Objective-C - incompatible pointer type assignment not detected when used with init method

[llvmbot]

Bugzilla Link	11292
Resolution	INVALID
Resolved on	Nov 08, 2011 13:12
Version	unspecified
OS	All
Attachments	example showing incompatible pointer type assignment not detected when used with init method
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@efriedma-quic

Extended Description

Objective-C - incompatible pointer type assignment not detected when used with init method

When an init method returns a pointer type and when that pointer is assigned to another incompatible pointer variable, the compiler doesn't throw an error / warning

Note - When tested with a method that doesn't have the prefix as "init", compiler manages to detect the error / warning

Actual Behavior

The compiler doesn't throw an error / warning, when pointer type returned by an init method is assigned to an incompatible variable.

Expected Behavior

The compiler should throw an error / warning, when pointer type returned by an init method is assigned to an incompatible variable.

Example (Sample Objective-C program)

Sample program showing incompatible pointer assignment by init method is attached

[efriedma-quic]

This is working as intended; see
http://clang.llvm.org/docs/LanguageExtensions.html#objc_instancetype .

[llvmbot]

Expected Behavior:

The compiler should enforce that all init methods to have the return type as "id". (see documentation) otherwise should throw a warning / error.

Enforcing this would mean that would mean that the init methods would no longer return an parent class pointer type.

Actual Behavior:

Presently the compiler allows init methods to have the return type of the class (class pointer type) that implements it.

Documentation:

The Objective‐C Programming Language (Page 51)

"The return type of an initializer method should be id.
The return type should be id because id gives an indication that the class is purposely not considered—that the class is unspecified and subject to change, depending on context of invocation. For example, NSString provides the method initWithFormat:. When sent to an instance of NSMutableString (a subclass of NSString), however, the message returns an instance of NSMutableString, not NSString."

Present Scenario:

Assignment of a parent class pointer to a derived class pointer variable is incorrect

This is currently being allowed in clang when the init method returns a parent class pointer (as shown in example).

The problem would be solved if the compiler should enforce that all init methods to have the return type as id. (see documentation) otherwise should throw a warning / error.

[efriedma-quic]

Cloned to rdar://problem/10387951 .

[DougGregor]

As noted by Eli, "-init" methods get a related result type, as documented here:

http://clang.llvm.org/docs/LanguageExtensions.html#objc_instancetype

Clang is behaving correctly.

Expected Behavior:

The compiler should enforce that all init methods to have the return type as
"id". (see documentation) otherwise should throw a warning / error.

Enforcing this would mean that would mean that the init methods would no longer
return an parent class pointer type.

Enforcing this would break lots of existing code. It's not feasible.

Actual Behavior:

Presently the compiler allows init methods to have the return type of the class
(class pointer type) that implements it.

... which is perfectly fine, so long as the "-init" methods follow the convention of returning 'self'. Related result type works on existing conventions.

Documentation:

The Objective‐C Programming Language (Page 51)

"The return type of an initializer method should be id.
The return type should be id because id gives an indication that the class is
purposely not considered—that the class is unspecified and subject to change,
depending on context of invocation. For example, NSString provides the method
initWithFormat:. When sent to an instance of NSMutableString (a subclass of
NSString), however, the message returns an instance of NSMutableString, not
NSString."

Present Scenario:

Assignment of a parent class pointer to a derived class pointer variable is
incorrect

This is currently being allowed in clang when the init method returns a parent
class pointer (as shown in example).

The problem would be solved if the compiler should enforce that all init
methods to have the return type as id. (see documentation) otherwise should
throw a warning / error.

Note that there is nothing "wrong" in the code you've pasted. [B alloc] ensures that we get an object of type "B". "initSomeLogic" returns "self", so when it gets a "B" object it returns a "B" object. That the initSomeLogic method is declared as A* doesn't matter here, due to related result type inference (linked above).