gershnik / objc-helpers

Utilities to make coding on Apple platforms in C++ or ObjectiveC++ more pleasant

ObjC-Helpers

An ever-growing collection of utilities to make coding on Apple platforms in C++ or ObjectiveC++ more pleasant

• What's included?
  • Convert ANY C++ callable to a block
  • Coroutines that execute on GCD dispatch queues
  • Boxing of any C++ objects in ObjectiveC ones
  • Comparators for ObjectiveC objects
  • Printing ObjectiveC objects to C++ streams and std::format
  • Accessing NSString/CFString as a char16_t container
  • Conversions between NSString/CFString and char/char16_t/char32_t/char8_t/wchar_t ranges
  • XCTest assertions for C++ objects

What's included?

The library is a collection of mostly independent header files. There is nothing to link with. Simply add these headers to your include path and include them as needed.

sample directory contains a sample that demonstrates the usage of main features.

Convert ANY C++ callable to a block

With modern Clang compiler you can seamlessly convert C++ lambdas to blocks like this:

dispatch_async(someQueue, []() { 
    //do something
})

This works and works great but there are a few things that don't:

• You can only pass a lambda as a block, not any other kind of callable. For example this does not compile:

  struct foo { void operator()() const {} };
  dispatch_async(someQueue, foo{});

• You cannot pass a mutable lambda this way. This doesn't compile either

  dispatch_async(someQueue, []() mutable { 
     //do something
  });

  Neither cannot you pass a block that captures anything mutable (like your lambda) - captured variables are all const
• Your lambda captured variables are always copied into the block, not moved. If you have captures that are expensive to copy - oh well...
• Because of the above you cannot have move-only thinks in your block. Forget about using std::unique_ptr for example.

The BlockUtils.h header gives you an ability to solve all of these problems.

It provides two functions: makeBlock and makeMutableBlock that take any C++ callable as an input and return an object that is implicitly convertible to a block and can be passed to any block-taking API. They (or rather the object they return) have the following features:

• You can wrap any C++ callable, not just a lambda.
• makeBlock returns a block that invokes operator() on a const callable and makeMutableBlock returns a block that invokes it on a non-const one. Thus makeMutableBlock can be used with mutable lambdas or any other callable that provides non-const operator().
• If callable is movable it will be moved into the block, not copied. It will also be moved if the block is "copied to heap" by ObjectiveC runtime or Block_copy in plain C++.
• It is possible to use move-only callables.
• All of this is accomplished with NO dynamic memory allocation

Some examples of their usage are as follows:

//Convert any callable
struct foo { void operator()() const {} };
dispatch_async(someQueue, makeBlock(foo{})); //this moves foo in since it's a temporary

//Copy or move a callable in
foo callable;
dispatch_async(someQueue, makeBlock(callable));
dispatch_async(someQueue, makeBlock(std::move(callable)));

//Convert mutable lambdas
int captureMeByValue;
dispatch_async(someQueue, makeMutableBlock([=]() mutable { 
    captureMeByValue = 5; //the local copy of captureMeByValue is mutable
}));

//Use move-only callables
auto ptr = std::make_unique<SomeType>();
dispatch_async(someQueue, makeBlock([ptr=str::move(ptr)]() {
    ptr->someMethod();
}));

One important thing to keep in mind is that the object returned from makeBlock/makeMutableBlock is the block. It is NOT a block pointer (e.g. Ret (^) (args)) and it doesn't "store" the block pointer inside. The block's lifetime is this object's lifetime and it ends when this object is destroyed. You can copy/move this object around and invoke it as any other C++ callable. You can also convert it to the block pointer as needed either using implicit conversion or a .get() member function. In ObjectiveC++ the block pointer lifetime is not-related to the block object's one. The objective C++ ARC machinery will do the necessary magic behind the scenes. For example:

//In ObjectiveC++
void (^block)(int) = makeBlock([](int){});
block(7); // this works even though the original block object is already destroyed

In plain C++ the code above would crash since there is no ARC magic. You need to manually manage block pointers lifecycle using copy and Block_release. For example:

//In plain C++ 
void (^block)() = copy(makeBlock([](int){}));
block(7); //this works because we made a copy
Block_release(block);

BlockUtil.h also provides two helpers: makeWeak and makeStrong that simplify the "strongSelf" casting dance around avoiding circular references when using blocks/lambdas.

Here is the intended usage:

dispatch_async(someQueue, [weakSelf = makeWeak(self)] () {
    auto self = makeStrong(weakSelf);
    if (!self)
        return;
    [self doSomething];
});

Coroutines that execute on GCD dispatch queues

Header CoDispatch.h allows you to use asynchronous C++ coroutines that execute on GCD dispatch queues. Yes there is this library but it is big, targeting Swift and ObjectiveC rather than C++/[Objective]C++ and has a library to integrate with. It also has more features, of course. Here you get basic powerful C++ coroutine support in a single not very large (~800 loc) header.

