C++20 is slated to receive Ranges, which is probably the most significant library update since the STL itself was introduced.

One of Ranges’ most prominent feature is that of pipeline style. Suppose we have an extremely simple task: We want to get the names of every child that is below the age of 14. Here’s the traditional rote implementation:

vector < string > child_names ; for ( auto & person : all_people ) { if ( person . age < 14 ) { child_names . push_back ( person . name ); } }

Looks pretty standard. We declare the vector outside of the loop, then we iterate over ever person object, and if it matches our criterion we will append that person’s name to our list.

Here is some equivalent code using ranges:

auto children_names = all_people | filter ([]( const auto & person ) { return person . age < 14 ; }) | transform ([]( const auto & person ) { return person . name ; }) | to_vector ;

There are a few significant differences, both technical and semantic:

Semantically: We are now very explicit about what we are doing. It is simple to understand this pipeline to be “filter” then “transform,” because that is literally the order that these operations appear in the source. We can consider that almost all of computing activity is about the manipulation of sequences. The ranges+pipeline style brings to the forefront the primary concepts of dealing with sequences or “ranges.” We can see that we begin with all_people The semantics of filter is that of “Allow some of the elements of the range, but ignore others.” Feed the filtered elements into transform , the semantics of which are that of “Replace each element in the sequence with the result of feeding it to some mapping/projection.” Take the result of the transform and generate a std::vector from every element. We use to_vector to immediately convert the range into a std::vector , but this step is not even necessary in some cases. Without to_vector , the range is “lazy” and individual values are fed from the first step ( all_people ) through the pipelines ( filter -> transform ) as a user walks through the range.

The concept of “feed value A into B ” is not new. We’ve been doing it since the creation of the subroutine:

auto user = get_user (); print_info ( user );

This is a simple pipeline from get_user() -> print_info() . We can, of course, compose additional operations that act on the intermediate values:

auto user = get_user (); auto group = get_primary_group ( user ); print_info ( group );

And now have a pipeline from get_user() -> get_primary_group() -> print_info() . You’ll also note that the print_info at the final step isn’t the same as the print_info from the prior sample: In this case, we are print_info -ing a “group” object rather than a “user” object. In this way, the pipeline not only transforms the value of the input but also the type of the input.

The intermediate variables can be a bit annoying to see, so perhaps we can fold the calls together?

print_info ( get_primary_group ( get_user ()));

Err…

That’s a bit ugly. We’ll get back to this one…

We’ve used get_primary_group , but what if we want to get all groups in which a user belongs?

auto user = get_user (); auto groups = get_user_groups ( user ); for ( const auto & grp : groups ) { print_info ( grp ); }

We have to introduce a loop here, as the print_info object only expects a single value. We have to “unwrap” the range with our for loop.

Ranges has an algorithm for this: The simplest algorithm of all: for_each :

auto user = get_user (); auto groups = get_user_groups ( user ); for_each ( groups , print_info );

We’ve flattened our code by removing our explicit for loop! In fact, I’d say this is preferable to the prior sample. The old std::for_each taking iterator pairs was often too cumbersome to work with when iterating over a full sequence, so this is a nice alternative.

(Keen-eyed users: Ignore that print_info is an overload set. This is slideware blogware, not software.)

Again, we can fold our operations into a single full-expression:

for_each ( get_user_groups ( get_user ()), print_info );

and it looks pretty terrible. Imagine if we had a dozen individual “pipeline” elements all folded into this single expression. Why would we want to do that to ourselves? We don’t, so we don’t write this way. It’s hard to understand, hard to write, hard to debug, and hard one the eyes.

Problem 1: Nested function calls are hard to read.

Pipelines in Ranges

Ranges generalizes the notion of a “pipeline” to operate on general iterable objects. Suppose the following pipeline:

// Pipeline auto primary_groups = get_all_users () | transform ( get_primary_group );

Read as: Feed get_all_users() into transform(get_primary_group) , where transform(get_primary_group) is a pipeline element that maps its inputs through its operand get_primary_group()

For each element of the range returned by get_all_users() , transform that element using get_primary_group() , and yield a new range from the results.

