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Reference WrappersDescription: When you want to pass a reference, but are only permitted to pass a copy.
Keywords: ecl reference wrappers
Tutorial Level: INTERMEDIATE
There are situations where you may wish to overload a function with both non-referenced and referenced arguments. Unfortunately, this is not directly possible with c++, it usually results in an ambiguity.
One way to resolve this is to create a wrapper with a pointer to the object inside it (we call this a reference wrapper) and pass it to the function as you would normally pass a copy. Once accepted, the function can use some metaprogramming logic to determine the fate of the accepted copy or reference wrapper.
Compiling & Linking
Include the following at the top of any translation unit:
If outside ros, you will also need to link to ecl_utilities.
The easiest way to generate a reference wrapper is via the convenience functions ref/cref. For example, below illustrates a referenced function object passed to a thread.
1 Thread thread(ref(function_object));
The cref call creates a constant reference, useful when your api will only accept a constant.
Accessing a reference wrapper is straight forward:
It can also be directly passed off as the type it wraps.
Testing for reference wrapper objects can be done via the is_reference_wrapper trait.
1 bool result = is_reference_wrapper<int>::value
This is most useful when used as a template argument metafunction.