弱NSStringvariables不是零后设置唯一强引用为零
我有这个代码的问题：
__strong NSString *yourString = @"Your String"; __weak NSString *myString = yourString; yourString = nil; __unsafe_unretained NSString *theirString = myString; NSLog(@"%p %@", yourString, yourString); NSLog(@"%p %@", myString, myString); NSLog(@"%p %@", theirString, theirString); 我期待所有的指针在这个时候是nil ，但他们不是，我不明白为什么。 第一个（强）指针是nil但其他两个不是。 这是为什么？
TL; 博士：问题是string文字永远不会被释放，所以你的弱指针仍然指向它。
理论
强大的variables将保留它们指向的值。
弱variables不会保留它们的值，当值被释放时，它们将把它们的指针设置为零（为了安全起见）。
不安全的不存在的值（如你可能可以读取的名称）将不会保留该值，如果它被释放，他们什么都不做，可能指向一个坏的内存
文字和常量
当你使用@"literal string"创build一个string时，它将变成一个永远不会改变的string。 如果您在应用程序的许多地方使用相同的string，则它始终是同一个对象。 string文字不会消失。 使用[[NSString alloc] initWithString:@"literal string"]不会有所作为。 因为它变成了一个指向string的指针。 不过值得注意的是[[NSString alloc] initWithFormat:@"literal string"]; 工作方式不同，将释放其string对象。
逐行：
__strong NSString *yourString = @"Your String";
您正在创build一个指向string的强指针。 这将确保价值不会消失。 在你的情况下，它有点特别，因为string是一个string，在技术上不会被释放 。
__weak NSString *myString = yourString;
你创build一个弱指针来指向与强指针相同的东西。 如果此时强指针指向其他的东西，它指向的值将被释放，然后弱指针将改变它的值，使其指向nil 。 现在它仍然指向强指针一样。
yourString = nil;
你强有力的指针指向nil 。 没有指向旧string，所以它应该被释放，如果它不是因为是一个文字string的事实 。 如果您尝试了与您自己创build的其他对象完全相同的内容，则弱variables将会更改，以便指向nil 。 但是，由于string文字是文字，并不会消失。 弱variables仍然指向它。
__unsafe_unretained NSString *theirString = myString;
创build一个新的未保留指针，指向指向string文字的弱指针。
NSLog(@"%p %@", yourString, yourString); NSLog(@"%p %@", myString, myString); NSLog(@"%p %@", theirString, theirString);
你打印所有的string，并弄糊涂为什么第一个值是nil但其他两个不是。
相关阅读：
string常量和string常量之间有什么区别？
大卫在他的回答中是100％正确的。 我刚刚使用GHUnit添加了四个明确的例子。
对象引用的生存期限定符行为。
使用NSObject作为所有对象的代理，生命周期限定符的行为是预期的。
- (void) test_usingNSObjects { NSObject *value1 = [[NSObject alloc] init]; NSObject *value2 = [[NSObject alloc] init]; NSObject *value3 = [[NSObject alloc] init]; __strong NSObject *sRefToValue = value1; __weak NSObject *wRefToValue = value2; __unsafe_unretained NSObject *uRefToValue = value3; value1 = value2 = value3 = nil; GHAssertNotNil(sRefToValue, @"Strong reference to the object that was originally \ assigned to value1. Even though value1 was set to nil, the \ strong reference to the object keeps the object from being \ destroyed."); GHAssertNil(wRefToValue, @"Weak reference to the object that was originally assigned to \ value2. When value2 was set to nil, the weak reference does \ not prevent the object from being destroyed. The weak \ reference is also set to nil."); // Removing the #ifdef and #endif lines will result in a EXC_BAD_ACCESS // signal. Receiving a EXC_BAD_ACCESS signal is the expected behavior for // that code. #ifdef RECIEVE_EXC_BAD_ACCESS GHAssertNotNil(uRefToValue, @"Unsafe unretained reference to the object that was \ originally assigned to value3. When value3 was set to nil, \ the unsafe unretained reference does not prevent the object \ from being destroyed. The unsafe unretained reference is \ unaltered and the reference is invalid. Accessing the \ reference will result in EXC_BAD_ACCESS signal."); #endif // To avoid future EXC_BAD_ACCESS signals. uRefToValue = nil; }
字面NSString s（@“something”）的生存期限定符行为。
这与test_usingNSObjects基本相同，但不是使用NSObject ，而是使用分配了文字string的NSString 。 由于文字string不像其他对象那样被破坏，因此可以观察到__weak和__unsafe_unretainedvariables的不同行为。
- (void) test_usingLiteralNSStrings { NSString *value1 = @"string 1"; NSString *value2 = @"string 2"; NSString *value3 = @"string 3"; __strong NSString *sRefToValue = value1; __weak NSString *wRefToValue = value2; __unsafe_unretained NSString *uRefToValue = value3; value1 = value2 = value3 = nil; GHAssertNotNil(sRefToValue, @"Strong reference to the object that was originally \ assigned to value1. Even though value1 was set to nil, \ literal strings are not destroyed."); GHAssertNotNil(wRefToValue, @"Weak reference to the object that was originally assigned \ to value2. Even though value2 was set to nil, \ literal strings are not destroyed so the weak reference is \ still valid."); GHAssertNotNil(uRefToValue, @"Unsafe unretained reference to the object that was \ originally assigned to value3. Even though value3 was set \ to nil, literal strings are not destroyed so the unsafe \ unretained reference is still valid."); }