This may sound familiar from our prior example. In fact, this pipeline-style composition is semantically equivalent to the following imperative style:

// Imperative auto users = get_all_users (); auto primary_groups = transform ( users , get_primary_group );

and semantically equivalent to the ugly nested style:

// Nested auto primary_groups = transform ( get_all_users (), get_primary_group );

So we have three roughly equivalent ways of representing the same computation. We can probably agree that the Nested style is pretty ugly, but what’s the benefit of the Pipeline style over Imperative ?

In this snippet, not much. Where ranges really shines is in the ability to define and compose the pipelines.

Kingdom of Sequences

You might have noticed that I moved to plurals in the prior section, while the previous has used singular user and group objects. This is because ranges acts on… ranges. A user is not a sequence, unless you count it as a sequence of one element (which is valid, but cumbersome). This means that the following is invalid:

auto groups = get_user () | get_user_groups (); // NOPE! for_each ( groups , print_info );

Obviously this code won’t work. The reason ranges can work with the vertical bar | (or “pipe”) operator is through the overloading of operator| on the intermediate types. The user object from get_user() defines no such operation, and there is no get_user_groups taking zero arguments. The above code is just nonsense.

Problem 2: “Pipeline” style only works with ranges.

Some Algorithms are More Equal than Others

So we can’t feed get_user() into get_user_groups() with a pipeline, but can we at least feed get_user_groups() into for_each() ?

auto user = get_user (); get_user_groups ( user ) | for_each ( print_info );

Nope! This won’t work in the current design and implementation of Ranges.

The way | works is special. When I call transform(some_fn) , what transform does is not “transform,” but rather return an intermediate value that has an overloaded operator| . So, it must be this operator| that performs the transform? Nope:

auto f = transform ( trim_whitespace ) | transform ( make_uppercase )

In this expression, there is still no actual range object that we have to operate on. So what is type of f ?

The overloaded operator| does not unconditionally perform a transform. Rather, it must choose whether it should “transform” or “compose” with another range adaptor. The type of f in the above is this new range adaptor created by the composition of the two transform s. It also has an overloaded operator| .

auto g = f | transform ( reverse )

Again, we have not yet fed a value to our range adaptor. The type of g is another composed range adaptor, now composed of three transforms (two are “stored” in f ).

vector < string > s = get_usernames () | g ;

Now! Now we have actually fed a range into our adaptor, and the actual computation takes place.

So, going back around, what about this snippet:

get_user_groups ( user ) | for_each ( print_info );

Why does this not work? The answer is simple: Unlike transform , there is no overload of for_each that yields a range adaptor. Calling for_each with a single argument is simply invalid. When we design a range adaptor, we must “opt-in” to the behavior that supports this “partial application” to support the operator| semantics.

Fair warning: the code to support this syntax is not insignificant.

Problem 3: “Pipeline” style is opt-in, and not all algorithms support it.

Building Reusable Pipelines

I’ve talked about a few downsides of ranges, but let’s talk about one of its greatest features: Composition.

Suppose we have a get_middle_name() function:

optional < string > get_middle_name ( user );

Not everyone has a middle name, so we return an optional , using nullopt to represent the “absence” of a user’s middle name.

Now, what if we want to collect users’ middle names?

vector < string > middle_names ; for ( const auto & user : get_users ()) { auto mid_name_opt = get_middle_name ( user ); if ( mid_name_opt ) { middle_names . push_back ( * mid_name_opt ); } }

Er… That’s gross. Can we use ranges?

vector < string > middle_names = get_users () | transform ( get_middle_name );

Not so fast! The range returned by our pipeline is a range of optional<string> , not a range of string . We need to encapsulate the body of the loop in the prior example into a new range adaptor. We could write all the boilerplate of a completely new range algorithm, or we could simply use what is existing in the Ranges library:

// dereferencable<T> ➞ T auto deref_item = []( const auto & item ) { return * item ; }; // range<dereferencable<T>> ➞ range<T> auto deref_each = transform ( deref_item );

We’ve defined a small function that dereferences values, and we’ve made a range that passes each element through our dereferencing function. Ready to use?

vector < string > middle_names = get_users () | transform ( get_middle_name ) | deref_each ;

Of course not!! We’re just blindly dereferencing each optional ! That’ll never do. We need to “filter” out nullopt objects:

// optional<T> ➞ bool auto is_engaged = []( const auto & opt ) { return opt . has_value (); }; // range<optional<T>> ➞ range<optional<T>> auto only_engaged = filter ( is_engaged );

Now we can glue our pieces together:

vector < string > middle_names = get_users () | transform ( get_middle_name ) | only_engaged | deref_each ;

Excellent! Any downstream element of only_engaged has a guarantee that each optional it receives is engaged.

