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Tuple unpacking5/7/2023 Unpacking a subset of fields with a, b = params works because by listing a, b = params you are providing enough information for the macro to create code like: a = params.a If there’s no way to write the code by hand, then there’s no way to implement it via a macro. A macro can’t do anything magical: it just makes it easier to write out code that you could otherwise write out by hand. You have to, somewhere, somehow, specify the set of local variables you want to create. To the original question, I think the answer is no: there’s no efficient way to do this. (this fails because the a created by the macro is a global variable). The solution also only works at global scope: julia> function foo() There is no need to use a macro in Parameters.jl or any other package. On the other hand, I know that we can unpack all the objects if instead of defining a named tuple we define a struct, such as: struct ParamsStructĪny unpacking strategy would be ok. However, I would like to unpack them all: params We can unpack particular components of a particular instance using: julia> params = ParamsNamedTuple() Define a named tuple using ParamsNamedTuple = ( I know that Parameters.jl addresses some related issues, but I have not been able to figure out if it can handle the following case. With this little helper we can implement index_apply as follows.I just wanted to know if it is possible to unpack named tuples in the following context. Conveniently, it also defines an implicit constexpr conversion operator such that we can use an integral_constant object in most places where we need a constant expression of the corresponding value type. Since it carries its value in a template parameter that value is a constant expression that can also be used as a parameter to other templates. The standard library defines the template class integral_constant which encapsulates a static constant of a specified type. Therefore, we cannot use them as template parameters.įortunately, there is an easy way around that problem. to the lambda are ordinary (run-time) values of type size_t. Template parameters can only be constant expressions. The get template takes the index as a template parameter. However, unfortunately, that code will not compile. We have eliminated the need for a specific helper function and can instead rely on one general helper for (hopefully) all cases. Template void function ( Tuple t, index_sequence ) Furthermore, since tuples can have heterogeneous values and C++ is a statically typed language there is no way to dynamically iterate over the values in a generic tuple. It cannot be dynamically generated as e.g. in a for-loop. The index has to be a constant expression. The standard library function get accepts the index as a template parameter (i.e. at compile time) and returns a reference to the value at that index. The difficulty with tuples in C++ is that they can only be indexed at compile time. Build instructions can be found in the Readme file. If you would like to follow along you can find the code examples on GitHub. In this post I will discuss how to deal with tuples with very compact code. The C++14 standard introduced a few features that greatly reduce the necessary boilerplate. Unfortunately, tuples can be a little bit tricky to deal with in a generic fashion. As the documentation says they offer a fixed-size collection of heterogeneous values. C++11 introduced tuples to the C++ standard library.
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