Tag Archives: murrayc-tuple-utils

C++17 in libsigc++ : invoke, apply, and constexpr if

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I’ve made a fairly minor 2.99.11 release of the unstable libsigc++, just to make use of C++17. Here are some brief descriptions of the C++17 features used in libsigc++ so far:

std::invoke()

std::invoke() invokes a callable object, such as a plain function pointer, pointer to a member method, or a functor, such as a sigc::slot, using one generic syntax. We provide the arguments, and possibly an object intance, in the std::invoke() call. For instance:

  • std::invoke(function, 1, 2);
  • std::invoke(method, object, 1, 2);
  • std::invoke(functor, 1, 2);

By using std::invoke in libsigc++ (and here), I hope that this makes the implementation more generic though it’s not immediately obvious how.

std::apply()

std::apply() is a little like std::invoke(), but lets us invoke a callable object with the arguments in a tuple. For instance:

  • std::apply(function, std::make_tuple(1, 2));
  • std::apply(method, std::make_tuple(object, 1, 2)); // I think.
  • std::apply(functor, std::make_tuple(1, 2));

This is useful for libsigc++ template code that uses tuples to concatenate or rearrange sets of variadic parameters. It lets us replace several slightly less generic helper functions that just called a  functor with a tuple unpacked as variadic arguments via a provided std::index_sequence. Receiving variadic parameters into a tuple is normal, so this complements that well, letting us use some of those parameters in calls to other functions.

constexpr if

I also used constexpr if to avoid the need for a helper template. Before C++17, templated code could sometimes need to use another helper template to perform different operations depending on the type, sometimes where some operations won’t even compile with the other type. The different operations would then be isolated in different template specializations of the helper template, and the main template could just have one generic call to that helper template.

But constexpr if lets you put it all in the main template. For instance, it let us remove the little with_type<> template, which only existed so we could call a functor for some types, but not for others.

I also updated the tuple-utils from my murrayc-tuple-utils project, including several  uses of constexpr (I also used the new C++17 nested namespace syntax when doing that).

C++ tuple utils

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I’ve recently been been writing lots of modern C++ code with variadic templates. For instance , I’ve been trying to make libsigc++ use variadic templates instead of being a mess of generated code.

I often find myself needing utility functions and type traits to manipulate tuples, but the C++ standard library still only offers std::tuple_cat(). Writing these is awkward and that often stops me from experimenting quickly.

So I’m gradually gathering this code together in a little murrayc-tuple-utils library. I’d gladly change the name if this gets any use. Really, I’m surprised that nothing like this seems to exist already, apart from as part of larger projects such as boost::hana and boost::fusion. But boost is a really awkward dependency and those are larger projects with much grander goals.

So far murrayc-tuple-utils has:

  • tuple_cdr(): Removes the first element.
  • tuple_start<N>(): Takes the first N elements.
  • tuple_end<N>(): Takes the last N elements.
  • tuple_subset<pos, len>: Takes len elements, starting at pos.
  • tuple_interlace<T1, T2>: Takes elements from each tuple, interlacing (or zipping) them together.

For each of them, there are also type traits, such as tuple_type_cdr<>::type, though these are not so necessary now that C++14 has decltype(auto) for return types.

These are just enough code to make things work enough for me when I’m in a rush. I’m sure they can be improved, and maybe this is how to get those patches and pull requests.

The project has a complete autotools build structure, with “make check” tests, Doxygen documentation building, a pkg-config .pc file, etc, so you can try it out, improve it, and add to it, without having to mess around with that stuff.