Actually, these two pieces together are pretty useful. Could we save them for later? Of course!

// range<optional<T>> ➞ range<T> auto unopt_each = only_engaged | deref_each ;

Our new unopt_each range adaptor is generic for any optional -like thing. It doesn’t even mention std::optional : Any type that supplies both has_value() and operator* can be used with unopt_each .

The Static Overhead

While ranges may be a zero-runtime-overhead abstraction (provided you have even a basic inliner running), it has great compile-time, write-time, read-time, and debug-time overhead. I’ll collect these into a term “static overhead.”

The range code, despite being beautifully easy to understand and read, has a large amount of complexity buried within. When all is well, we don’t have to worry much about this. Unfortunately, the “happy path” is very often the path less travelled by.

You’ll also note my qualification that you need a inliner to have the ranges boilerplate evaporate at runtime. When working with unoptimized builds, all of that boilerplate needs to be walked through at runtime (and especially at debug time!).

Zero-Overhead Composition

You’ll remember our straightforward imperative style of composition:

auto a = foo (); auto b = bar ( a ); auto c = baz ( b );

This is incredibly easy to debug, and has zero overhead, both at runtime or “static” overhead.

At the apex of the range-v3 library is the Calendar example. At the apex of the Calendar Example is the format_calendar function:

// In: range<date> // Out: range<string>, lines of formatted output auto format_calendar ( std :: size_t months_per_line ) { return // Group the dates by month: by_month () // Format the month into a range of strings: | layout_months () // Group the months that belong side-by-side: | views :: chunk ( months_per_line ) // Transpose the rows and columns of the size-by-side months: | transpose_months () // Ungroup the side-by-side months: | views :: join // Join the strings of the transposed months: | join_months (); }

Understanding the individual elements of the above pipeline isn’t necessary, but a few things to note:

The function format_calendar accepts a size_t specifying how many months we want on an output line, and returns a range adaptor. The range adaptor returned by format_calendar accepts a range<date> as input and yields a range<string> as output. The type of the range adaptor is not specified in the function. A violation of the input constraints of the adaptor will be reported somewhere seemingly unrelated to format_calendar . Ouch.

Problem 4: Without redesigns, naive custom pipelines with input constraints will disassociate the point-of-error from point-of-requirement.

Walking through this code in a debugger will be an exercise in pain. The imperative style, by contrast, is trivial to debug:

template < range < date > Calendar > auto format_calendar ( const Calendar & cal , size_t months_per_line ) { // Group the dates by month auto months = by_month ( cal ); // Format the months into a range of strings auto month_strings = layout_months ( months ); // Group the months that belong side-by-side auto chunked_months = chunk ( month_strings , months_per_line ); // Transpose the rows and columns of side-by-side months auto transposed = transpose_months ( chunked_months ); // Ungroup the side-by-side months auto joined_view = view :: join ( transposed ); // Join the strings of the transposed months return join_months ( joined_view ); }

Simple, easy to understand, easy to debug, full of tautological variable names, incredibly verbose, and most significantly: not API compatible at all. We’re now taking the range of dates as an input to format_calendar , rather than returning a range adaptor! format_calendar can no longer be used in a pipeline, as the “input” to this “adaptor” is actually a function parameter!

Problem 5: Pipeline style is difficult to debug.

Just for fun, let’s write the code using the nested style:

template < range < date > Calendar > auto format_calendar ( Calendar && cal , size_t months_per_line ) { // Join the strings of the transposed months return join_months ( // Ungroup the side-by-side months view :: join ( // Transpose the rows and columns of side-by-side months transpose_months ( // Group the months that belong side-by-side chunk ( // Format the months into a range of strings layout_months ( // Group the dates by month by_month ( cal ) ), months_per_line ) ) ) ); }

Okay, I lied about the “fun” part. This is awful, but it gives us a hint at a possible solution, and not just a solution to the debuggability, but the need to write boilerplate, the unoptimized runtime overhead, and the ability to work with non-range values in a “pipeline” style.

