不兼容的指针类型，初始化'SubClass * __ strong'，类型为'BaseClass *' [英] Incompatible pointer types initializing 'SubClass *__strong' with an expression of type'BaseClass *'

问题描述

在objective-C中，为什么我们不能 alloc + init 或 / code>一个具有超类的基类对象，同时我们可以使用超类的构造函数来初始化

下面是一些代码： p>

s1 可以非常舒适地创建。

  NSMutableString * s1 = [NSString string]; 
 NSLog（@％@，s1）; 
  

但 s2 和 s3 不能并且给出警告

不兼容的指针类型用表达式初始化SubClass * __ strong类型'BaseClass *'

  NSMutableString * s2 = [[NSString alloc] init]; 
 NSLog（@％@，s2）; 
 
 NSMutableString * s3 = [NSString new]; 
 NSLog（@％@，s3）; 
 
 //这里没有警告。 
 id mem = [NSString alloc]; 
 NSMutableString * s4 = [mem init]; 
 NSLog（@％@，s4）; 
  

当我们将alloc + init断开为两个不同的语句时会发生什么？

解决方案

答案可以在

相关结果类型

strong>

根据Cocoa约定，具有某些名称
（init，alloc等）的Objective-C方法总是返回
的实例接收类的类型。这样的方法被称为具有相关
结果类型，意味着发送到这些方法中的一个的消息将
具有与接收器类的实例相同的静态类型。

因此在

  NSMutableString * s2 = [[ NSString alloc] init]; 
  

右侧的类型实际上是 NSString * ，而不是 id ，并将其分配给 NSMutableString * ，则会产生一个不兼容的指针类型警告。 / p>

另一方面， string 方法

  NSMutableString * s1 = [NSString string]; 
  

没有相关结果类型，所以它只返回可以分配给 NSMutableString * 。

单独的语句仅在使用 id 作为中间类型时抑制警告。使用 NSString 或 NSMutableString ，您仍会收到警告：

  NSString * tmp4 = [NSString alloc]; 
 NSMutableString * s4 = [tmp4 init]; //<  -  Warning here 
 
 NSMutableString * tmp5 = [NSString alloc]; //<  -  Warning here 
 NSMutableString * s5 = [tmp5 init]; 
  
 
 
 
 
 
 
根据文档，相关结果类型，如果其返回类型与其类的类型兼容并且如果：
 
 
 
 
 
第一个单词是alloc new，并且方法是类方法，或
 
 
第一个词是autorelease，init，retain或self，并且方法是一个实例方法。
 
 
 
In objective-C, why we can not alloc+init or new a base-class object with super-class, whilst we can use constructor of super-class to initialize?

Below is some code :

s1 can be created quite comfortably.
NSMutableString *s1=[NSString string];
NSLog(@"%@",s1);
But s2 and s3 can not be, and gives a warning 

Incompatible pointer types initializing 'SubClass *__strong' with an expression of type'BaseClass *'
NSMutableString *s2=[[NSString alloc] init];
NSLog(@"%@",s2);

NSMutableString *s3=[NSString new];
NSLog(@"%@",s3);

//here no warning.
id mem=[NSString alloc];
NSMutableString *s4=[mem init];
NSLog(@"%@",s4);
What happens when we break alloc + init to two different statement?
解决方案
The answer can be found in Objective-C Features of the Clang 3.3 documentation:

  
Related result types
  
  
According to Cocoa conventions, Objective-C methods with certain names
  ("init", "alloc", etc.) always return objects that are an instance of
  the receiving class’s type. Such methods are said to have a "related
  result type", meaning that a message send to one of these methods will
  have the same static type as an instance of the receiver class.
Therefore in
NSMutableString *s2 = [[NSString alloc] init];
the type of the right hand side is actually NSString * and not id, and assigning that to an NSMutableString * gives a "Incompatible pointer types" warning.

On the other hand, the string method in
NSMutableString *s1 = [NSString string];
does not have a "related result type", so it just returns an id which can be assigned to the NSMutableString *.

Breaking alloc/init into separate statements suppresses the warning only if you use id as intermediate type. With NSString or NSMutableString you still get the warnings:
NSString *tmp4 = [NSString alloc];
NSMutableString *s4 = [tmp4 init]; // <-- Warning here

NSMutableString *tmp5 = [NSString alloc]; // <-- Warning here
NSMutableString *s5 = [tmp5 init];




According to the documentation, a method has a "related result type" if its return type is compatible with the type of its class and if:


the first word is "alloc" or "new", and the method is a class method, or
the first word is "autorelease", "init", "retain", or "self", and the method is an instance method.


                        
这篇关于不兼容的指针类型，初始化'SubClass * __ strong'，类型为'BaseClass *'的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！