Working with coroutines is discussed in greater detail in a separate doc.

Here is a small sample of what you can do:

DispatchTask<int> coro() {

    //this will execute asyncronously on the main queue
    int i = co_await co_dispatch([]() {
        return 7;
    });

    //you can specify a different queue of course
    auto queue = dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0);
    int j = co_await co_dispatch(queue, []() {
        return 42;
    }).resumeOnMainQueue(); //add this to resume back on main queue

    //you can convert ObjC APIs with asynchronous callbacks to couroutines
    auto status = co_await makeAwaitable<int>([](auto promise) {
        NSError * err;
        [NSTask launchedTaskWithExecutableURL:[NSURL fileURLWithPath:@"/bin/bash"]
                                    arguments:@[@"-c", @"ls"]
                                        error:&err
                           terminationHandler:^(NSTask * res){
            promise.success(res.terminationStatus);
        }];
        if (err)
            throw std::runtime_error(err.description.UTF8String);
    }).resumeOnMainQueue();

    //this will switch execution to a different queue
    co_await resumeOn(queue);
}

//coroutines can await other corotines
DispatchTask<int> anotherCoro() {
    int res = co_await coro();

    co_return res;
}

//you can also have asynchronous generators
DispatchGenerator<std::string> generator() {
    co_yield "Hello";
    co_yield "World";
    //in real life you probably will use something like
    //co_yield co_await somethingAsync(); 
}

DispatchTask<int> useGenerator() {
    std::vector<std::string> dest;
    //this will run generator asynchrnously on the main queue
    for (auto it = co_await generator().begin(); it; co_await it.next()) {
        res.push_back(*it);
    }

    //you can also say things like
    //auto it = generator().resumingOnMainQueue().beginOn(queue)
    //to control the running and resuming queues
}

int main() {

    //fire and forget
    anotherCoro();
    useGenerator();

    dispatch_main();
}

This facility can also be used both from plain C++ (.cpp) and ObjectiveC++ (.mm) files.

Boxing of any C++ objects in ObjectiveC ones

Sometimes you want to store a C++ object where an ObjectiveC object is expected. Perhaps there is some NSObject * tag which you really want to put an std::vector in or something similar. You can, of course, do that by creating a wrapper ObjectiveC class that stores std::vector but it is a huge annoyance. Yet another ObjectiveC class to write (so a new header and a .mm file) lots of boilerplate code for init and value access and, after all that, it is going to to be std::vector specific. If you later need to wrap another C++ class you need yet another wrapper.

For plain C structs ObjectiveC has a solution: NSValue that can store any C struct and let you retrieve it back later. Unfortunately in C++ this only works for "trivially copyable" types (which more or less correspond to "plain C structs"). Trying to stick anything else in NSValue will appear to work but likely do very bad things - it simply copies object bytes into it and out! Whether bytes copied out will work as the original object is undefined.

To solve this issue BoxUtil.h provides generic facilities for wrapping and unwrapping of any C++ object in an NSObject-derived classes without writing any code. Such wrapping and unwrapping of native objects in higher-level language ones are usually called "boxing" and "unboxing", hence the name of the header and it's APIs.

The only requirement for the C++ class to be wrappable is having a public destructor and at least one public constructor. The constructor doesn't need to be default - boxing works with objects that need to be "emplaced".

You use it like this:

std::vector<int> someVector{1,2,3};
//this copies the vector into the wrapper
NSObject * obj1 = box(someVector);
//and this moves it
NSObject * obj2 = box(std::move(someVector));
//you can also do this
NSObject * obj3 = box(std::vector<int>{1,2,3});
//and you can emplace the object directly rather than copy or move it
NSObject * obj4 = box<std::vector<int>>(5, 3); //emplaces {3,3,3,3,3}

//You can get a reference to wrapped object
//This will raise an ObjectiveC exception if the type doesn't macth

auto & vec = boxedValue<std::vector<int>>(obj1);
assert(vec.size() == 3);
assert(vec[1] == 2);

The reference you get back is mutable by default. If you want immutability do this
NSObject * immuatbleObj = box<const std::vector<int>>(...any of the stuff above...);

//if your C++ object has a copy constructor the wrapper 
//will implement NSCopying
auto * obj5 = (NSObject *)[obj1 copy];

//this uses operator== if available, which it is
assert([obj1 isEqual:obj3]);

//and this uses std::hash if available
//it will raise an exception if you have operator== but not std::hash!
//as incositent equality and hashing is one of the most common ObjectiveC errors
auto hash = obj1.hash

//you can obtain a sensible description
//it will try to use:
//std::to_string 
//iostream << 
//fall back on "boxed object of type <name of the class>"

auto desc = obj1.description;

//if your object supports <=> operator that returns std::strong_ordering
//you can use compare: method
assert([box(5) compare:box(6)] == NSOrderingAscending);

Comparators for ObjectiveC objects

Header NSObjectUtil.h provides NSObjectEqual and NSObjectHash - functors that evaluate equality and hash code for any NSObject and allow them to be used as keys in std::unordered_map and std::unordered_set for example. These are implemented in terms of isEqual and hash methods of NSObject.