Eliminating the Static Overhead

For reference, the five issues I’ve outlined:

Nested function calls are hard to read. (Not really an issue with Ranges, but will be relevant.) Pipeline style only works on range objects and range adaptors. Pipeline style is opt-in, and not all range algorithms have opted-in. Naive custom range adaptors made from pipelines that carry input constraints will disassociate the point-of-error from point-of-requirement. Pipeline style is difficult to debug.

And now, let’s fix them all with a single feature: The pipeline-rewrite operator.

A Unique Operator

Many languages have a “pipe” operator, and they have different semantics depending on the language. For the spelling, I’ve chosen |> . For the semantics I’ve chosen a very specific behavior that will specifically work to the benefit of solving the above five problems. To understand what |> does, it is best seen with a code sample:

// Replace all occurrences of `needle` found in `str` with `repl` string replace ( string str , string needle , string repl ); // "sanitize" the given string string sanitize ( string s ) { return s |> replace ( "eval" , "review" ); }

The sanitize function will replace any occurrence of eval with review . It does this by feeding the input string s into replace .

We call replace with two arguments, and that… wait. Where is the overload of replace taking two arguments?

Hint: It doesn’t exist. There is no overload of replace taking two arguments in this sample. In fact, there is no invocation of replace with two arguments. sanitize is calling replace with three arguments: s , "eval" , and "review" .

The |> operator is not a runtime operator. It is not overloadable. It does not produce any changes to the generated code. It has no semantics in the abstract machine. The |> operator manipulates the very syntax of the language to generate pipeline style for free. There is nothing at all special about replace that allows it to support |> . There are no changes to string needed support |> . It Just Works™ out of the box.

Here are the rules and semantics of the |> operator:

The right-hand side of |> must be a call expression. Important: It is not a callable expression, but a call expression. The resulting expression is as-if the user has placed the left-hand operand as the first argument of the function call expression on the right-hand side. Any arguments appearing in the right-hand function call expression are “shifted over” to accommodate the left-hand expression begin passed as the first argument. The |> is left-associative.

Thus the following two snippets are not just semantically equivalent, but semantically identical:

s |> replace ( "foo" , "bar" );

replace ( s , "foo" , "bar" );

Solve #1: Nested function calls are hard to read.

Take our replace function. Suppose we want to make multiple replacements. The code would look something like this:

string replace ( string str , string needle , string repl ); string change_string ( string s ) { s = replace ( s , "foo" , "bar" ); s = replace ( s , "baz" , "qux" ); s = replace ( s , " \r

" , "

" ); s = replace ( s , " \t " , " " ); s = replace ( s , " \x1b " , "ESC" ); return s ; }

I mean… It doesn’t look horrible, but it’s not very nice. We keep re-assigning into s just to pass it to the next call to replace . This necessitates that s cannot be const , despite it being used for nothing other than intermediate storage. We could embed reach s into the following expression in the nested style. This gives us a single return statement, and we can const the argument s :

string replace ( string str , string needle , string repl ); string change_string ( const string & s ) { return replace ( replace ( replace ( replace ( replace ( s , "foo" , "bar" ), "baz" , "qux" , ), " \r

" , "

" ), " \t " , " " ), " \x1b " , "ESC" ); }

(I feel like I need to wash my hands after writing that code.)

This is horrible, but it more closely represents the semantics of the what the function really does. Let’s try again, now with the power of |> :

string replace ( string str , string needle , string repl ); string change_string ( string s ) { return s |> replace ( "foo" , "bar" ) |> replace ( "baz" , "qux" ) |> replace ( " \r

" , "

" ) |> replace ( " \t " , " " ) |> replace ( " \x1b " , "ESC" ); }

By the meaning of |> , this code is semantically identical to the code using the nested calls, but I’d say it is much more legible, would you agree?

Here is the support code we need to make replace work with the |> operator:

// [This space intentionally left blank]

Solve #2: Pipeline style only works on range objects and adaptors

The prior example used string s, which haven’t opted-in to support any special semantics, despite being “range-like.”

Let’s go back to our earlier sample:

auto groups = get_user () | get_user_groups ();

This does not work because 1) a user is not a range, and 2) get_user_groups() is not callable with zero arguments, and does not return any kind of “adaptor” type.

We can use pipeline style via |> if we add this additional support code:

// [This space intentionally left blank]

and now:

auto groups = get_user () |> get_user_groups ();

It all Just Works™! The value of the left-hand expression get_user() is inserted as the first argument to the function call on the right-hand side, get_user_groups() , thus get_user_groups(get_user()) .

