In the process of development, I have been skeptical of the knowledge of modifying external objects inside functions, and I do not know under what circumstances can be modified, and under what circumstances can not be modified, so I carried out practical tests by way of demo.

Related knowledge: deep copy, shallow copy, mutable object, immutable object, container, etc.

Shallow copy means that the copied object and the original object point to the same address, that is, the same memory space.

A deep copy is a true copy of an object that points to a new address, that is, a different memory space.

Here are a few representative examples

string

NSString *str = @" string";
NSLog(STR =%@, object address =%p, pointer address =%p \n",str,str,&str);
[self changeStrValue:str];
NSLog(@" after method call STR =%@, object address =%p, pointer address =%p \n",str,str,&str);

- (void)changeStrValue:(NSString *)string{
    NSLog(STR =%@, object address =%p, pointer address =%p \n",string,string,&string);
    string = @" Changed string";
    NSLog(STR =%@, object address =%p, pointer address =%p \n",string,string,&string);
}
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Print:

2021- 10- 19Method call before STR = string, object address =0x1010e6068, pointer address =0x7ffeeeb1a0f0 
2021- 10- 19Method before executing STR = string, object address =0x1010e6068, pointer address =0x7ffeeeb1a0b8 
2021- 10- 19Method STR = changed string, object address =0x1010e61c8, pointer address =0x7ffeeeb1a0b8 
2021- 10- 19After the method call STR = string, object address =0x1010e6068, pointer address =0x7ffeeeb1a0f0 
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An array of

NSMutableArray *arr = [NSMutableArray arrayWithArray:@[@1The @2]].NSLog(STR =%@, object address =%p, pointer address =%p \n",arr,arr,&arr);
[self changeArray:arr];
NSLog(@" after method call STR =%@, object address =%p, pointer address =%p \n",arr,arr,&arr);
NSLog(@ "% @",arr);

- (void)changeArray:(NSMutableArray *)arr{
    NSLog(STR =%@, object address =%p, pointer address =%p \n",arr,arr,&arr);
    [arr addObject:@3];
    NSLog(STR =%@, object address =%p, pointer address =%p \n",arr,arr,&arr);
}
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Print:

2021- 10- 19Before method call STR =(1.2), object address =0x60000248cf00, pointer address =0x7ffeee6280e0 

2021- 10- 19Method before executing STR =(1.2), object address =0x60000248cf00, pointer address =0x7ffeee628088 

2021- 10- 19Method after executing STR =(1.2.3), object address =0x60000248cf00, pointer address =0x7ffeee628088 

2021- 10- 19After method call STR =(1.2.3), object address =0x60000248cf00, pointer address =0x7ffeee6280e0 

2021- 10- 19  ( 1.2.3 )
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Custom object

@interface Model : NSObject

@property (nonatomic , strong) NSString  * name;
@property (nonatomic , assign) NSInteger    age;

@end
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 Model *m = [[Model alloc] init];
 m.name = @"jim";
 m.age = 1;
 NSLog(@" m=%@, object address =%p, pointer address =%p \n",m,m,&m);
 [self changeModel:m];
 NSLog(@" m=%@, object address =%p, pointer address =%p \n",m,m,&m);

- (void)changeModel:(Model *)model{
    NSLog(STR =%@, object address =%p, pointer address =%p \n",model,model,&model);
    model.age = 2;
    model.name = @"ddd";
    NSLog(STR =%@, object address =%p, pointer address =%p \n",model,model,&model);
}

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Print:

2021- 10- 19M =<Model:0x60000284cca0>, object address =0x60000284cca0, pointer address =0x7ffeebde50f8 

2021- 10- 19STR =<Model:0x60000284cca0>, object address =0x60000284cca0, pointer address =0x7ffeebde50c8 

2021- 10- 19Method after execution STR =<Model:0x60000284cca0>, object address =0x60000284cca0, pointer address =0x7ffeebde50c8 

2021- 10- 19M =<Model:0x60000284cca0>, object address =0x60000284cca0, pointer address =0x7ffeebde50f8 
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Deep understanding of “deep copy” and “shallow copy” in iOS

On the OC by reference to pass the skills

Value passing and reference passing in Objective-C