"Unrecognized selector sent to instance", even though XCode says the instance is the right type

Question

In a mac desktop app, I'm getting an "Unrecognized selector sent to instance" error.

Here's the line that causes the error:

    My_WebView *mvw = [mWebView LoadHtml];

And here's the relevant class definition:

My_WebView.h

    #import <WebKit/WebKit.h>  

    @interface My_WebView : WebView
    {
    }

    - (My_WebView *) LoadHtml; // initialize the WebView with a page

    @end

My_WebView.mm

    #import  "My_WebView.h"

    @implementation My_WebView

    - (My_WebView *) LoadHtml
    {
        NSLog(@"Loading HTML...");
        // do stuff...
        return self;
    }

    @end

When I run my code in the XCode debugger and break at the offending line, I'm told that mWebView is of the expected type (My_WebView). This rules out the problems identified in the many similar questions I've seen here.

Does anyone know what I'm doing wrong?

How are you instantiating mWebView? Are you checking that mWebView is not nil? — neilco, Sep 24 '13 at 20:30
Did you actually log the instance, the type you saw may be a lie... — Wain, Sep 24 '13 at 20:32
I'll try that... how do I log the instance? (I'm a major Cocoa newbie.) — dB', Sep 24 '13 at 20:32
Re: logging, ok, got it: `NSLog(@"mWebView: %@", mWebView);` I'll try that and get back to you. — dB', Sep 24 '13 at 20:46
Or from the debugger, `po mWebView` (assuming that you're in a stack frame where `mWebView` is accessible). You can log the result of any expression using either `po` (for objects) or `p` (for non-objects). — Chris Devereux, Sep 24 '13 at 20:47

dB' · Accepted Answer · 2013-09-26T18:39:32.913

I figured it out. I was confused. The instance wasn't the right type!

The XCode debugger said that mWebView was a pointer to a My_WebView.

    mWebView    My_WebView *    0x000000010e0884c0

This led me to think, wrongly, that My_WebView was also the runtime type of mWebView. It wasn't, as NSLog(@"mWebView: %@", mWebView); and po mWebView revealed.

     mWebView: <WebView: 0x1180ca4c0>

mWebView was actually just a plain old Cocoa WebView. That's why it wasn't responding to a LoadHTML message. Problem solved.

"Unrecognized selector sent to instance", even though XCode says the instance is the right type

1 Answers1