[llvm] r208067 - Update programmers manual to cover llvm::function_ref, and add a note to the

Richard Smith richard-llvm at metafoo.co.uk
Tue May 6 00:45:39 PDT 2014

Author: rsmith
Date: Tue May  6 02:45:39 2014
New Revision: 208067

URL: http://llvm.org/viewvc/llvm-project?rev=208067&view=rev
Update programmers manual to cover llvm::function_ref, and add a note to the
coding standard suggesting using it instead of the (unavailable) std::function.


Modified: llvm/trunk/docs/CodingStandards.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/CodingStandards.rst?rev=208067&r1=208066&r2=208067&view=diff
--- llvm/trunk/docs/CodingStandards.rst (original)
+++ llvm/trunk/docs/CodingStandards.rst Tue May  6 02:45:39 2014
@@ -108,6 +108,8 @@ unlikely to be supported by our host com
 * Lambdas: N2927_
   * But *not* ``std::function``, until Clang implements `MSVC-compatible RTTI`_.
+    In many cases, you may be able to use ``llvm::function_ref`` instead, and it
+    is a superior choice in those cases.
   * And *not* lambdas with default arguments.
 * ``decltype``: N2343_

Modified: llvm/trunk/docs/ProgrammersManual.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/ProgrammersManual.rst?rev=208067&r1=208066&r2=208067&view=diff
--- llvm/trunk/docs/ProgrammersManual.rst (original)
+++ llvm/trunk/docs/ProgrammersManual.rst Tue May  6 02:45:39 2014
@@ -263,6 +263,78 @@ almost never be stored or mentioned dire
 when defining a function which should be able to efficiently accept concatenated
+.. _function_apis:
+Passing functions and other callable objects
+Sometimes you may want a function to be passed a callback object. In order to
+support lambda expressions and other function objects, you should not use the
+traditional C approach of taking a function pointer and an opaque cookie:
+.. code-block:: c++
+    void takeCallback(bool (*Callback)(Function *, void *), void *Cookie);
+Instead, use one of the following approaches:
+Function template
+If you don't mind putting the definition of your function into a header file,
+make it a function template that is templated on the callable type.
+.. code-block:: c++
+    template<typename Callable>
+    void takeCallback(Callable Callback) {
+      Callback(1, 2, 3);
+    }
+The ``function_ref`` class template
+The ``function_ref``
+(`doxygen <http://llvm.org/doxygen/classllvm_1_1function_ref.html>`__) class
+template represents a reference to a callable object, templated over the type
+of the callable. This is a good choice for passing a callback to a function,
+if you don't need to hold onto the callback after the function returns.
+``function_ref<Ret(Param1, Param2, ...)>`` can be implicitly constructed from
+any callable object that can be called with arguments of type ``Param1``,
+``Param2``, ..., and returns a value that can be converted to type ``Ret``.
+For example:
+.. code-block:: c++
+    void visitBasicBlocks(Function *F, function_ref<bool (BasicBlock*)> Callback) {
+      for (BasicBlock &BB : *F)
+        if (Callback(&BB))
+          return;
+    }
+can be called using:
+.. code-block:: c++
+    visitBasicBlocks(F, [&](BasicBlock *BB) {
+      if (process(BB))
+        return isEmpty(BB);
+      return false;
+    });
+Note that a ``function_ref`` object contains pointers to external memory, so
+it is not generally safe to store an instance of the class (unless you know
+that the external storage will not be freed).
+``function_ref`` is small enough that it should always be passed by value.
+You cannot use ``std::function`` within LLVM code, because it is not supported
+by all our target toolchains.
 .. _DEBUG:
 The ``DEBUG()`` macro and ``-debug`` option

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