Want a (std::) slice?

1. Intro

Whenever we use data structures that are compile time restricted in terms of extends, we allow for significant optimizations and early error detection in the expense of more cumbersome access style and code maintenance, due to the inevitable introduction of templated code. Users of such data types, algebraic (tuple, pair) or aggregate (array), breathed a sigh of relief (well… maybe not all of them) with the introduction of std::apply in C++17, that handles the boilerplate of unpacking their contents into a callable. To give an example, you should now be able to write your callable like so


  • Remain blissfully unaware of the structure that packs your arguments (the back end doesn’t need to specialize for different argument sources)
  • Use the callable with any of the aforementioned data types without special treatment (the front end needs no boilerplate to unpack the data structures)

So, for example, you can know invoke your callable like so

2. Creating slices

Handy as it may be, this ecosystem of data types and tools, leaves a lot to be desired. First and foremost, the slice operation i.e. the creation of  an entity that captures a subset of some given data. My main motivation for this was to be able to pass slices to the apply function although while researching the topic I found an old freestanding request, which I’ve just answered here (isn’t it a bummer when you invent something only to discover other people have had similar musings?) Below we define the tuple_slice operation that fills this gap and elaborate on the code:

Not to be confused with valarray’s slice, this piece of code does 3 simple things:

  • Protects from invalid instantiations with static asserts
  • Forwards the container (tuple, array, pair) + an integer sequence to the implementation function
  • The implementation function gets every element indexed by the sequence and forwards it as a tuple i.e. lvalues are referenced and rvalues are move constructed

A slice of t can be obtained like so

tuple_slice<I1, I2>(t);

Where [I1, I2) is the exclusive range of the subset.

To be able to create slices that are not zero-based, all we do is create the index sequence out of the cardinality of the desired sequence and then offset the values. This works but a helper “make_integer_range” type would be nice to have:

You can experiment with integer ranges here. Back to the main act then (using make_index_range to simplify the definition of tuple_slice is trivial so we’ll omit it), here are some example uses of tuple_slice:

3. Design considerations

One might ask “Shouldn’t a slice operation on an array create another array?” and this brings us to a generic code paradox: While trying to handle more cases we may end up being more restrictive; The extra effort to create respective make_array and forward_as_array functions is not the problem (there you go if you don’t believe me). Here are the considerations:

  • We cannot create an array of references, hence we’d fall back to creating an array of reference wrappers. Not only does this create an interface mismatch with the tuple case (we need to call get() on a reference wrapper to obtain the value) but between lvalue and rvalue arrays as well: slicing the later would give us move constructed values, hence no  get() method.
  • How would this logic extend to the pair case? Slicing a pair does not give us yet another pair (only in the dummy case where the slice is identical to the original) so the fallback of producing a tuple would be viewed as an exception. To me, the frequency of the word “exception” in the description of a design is inversely proportional to its quality.
  • Slices should handle uniformly the empty case and tuples do this .
  • If there’s a unique sliced type, pattern matching it would require minimum effort.
  • There’s always the option to create arrays out of the slices if we want to (would be interesting to see if the intermediate tuple can be optimized away) .

So, there you go … less is more

4. References

Compose and curry as folds

1. Intro

In a previous post we introduced C++17 fold expressions and described a way to extend them for arbitrary callables. Implementation details don’t matter for what we’re elaborating on here but it should be clear that (given the tools we developed) the following is possible:

(Op<F>(args) + ...)
(... + Op<F>(args))

i.e. performing a right or left fold respectively using the “F” type as the folding operator (the “+” operator is used just to generate the underlying expression templates). By “F” we mean a type that will serve as the reducing function (it generates callable instances) e.g. this:

Such tools enhance the notion of “reduce” being the ultimate production line where new algorithmic skeletons and processes can be forged sometimes to a surprising extend as we’ll see below.

2. On composition

In a nutshell composition is the pointwise application of one function to the result of another to produce a third one. Add another one to the pipeline and the mathematic notation goes something like this:

(\;f\; \circ\ g\; \circ\ y\;)\;(\;x\;)\; \equiv f\;(\;g\;(\;y\;(\;x\;)\;)\;)\;

…and so on and so forth, we should be able to compose as many functions as we want provided that the result of the “inner” functions can be a valid input to the outer.

Now, look at the mathematical notation again. What does the right hand side look like? It’s just folding over the function call operator! Written in pseudo C++ (I say pseudo because C++ folds don’t support arbitrary operators out of the box) it would look like this:

args () ...

where args is a pack of callables we want to compose. Now it all boils down to synthesizing the reducer, and composing two callables is easy enough, by creating a lambda that applies the left side to the result of calling the right side with whatever arguments you pass to it:

We’ve also added a helper compose function to hide the fold generation even though it’s good to know that it takes a right fold to do this. Having this in place you can write things like

One more thing

I might have oversimplified the definition of the Comp structure. Taking move semantics into consideration is not only a matter of efficiency but correctness as well, since preserving value category is essential when using forwarded callables. To ammend this, the returned lambda will do a perfect capture using this helper type:

This type will be used by the (C++14 generalized) lambda capture to acquire either a reference (wrapper) or a moved callable (depending on its value category). So now Comp is written as follows

3. On currying

Currying is the technique of translating the evaluation of a function that takes multiple arguments (or a tuple of arguments) into evaluating a sequence of functions, each with a single argument. In other words it could be described (nitpickers pardon me) by the production of partially applied functions something you see in C++ with certain applications of std::bind. Following the same logic as above we’ll be using the left fold pipeline with the following reducer

Now this is possible

The last piece of the puzzle is constexpr lambdas that would really allow for this to extend to synthesizing constexpr callables and this is on its way for C++17


A runnable example can be found here. Note that this particular example only runs for g++ and I’ll be diving into clang in a later endeavor.