Scoping: Local vs Var

问题:

I'm new to CF so this may be a basic question. But I've heard I should use local for objects inside functions due to how scoping works in CF. But what about 'var'? Is var the same as using local?

e.g.

function MyFunction()
{
    local.obj = {};
}

Is this the same as:

function MyFunction()
{
    var obj = {};
}

If they aren't the same, what is the difference between them? And when should I be using either of them?

回答1:

They are very similar, but not exactly the same. Both only exist inside of a function but they work slightly differently.

The var version works it way through all the default variable scopes. See http://help.adobe.com/en_US/ColdFusion/9.0/Developing/WSc3ff6d0ea77859461172e0811cbec09af4-7fdf.html

Local will match only a variable in a local scope. Consider the following

<cffunction name="himom">


    <cfoutput>
      <p><b>try 0:</b> #request_method#</p>

    <!--- you might think that the variable does not exist, but it does because it came from cgi scope --->
</cfoutput> 


   <cfquery name="myData" datasource="Scorecard3">
        SELECT 'This is via query' AS request_method
   </cfquery>
   <!--- Data is now being loaded into a query --->


   <cfoutput query="myData">
      <p><b>try 1:</b> #request_method#</p>
   </cfoutput>
   <!--- This one now came from the query --->        


    <cfset var request_method = "This is Var">
   <!--- We now declare via var ---> 



   <cfoutput query="myData">
      <p><b>try 2:</b> #request_method#</p>
   </cfoutput> 
   <!--- the query version disappears and now the var version is takes presidence --->


    <cfset local.request_method = "This is local">
    <!--- now we declare via local --->

   <cfoutput query="myData">
      <p><b>try 3:</b> #request_method#</p>
   </cfoutput>
<!--- The local method takes presidence --->




   <cfoutput>
   <p><b>try 4:</b> #request_method#</p>
   <!--- in fact it even takes presidence over the var --->


   <p><b>try 5:</b> #local.request_method#</p>
<!--- there is no question where this comes from --->   
   </cfoutput>

</cffunction>


<cfset himom()>

Results of the above

try 0: GET

try 1: This is via query

try 2: This is Var

try 3: This is local

try 4: This is local

try 5: This is local

In summary

When developing, you could use either to make sure that variables only exist inside of a function, but always prefixing your variables with local goes a long way in making sure that your code is clearly understood

回答2:

In ColdFusion 9+, using the local scope and the var directive in a ColdFusion CFC provide the same result.

Ben Forta explains it here: http://forta.com/blog/index.cfm/2009/6/21/The-New-ColdFusion-LOCAL-Scope

I would recommend using the local. notation as it is more explicit.