The type of user is not tweaked in any way, and there are no ranges involved (other than the return value of get_user_groups() . Speaking of…)

Solve #3: Pipeline style is opt-in for algorithms

So we got our user’s groups, and now we want to send them to print_info . Remember that for_each() does not have an overload returning a range adaptor. It expects a range and a unary function immediately:

auto groups = get_user () |> get_user_groups (); groups | for_each ( print_info ()); // NOPE!

You can probably tell where this is going. Yadda yadda, additional support code intentionally blank, skip to the point:

get_user () |> get_user_groups () |> for_each ( print_info );

for_each is completely unchanged. In fact, every function ever written now supports |> for free.

Remember that transform has two overloads?

auto transform ( fn ); // [1] Returns a range adaptor auto transform ( range , fn ); // [2] Transforms the given range by `fn`

In the first example with transform , we used overload [1] to produce a range adaptor. Here’s that example rewritten with |> :

// Pipeline auto primary_groups = get_all_users () |> transform ( get_primary_group );

Despite appearances, this is using overload [2] . In fact, if we only use |> with the existing ranges library, we can simply discard the overload [1] altogether. We don’t need it! Library maintainers don’t need to write any boilerplate to produce range adaptors. In fact, the “range adaptor” objects disappear entirely. All we have left are regular functions that operate on ranges immediately upon invocation.

Solve #4: Custom range adaptors build from pipelines don’t express constraints

The solution to #4 comes as a byproduct of dropping range adaptors. Because our range operations are now all regular functions, we can express the constraints and concepts on those functions naturally. Let’s bring back our format_calendar() example, now using the pipeline style:

template < range < date > Calendar > range < string > format_calendar ( const Calendar & cal , std :: size_t months_per_line ) { return cal // Group the dates by month: |> by_month ( cal ) // Format the month into a range of strings: |> layout_months () // Group the months that belong side-by-side: |> views :: chunk ( months_per_line ) // Transpose the rows and columns of the size-by-side months: |> transpose_months () // Ungroup the side-by-side months: |> views :: join () // Join the strings of the transposed months: |> join_months (); }

Because we are using a constrained template parameter, violations of those constraints will refer directly to the call site where that violation occurred, rather than deep within the template machinery of the range adaptor.

We’ve still broken the interface of format_calendar (it now takes the calendar immediately), but with the help of |> , the code that was previously:

dates_of_year ( 2020 ) | format_calendar ( 3 );

can be written as

dates_of_year ( 2020 ) |> format_calendar ( 3 );

which is minimally disruptive.

Solve #5: Pipeline style is difficult to debug

With the loss of the boilerplate underlying the work done by range adaptors and operator| , we get debuggability close to that of the imperative style. We do not need step-in-out-in-out-in-out of the pipeline support code for |> , because there is none.

In fact, I could hypothesize a debugger taking advantage of |> . We have all been in that situation where we have a nested call:

foo ( bar ());

We hit it in the debugger, and we want to see the return value of bar() before it gets passed to foo() . How do we do that? Step in? That will just send us into bar() , and if we then step-out we might see a “return value” in the watch window, or we might see nothing. Step over? No: That will just send us flying across foo() . The only reliable way is to step-in to bar() , step-out, then step-in to foo() , where we can see the value of the first argument (assuming you can step-in to foo() : You might not have the debug symbols for it). Don’t even ask if we have foo(bar(baz())) . Uggh.

Imagine a smart compiler generating debug information such that this code:

baz () |> bar () |> foo ();

Will result in meaningful step-over calls over each step in the pipeline. Such smart debug information would be difficult to generate with operator| and range adaptors, since the meaning behind such things is completely opaque to a compiler/debugger.

Additional Goodies

There are two more great benefits to using |> :

Compiling code with |> is far less taxing on a compiler/linker than the code required to support the overloaded operator| . Reduced compile/link time, and smaller codegen. Writing code to support |> is the same as writing regular code. There is no need to support two overloads of each algorithm when a single one will work with both pipeline style and immediate style.

