Type qualifiers are heavily used in C and Objective C. In C99 there are three type qualifiers: const, restrict, and volatile. In objective C, Apple introduced __weak, __strong, __unsafe_unretained, and __autoreleasing for automatic reference counting.

It is easy to get confused with complicated type qualifiers. For example:

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char* const * x;  // x is a pointer to const pointer to char

In this post I’ll go through what type qualifiers are, and how do we read and write it in correct way.

Names and definitions

Type qualifiers should not be confused with storage specifiers like static auto, extern and register. To illustrate this, allow me to use Mike Ash’s example:

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static const int x;

Here, static is a storage specifier to tell complier how x is stored, and const is a type qualifier to tell complier the type of x is read-only data. Since const is part of the type, you can write:

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typedef const int MyInt;

but you cannot write:

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typedef static int MyInt;

This is because static is not part of the type.

You might wonder: is __block a type qualifier or storage specifier? The clang block language spec said that it is a storage qualifier. Don’t get confused. __block is a storage qualifer/specifer which modifies how variable is stored. For more curious on __block, you can check out my previous post Block byref internals.

ARC ownership qualifiers

What about __strong, __weak, __unsafe_unretained, and __autoreleasing? You can use it with typedef, and they are truly part of the type. However, they are a bit different from C type qualifiers. ARC generated code have an runtime API supports it. You can manually use some of those: objc_storeWeak, objc_destroyWeak…etc. In Clang specification, these qualifiers are named ownership qualifiers. Luckily, they share the same rule of type qualifiers.

The grammar

C declaration can be really complicated. In this section I’ll just cover the basics and the most commonly seen ones.

• Rule 1: find the identifier (the variable), read from right to left.

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int x;                // x is int
int * x;              // x is a 'pointer to' int
int * * x;            // x is a 'pointer to' 'pointer to' int

When there is a type qualifier, it applies to its immediate left:

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int const x;          // x is a const int
int * const x;        // x is a 'const pointer to' int
int const * x;        // x is a 'pointer to' const int
int * const * x;      // x is a 'pointer to' 'const pointer to' int

• Rule 2: If next to type specifier, it applies to type-specifier

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const int x;          // x is a const int
const int * const x;  // x is a 'const pointer to' const int

• Rule 3: If there are parenthesis or bracelets, reorder it to postfix form:

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int * const * ( * p)();
// postfix: p * () * const * int
// p is a pointer to a function returning a
// pointer to const-pointer-to-int

For more curious, checkout Deciphering Complex C Declarations and cdecl.

volatile, restrict

volatile

Every reference to the variable will reload the contents from memory rather than take advantage of situations where a copy can be in a register.

The volatile qualifier maintains consistency of memory access to data objects. Volatile variable are read from memory each time their values is needed, and writen back to memory each time they are changed. However, volatile variables are not automic. If you want to write thread safe operation, you can write something like:

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static __inline bool OSAtomicCompareAndSwapInt(int oldi, int newi, int volatile *dst) {
    int original = InterlockedCompareExchange(dst, newi, oldi);
    return (original == oldi);
}

This is a function that is thread and multiprocessor safe to swap/update an integer. Objective-C runtime uses these functions defined in OSAtomic.h to manage retain counts.

restrict

restrict is a keyword purely for the purpose of optimization.

In the C programming language, as of the C99 standard, restrict is a keyword that can be used in pointer declarations. The restrict keyword is a declaration of intent given by the programmer to the compiler. It says that for the lifetime of the pointer, only it or a value directly derived from it (such as pointer + 1) will be used to access the object to which it points. This limits the effects of pointer aliasing, aiding caching optimizations.

restrict is a qualifier for pointers. It claims that the memory that pointer points to can only be accessed by this pointer. Consider this case:

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char *src, *dst;
for(int i = 0; i < len; i++)
    dst[i] = src[i];

If the *dst overlapped with *src, compiler can only generate code that load a small piece of memory and operate it once at a time. Fortran does not have this problem because it does not have pointers. Thus Fortran can do ambitious optimization to load a big chunck of memory and operate it all at once. restrict is a new keyword defined in C99 to address this problem. The original code can be rewritten as:

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void *memcpy(void *restrict s1, const void *restrict s2, size_t n);

and compiler can optimize this code like Fortran does!

Note that restrict is a type qualifier for pointers.

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int * restrict x; // correct
int restrict * x; // wrong
restrict int * x; // wrong

ARC ownership qualifiers

If you understand all the above, then Objective-C Automatic Reference Counting qualifiers should be easy to you! Here is the definition from Apple:

1. __strong is the default. An object remains alive as long as there is a strong pointer to it.
2. __weak specifies a reference that does not keep the referenced object alive. A weak reference is set to nil when there are no strong references to the object.
3. __unsafe_unretained specifies a reference that does not keep the referenced object alive and is not set to nil when there are no strong references to the object. If the object it references is deallocated, the pointer is left dangling.
4. __autoreleasing is used to denote arguments that are passed by reference (id *) and are autoreleased on return.

All ownership qualifiers should decorate on Objective-C object pointers.

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MyClass * __weak _weakSelf = self;    // correct
MyClass __weak * _weakSelf = self;    // wrong! May cause serious bugs!
__weak MyClass * _weakSelf = self;    // wrong!

// Explicitly transfer CGColorRef ownership from UIColor to layer's backgroundColor
CALayer* layer = [CALayer layer];
CGColorRef redRef;
@autorelease{
    UIColor * __autoreleasing redColor = [[UIColor redColor] colorWithAlphaComponent:.5f];
    redRef = CFRetain([redColor CGColor]);
} // UIColor released
layer.backgroundColor = redRef;
CFRelease(redRef);

For more curious on why do we need to write verbose code for CGColor, you can take a look at Big Nerd Ranch’s ARC Gotcha – Unexpectedly Short Lifetimes and Amatten’s ARC Best Practices.

References

• Deciphering complex c declarations
• Mike Ash Friday QA: Type qualifiers in C part 1
• ARC Gotcha – Unexpectedly Short Lifetimes and Amatten’s
• ARC Best Practices
• Block language spec
• Clang ARC spec
• Transitioning to ARC Release Notes