What happens if OC sends a message to an object that has already been disposed of

What happens if OC sends a message to an object that has already been disposed of

Keywords: MRC, ARC, Exc_bad_instruction, deallocated

First, ask questions

What happens when OC sends a message to an already-freed object.

second, make assumptions

Sends a message to an already freed object, and the program will flash back.

third, verification issues

1. Create MRC Project

To facilitate verification, we use MRC to test.

Create a project, switch the environment to Mrc,target's build settings search, Objective-c Automatic Reference counting, objective-c Automatic Reference Counting is set to No.

2. Create an Object person

Add a Eat method to print "Eat"

3. Send a EAT message to the released Person object

Inside the Viewcontroller.

-(void) viewdidload {
    [super viewdidload];
    
    Person *p = [[Person alloc] init];
    [P eat];
    [P release]; Call release, Release P
    [P eat];//Send a EAT message to the already freed P object
}

(1) Run the program;

(2) again in [P release] This line break point, let the program execute to the previous line, p is not released when the Lldb inside PO p.

4. Results

(1) The running program Flash back to [P eat] this line, the flashback information is as follows:

[P Eat] line error message

Thread 1:exc_bad_instruction (CODE=EXC_I386_INVOP, subcode=0x0)

Lldb inside Log

2018-03-27 16:20:12.558905+0800 runloop[2930:216258] * * *-[person Eat]: message sent to deallocated instance 0x60000000b6 D0

(2) Breakpoint debugging, PO p return

(LLDB) Po P
2018-03-27 16:31:50.075191+0800 runloop[3052:225294] * *-[person respondstoselector:]: Message sent to D eallocated instance 0x60000000b060
0x000060000000b060

(LLDB) 

Iv. Draw Conclusions

Sends a message to an object that has been released in OC, the program flashes directly and reports the following main error message: exc_bad_instruction, message sent to deallocated instance

[Reference exc_bad_access] (http://www.cocoachina.com/ios/20160226/15324.html)