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http://clang.llvm.org/docs/BlockLanguageSpec.txt
Looks really cool.
However,
Can anyone enlighten me on this?
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__block is a storage qualifier that can be used in two ways: Marks that a variable lives in a storage that is shared between the lexical scope of the original variable and any blocks declared within that scope. And clang will generate a struct to represent this variable, and use this struct by reference(not by value).
A C block is a block that is shaped like a “C”, so other blocks can fit inside it. These blocks perform the conditions and loops.
A block definition produces an opaque value which contains both a reference to the code within the block and a snapshot of the current state of local stack variables at the time of its definition. The block may be later invoked in the same manner as a function pointer.
Block Literal Expressions produces a reference to a Block with no arguments with no return value. The return type is optional and is inferred from the return statements.
Blocks are, essentially, a way to pass code and scope around as data. They're known in some other languages as closures and anonymous functions.
Here's an article with more details and code examples.
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NanoTech already linked to an explanation of blocks. As for how this relates to C++ let me state my personal opinion: This extension is not useful in C++. Here's why:
Regarding the block reference type: We already have "polymorphic functions" which can carry some state around, see boost::function, tr1::function. C++ will include a polished version of this in its next standard library. The advantage over "C Blocks" is that you don't need to mess with things like Block_copy and Block_release. These polymorphic functions objects are smart enough to do their own memory managing.
Regarding the block literal syntax: It's a nice syntax that allows you to put the code where it "belongs" without the need for much boilerplate code. But the same applies to its C++ counter part: C++0x lambdas. But C++0x lambda feature also allows you to use lambda objects in tight inner loops without high performance costs of function calls due to possible inlining.
Since C++0x lambdas can be also used in situations where performance is an issue and std::function is easier to handle w.r.t. memory management the addition of "C Blocks" to C++ seems redundant. "C blocks" seem to be more tailored to languages that don't support templates or destructors.
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