Header NSStringUtil.h provides NSStringLess and NSStringLocaleLess comparators. These allow NSString objects to be used as keys in std::map or std::set as well as used in STL sorting and searching algorithms.

Additionally it provides NSStringEqual comparator. This is more efficient than NSObjectEqual and is implemented in terms of isEqualToString.

Header NSNumberUtil.h provides NSNumberLess comparator. This allows NSNumber objects to be used as keys in std::map or std::set as well as used in STL sorting and searching algorithms.

Additionally it provides NSNumberEqual comparator. This is more efficient than NSObjectEqual and is implemented in terms of isEqualToNumber.

For all comparators nils are handled properly. A nil is equal to nil and is less than any non-nil object.

Printing ObjectiveC objects to C++ streams and std::format

Header NSObjectUtil.h provides operator<< for any NSObject to print it to an std::ostream. This behaves similarly to %@ formatting flag by delegating either to descriptionWithLocale: or to description.

Header NSStringUtil.h provides additional operator<< to print an NSString to an std::ostream. This outputs UTF8String.

Both headers also provide std::formatters with the same functionality if std::format is available in the standard library and fmt::formatter if a macro NS_OBJECT_UTIL_USE_FMT is defined. In the later case presence of <fmt/format.h> or "fmt/format.h" include file is required.

Accessing NSString/CFString as a char16_t container

Header NSStringUtil.h provides NSStringCharAccess - a fast accessor for NSString characters (as char16_t) via an STL container interface. This uses approach similar to CFStringInlineBuffer one. This facility can be used both from ObjectiveC++ and plain C++.

Here are some examples of usage

for (char16_t c: NSStringCharAccess(@"abc")) {
    ...
}

std::ranges::for_each(NSStringCharAccess(@"abc") | std::views::take(2), [](char16_t c) {
    ...
});

Note that NSStringCharAccess is a reference class (akin in spirit to std::string_view). It does not hold a strong reference to the NSString/CFString it uses and is only valid as long as that string exists.

Conversions between NSString/CFString and char/char16_t/char32_t/char8_t/wchar_t ranges

Header NSStringUtil.h provides makeNSString and makeCFString functions that accept:

• Any contiguous range of Chars (including std::basic_string_view, std::basic_string, std::span etc. etc.)
• A pointer to a null-terminated C string of Chars
• An std::initializer_list<Char>

where Char can be any of char, char16_t, char32_t, char8_t, wchar_t

and converts it to NSString/CFString. They return nil on failure.

Conversions from char16_t are exact and can only fail when out of memory. Conversions from other formats will fail also when encoding is invalid. Conversions from char assume UTF-8 and from wchar_t, UTF-32.

To convert in the opposite direction the header provides makeStdString<Char> overloads. These accept:

• NSString */CFStringRef, optional start position (0 by default) and optional length (whole string by default)
• A pair of NSStringCharAccess iterators
• Any range of NSStringCharAccess iterators

They return an std::basic_string<Char>. A nil input produces an empty string. Similar to above conversions from char16_t are exact and conversions to other char types transcode from an appropriate UTF encoding. If the source NSString */CFStringRef contains invalid UTF-16 the output is an empty string.

This functionality is available in both ObjectiveC++ and plain C++

XCTest assertions for C++ objects

When using XCTest framework you might be tempted to use XCTAssertEqual and similar on C++ objects. While this works and works safely you will quickly discover that when the tests fail you get a less than useful failure message that shows raw bytes of the C++ object instead of any kind of logical description. This happens because in order to obtain the textual description of the value XCTAssertEqual and friends stuff it into an NSValue and then query its description. And, as mentioned in BoxUtil.h section, NSValue simply copies raw bytes of a C++ object.

While this is still safe, because nothing except the description is ever done with those bytes the end result is hardly usable. To fix this XCTestUtil.h header provides the following replacement macros:

• XCTAssertCppEqual
• XCTAssertCppNotEqual
• XCTAssertCppGreaterThan
• XCTAssertCppGreaterThanOrEqual
• XCTAssertCppLessThan
• XCTAssertCppLessThanOrEqual

That, in the case of failure, try to obtain description using the following methods:

• If there is an ADL call testDescription(obj) that produces NSString *, use that.
• Otherwise, if there is an ADL call to_string(obj) in using std::to_string scope, use that
• Otherwise, if it is possible to do ostream << obj, use that
• Finally produce "<full name of the type> object" string.

Thus if an object is printable using the typical means those will be automatically used. You can also make your own objects printable using either of the means above. The testDescription approach specifically exists to allow you to print something different for tests than in normal code.