parameter separator = semicolon [OK]

Hint: Use semicolons between parameters in function declarations [OK]

Common Mistakes:

• Using commas instead of semicolons between parameters
• Removing quotes around operator symbol
• Confusing function with procedure

5. You want to overload the * operator for a custom record type vector2d to perform dot product. Which of the following is the correct approach?

hard

A. Define a function "*" that returns a vector2d by adding the inputs

B. Define a procedure "*" that modifies one vector2d input

C. Define a function "*" that takes two vector2d inputs and returns a scalar result

D. Use the built-in * operator without overloading

Solution

1. Step 1: Understand dot product operation

   Dot product takes two vectors and returns a scalar (number), not a vector.

2. Step 2: Match correct function signature

   Overloaded operator * should be a function taking two vector2d and returning scalar.

3. Step 3: Eliminate incorrect options

   Procedure cannot return value; built-in * won't work for custom type; returning vector by adding is wrong.

4. Final Answer:

   Define a function "*" that takes two vector2d inputs and returns a scalar result -> Option C

5. Quick Check:

   Dot product = function * returning scalar [OK]

Hint: Dot product returns scalar, so overload * as function returning scalar [OK]

Common Mistakes:

• Using procedure instead of function for operator
• Returning vector instead of scalar for dot product
• Assuming built-in operator works for custom types