Weak NSString variable is not nil after setting th

问题:

I have a problem with this code :

__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);

I'm expecting all pointers to be nil at this time, but they are not and I don't understand why. The first (strong) pointer is nil but the other two are not. Why is that?

回答1:

tl; dr: The problem is that the string literal never gets released so your weak pointer still points to it.

---

Theory

Strong variables will retain the value they point to.

Weak variables won't retain their value and when the value is deallocated they will set their pointer to nil (to be safe).

Unsafe unretained values (as you probably can read by the name) won't retain the value and if it gets deallocated they do nothing about it, potentially pointing to a bad piece of memory

---

Literals and constants

When you create a string using @"literal string" it becomes a string literal that never will change. If you use the same string in many places in your application, it is always the same object. String literals doesn't go away. Using [[NSString alloc] initWithString:@"literal string"] won't make a difference. Since it becomes a pointer to the literal string. It is however worth noting that [[NSString alloc] initWithFormat:@"literal string"]; works differently and will the release its string object.

Line by line:

__strong NSString *yourString = @"Your String"; 

You are creating a strong pointer to a string. This will ensure that the value does not go away. In your case it's a little bit special since the string is a string literal that technically won't be released.

__weak NSString *myString = yourString;

You create a weak pointer to the same thing as your strong pointer. If at this time the strong pointer would point to something else, the value it is pointing to would get deallocated, then the weak pointer would change its value so that it points to nil. Now it still points to the same as the strong pointer.

yourString = nil;

Your strong pointer points to nil. Nothing points to the old string so it should get released if it wasn't for the fact that is was a literal string. If you tried the exact same thing with other objects that you created yourself, the weak variable would change so that it points to nil. But, since the string literal is literal and doesn't go away. The weak variable will still point to it.

__unsafe_unretained NSString *theirString = myString;

A new unretained pointer is created, pointing to your weak pointer which is pointing to the string literal.

NSLog(@"%p %@", yourString, yourString);
NSLog(@"%p %@", myString, myString);
NSLog(@"%p %@", theirString, theirString);

You print all your strings and get confused why the first value is nil but the other two aren't.

---

Related reading:

What's the difference between a string constant and a string literal?

回答2:

David is 100% correct in his answer. I just added four explicit examples using GHUnit.

The lifetime qualifier behavior for object references.

Using NSObject as a proxy for all objects, the behavior of the lifetime qualifiers is as expected.

- (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;
}

The lifetime qualifier behavior for literal NSStrings (@"something").

This is basically the same as test_usingNSObjects, but instead of using a NSObject, a NSString that is assigned a literal string is used. Since literal strings are not destroyed like other objects, different behaviors for __weak and __unsafe_unretained variables are observed.

- (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.");
}

The lifetime qualifier behavior for nonliteral NSStrings.

This is basically the same as test_usingNSObjects, but instead of using a NSObject, a NSString that is assigned a nonliteral string is used. Since the nonliteral strings are destroyed like other objects, the behaviors are the same as observed in test_usingNSObjects.

- (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 creation - literal vs nonliteral.

Shows strings created in various ways if they are literal nor nonliteral.

- (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, i.e. 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, i.e. isEqualToString: .
    GHAssertEqualStrings(literalString, @"literalString", nil);
    GHAssertEqualStrings(referenced, @"literalString", nil);
    GHAssertEqualStrings(copy, @"literalString", nil);
    GHAssertEqualStrings(initWithString, @"literalString", nil);
    GHAssertEqualStrings(initWithFormat, @"literalString", nil);
}

回答3:

the weak property will only be set to nil after the autorelease pool was drained.

try:

@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);