nonliteral NSString的生命周期限定符行为。
这与test_usingNSObjects基本相同，但不是使用NSObject ，而是使用分配了非字串的NSString 。 由于nonliteralstring像其他对象一样被销毁，行为与test_usingNSObjects的行为相同。
- (void) test_usingNonliteralNSStrings { NSString *value1 = [[NSString alloc] initWithFormat:@"string 1"]; NSString *value2 = [[NSString alloc] initWithFormat:@"string 2"]; NSString *value3 = [[NSString alloc] initWithFormat:@"string 3"]; __strong NSString *sRefToValue = value1; __weak NSString *wRefToValue = value2; __unsafe_unretained NSString *uRefToValue = value3; value1 = value2 = value3 = nil; GHAssertNotNil(sRefToValue, @"Strong reference to the object that was originally \ assigned to value1. Even though value1 was set to nil, the \ strong reference to the object keeps the object from being \ destroyed."); GHAssertNil(wRefToValue, @"Weak reference to the object that was originally assigned to \ value2. When value2 was set to nil, the weak reference does \ not prevent the object from being destroyed. The weak \ reference is also set to nil."); // Removing the #ifdef and #endif lines will result in a EXC_BAD_ACCESS // signal. Receiving a EXC_BAD_ACCESS signal is the expected behavior for // that code. #ifdef RECIEVE_EXC_BAD_ACCESS GHAssertNotNil(uRefToValue, @"Unsafe unretained reference to the object that was \ originally assigned to value3. When value3 was set to nil, \ the unsafe unretained reference does not prevent the object \ from being destroyed. The unsafe unretained reference is \ unaltered and the reference is invalid. Accessing the \ reference will result in EXC_BAD_ACCESS signal."); #endif // To avoid future EXC_BAD_ACCESS signals. uRefToValue = nil; }
NSString创build – 字面与nonliteral。
显示以各种方式创build的string，如果它们是文字或非文字的。
- (void) test_stringCreation { NSString *literalString = @"literalString"; NSString *referenced = literalString; NSString *copy = [literalString copy]; NSString *initWithString = [[NSString alloc] initWithString:literalString]; NSString *initWithFormat = [[NSString alloc] initWithFormat:@"%@", literalString]; // Testing that the memory addresses of referenced objects are the same. GHAssertEquals(literalString, @"literalString", @"literal"); GHAssertEquals(referenced, @"literalString", @"literal"); GHAssertEquals(copy, @"literalString", @"literal"); GHAssertEquals(initWithString, @"literalString", @"literal"); GHAssertNotEquals(initWithFormat, @"literalString", @"nonliteral - referenced objects' memory addresses are \ different."); // Testing that the objects referenced are equal, ie isEqual: . GHAssertEqualObjects(literalString, @"literalString", nil); GHAssertEqualObjects(referenced, @"literalString", nil); GHAssertEqualObjects(copy, @"literalString", nil); GHAssertEqualObjects(initWithString, @"literalString", nil); GHAssertEqualObjects(initWithFormat, @"literalString", nil); // Testing that the strings referenced are the same, ie isEqualToString: . GHAssertEqualStrings(literalString, @"literalString", nil); GHAssertEqualStrings(referenced, @"literalString", nil); GHAssertEqualStrings(copy, @"literalString", nil); GHAssertEqualStrings(initWithString, @"literalString", nil); GHAssertEqualStrings(initWithFormat, @"literalString", nil); }
在自动释放池被排空之后，弱财产将只设置为零。
尝试：
@autoreleasepool { _strong NSString *yourString = @"Your String"; __weak NSString *myString = yourString; yourString = nil; __unsafe_unretained NSString *theirString = myString; } NSLog(@"%p %@", yourString, yourString); NSLog(@"%p %@", myString, myString); NSLog(@"%p %@", theirString, theirString);