Solving unopt_each

You may recall this code from a prior example:

vector < user > get_users (); optional < string > get_middle_name ( user ); vector < string > all_middle_names () { // dereferencable<T> ➞ T auto deref_item = []( const auto & item ) { return * item ; }; // range<dereferencable<T>> ➞ range<T> auto deref_each = transform ( deref_item ); // optional<T> ➞ bool auto is_engaged = []( const auto & opt ) { return opt . has_value (); }; // range<optional<T>> ➞ range<optional<T>> auto only_engaged = filter ( is_engaged ); // range<optional<T>> ➞ range<T> auto unopt_each = only_engaged | deref_each ; return get_users () | transform ( get_middle_name ) | unopt_each ; }

If we remove range adaptors and the overloaded operator| , this won’t work anymore! Let’s take them one at a time.

deref_each

We have deref_each , a handy little range transformer:

auto deref_each = transform ( deref_item );

Without the overload of transform accepting a single argument and returning a range adaptor, this code is broken. The solution? EZ PZ:

auto deref_each = []( auto && range ) { return range |> transform ( deref_item ); };

Err… that’s a lot more code, but it definitely works! Now, when we use deref_each in a pipeline, we must call it: rng |> deref_each() . Additionally, we get immediate-style for free: deref_each(rng) .

Still, it’s so ugly! We’ll come back to this…

only_engaged

only_engaged follows a very similar pattern:

auto only_engaged = filter ( is_engaged );

becomes:

auto only_engaged = []( auto && rng ) { return range |> filter ( is_engaged ); };

Noticing a pattern?

The Composition: unopt_each

Our composition of adaptors, unopt_each , made use of operator| , and we can’t simply convert it to use |> . Like deref_each and only_engaged , we must wrap it in a lambda expression:

auto unopt_each = [ & ]( auto && rng ) { return rng |> only_engaged () |> unopt_each (); };

The Result

The resulting pipeline expression looks markedly similar to the original:

return get_users () |> transform ( get_middle_name ) |> unopt_each ();

We must actually invoke the closure unopt_each , but that is not a huge difference.

Curing The Ugly

Having to wrap our in-situ custom range algorithms with lambda expressions is a lot of typing and quite an eyesore. What can we do to fix it?

Well: Nobody expects the Spanish Inquisition expression lambdas!

Readers may be familiar with my prior blog post on my informal proposal for expression lambdas, which are lambda expressions that consist of a single expression that stands in for the return value, using placeholder names for the implicit arguments:

// Expression lambda: auto l1 = [][ _1 . foo () + baz ( _2 )]; // Roughly equivalent to: auto l2 = []( auto && __1 , auto && __2 ) { return __1 . foo () + baz ( __2 ); };

(In my prior post I used &N as the placeholder, but I’m considering proposing with _N instead, just to be less punctuation-dense.)

If we introduce expression lambdas into our example, this:

auto deref_item = []( const auto & item ) { return * item ; }; auto deref_each = [ & ]( auto && range ) { return range |> transform ( deref_item ); }; auto is_engaged = []( const auto & opt ) { return opt . has_value (); }; auto only_engaged = [ & ]( auto && rng ) { return range |> filter ( is_engaged ); }; auto unopt_each = [ & ]( auto && rng ) { return rng |> only_engaged () |> unopt_each (); };

can be rewritten:

auto deref_item = [][ * _1 ]; auto deref_each = [ & ][ _1 |> transform ( deref_item )]; auto is_engaged = [][ _1 . has_value ()]; auto only_engaged = [ & ][ _1 |> filter ( is_engaged )]; auto unopt_each = [ & ][ _1 |> only_engaged () |> deref_each ()];

No magic range adaptor objects, just functions.

Bringing it Back Around

Ignoring the possibility of |> for a moment, remember the two snippets at the top of this post?

// Imperative style vector < string > child_names ; for ( auto & person : all_people ) { if ( person . age < 14 ) { child_names . push_back ( person . name ); } } // Pipeline style auto children_names = all_people | filter ([]( const auto & person ) { return person . age < 14 ; }) | transform ([]( const auto & person ) { return person . name ; }) | to_vector ;

Despite the pipeline style being declarative and very ~fancy~, we’d probably all agree that it is way more verbose, and dare I say… “ugly”? Let’s bring in expression lambdas, and see how they can fit into Ranges without a |> operator involved:

auto children_names = all_people | filter ([][ _1 . age < 14 ]) | transform ([][ _1 . name ]) | to_vector ;

This, in my opinion, is beautiful code.