Detecting BugsUsing Assertions

Ben Scribner

Defining the Problem Bugs exist Unexpected errors happen

Hardware failures Loss of data Data may exist but may be in the wrong format Complex code sometimes has unforeseen effects Users enter incorrect data

The Familiar Solution Standard error checking

Check if a file exists Make sure variables are defined Ensure that data is in the correct format

Ways of handling errors Exceptions Messages to the user Form Validation Method

Error Checking Limitations Cannot check everything

Already takes up a lot of code, using for every situation is impractical

Situations most people do not use error checking Function output Default case in switch statement Else in an if statement Pre / Post Conditions

To Sum Up the Problem Programmers assume many things to be true Often these assumptions are proven incorrect Assumption errors can be difficult to catch These errors can cause other modules to fail

making the problem difficult to trace

Is There Another Method? Assertions

A Boolean expression that is true if an assumption is correct

Could be a comment Often used as actual code to test conditions Strictly used for testing

What Does It Look Like?<cf_assert assertion = “

myAge as a: IsNumeric( |a| ); |a| GT 0; |a| LT 100 myName as b: IsDefined( ‘|b|’ ); Len( |b| ) GT 4”>

<cf_assert assertion = “URL.test as u: IsDefined( ‘|u|’ )”>

<cf_assert assertion = “FORM.test as f: IsDefined( ‘|f|’ )”>

<cf_assert assertion = “todayDate as t: IsNumericDate( |t| )”>

<cf_assert assertion = “myArray as a: NOT ArrayIsEmpty( |a| )”>

How Do I Put One Together?

Assertion := <cf_assert assertion = “Statement”>

Statement := any_variable as any_char: Boolean | Statement Statement

ex: myVar as v

Boolean := (statement returning true or false) | Boolean; Boolean

Note: the character representing the variable is always referred to using pipes

ex. |a|

How Is It Implemented? cf_assert sends whole expression to

assert.cfm assert.cfm is in the custom tags folder

It parses the expression and evaluates it If expression = true, assertion does nothing If expression = false, assertion throws an

exception Exceptions can be caught manually, or

Coldfusion will catch them and display the errors

What Does an Error Return? Without exceptions:

<cfset myVar = “Ben”>

<cf_assert assertion = “myVar as v: IsNumeric( |v| )”>

When I test the code:

Error Occurred While Processing Request

The assertion IsNumeric( myVar ) failed

What Does an Error Return? (cont.) With exceptions

<cfset myVar = “Ben”>

<cftry>

<cf_assert assertion = “myVar as v: IsNumeric( |v| )”>

<cfcatch type=“any”>

your variable is not a number

</cfcatch

</cftry>

When I test the code:

your variable is not a number

Database ExampleData:

PID displayOrder

1 5

3 2

2 7

9 6

<cfquery name=“select” datasource=“myDatabase”>

Select * From myTable

Order By displayOrder

</cfquery>

Database Example (cont.)<cfset array=ArrayNew(2)>

<cfloop query=“select”>

<cfif CurrentRow GT 1>

<cfset dispOrd=array[CurrentRow – 1][2]>

<cf_assert assertion = “array as a: #select.displayOrder# GT #dispOrd#”>

<cfelse>

<cf_assert assertion = “array as a: ArrayIsEmpty( |a| )”>

</cfif>

<cfset array[CurrentRow][1] = #select.PID#>

<cfset array[CurrentRow][2] = #select.displayOrder#>

</cfloop>

Switch ExamplemyVar can be either 1 or 2, nothing else

<cfswitch expression=“#myVar#”>

<cfcase value=1>

<cfinclude template=“myPage1.cfm”>

</cfcase>

<cfcase value=2>

<cfinclude template=“myPage2.cfm”>

</cfcase>

Switch Example (cont.)<cfdefaultcase>

<cf_assert assertion = “myVar as v: |v| IS 1 OR |v| IS 2”>

</cfdefaultcase>

</cfswitch>

Sometimes it is assumed that there is no need for a default case. This is a good place for an assertion

Form ExamplePage1.cfm:

<Form Action=“Page2.cfm” Method=“post”>

Name: <Input Type=“text” Name=“name”><br>

Date of Birth: <Input Type=“text” Name=“dob”><br>

<Input Type=“submit” value=“Calculate”>

</Form>

Form Example (cont.)Page2.cfm

<cfmodule template=“/wwwAdmin/CF_Tags/Validate_Forms_XML.cfm”

FDF_URL=“/benTest/page1.xml”>

<cf_assert assertion = “FORM.name as n: IsDefined( ‘|n|’ );

Len( |n| ) GTE 3

FORM.dob as d: IsDefined( ‘|d|’ ); IsNumericDate( |d| )”>

<cfoutput>

Hello #FORM.name#,

you are #DateDiff(‘YYYY’, FORM.dob, Now())#.

</cfoutput>

Assertion Benefits Can be used to:

Catch logic errors Check results of an operation Detect errors soon after they occur Make statements about the code that are

guaranteed to be true Serve much the same purpose as comments

How They Should Be Used Solely for testing

They are designed to help the programmer find bugs, not to aide the flow of execution

Should be turned off in production code assert.cfm can be an empty file on production, so

there is no need to comment assertions out Should be used in any situation where the

programmer assumes something to be true

The Other Side Assertion Limitations

Errors in assertion logic can lead to misleading problems

They can impact performance Some cases are difficult to test for, some

assertions may never execute They take time to write

What Does All This Mean to Me? The use of assertions can lead to well-tested,

correct code They self-document the code, allowing others

to understand modules more efficiently Programmers will better understand pre/post

conditions Some bugs may be caught before the code is even

tested

In Conclusion Assertions are a great way to aide testing They are by no means a “Silver Bullet”

Using them will not eliminate all bugs Questions?