Flux

Flux is an experimental C++20 library for working with sequences of values. It offers similar facilities to C++20 ranges, D ranges, Python itertools, Rust iterators and related libraries for other languages.

Quick Example

constexpr auto result = flux::from(std::array{1, 2, 3, 4, 5})
                         .filter(flux::pred::even)
                         .map([](int i) { return i * 2; })
                         .sum();
static_assert(result == 12);

Try it on Compiler Explorer

Getting Started

Right now the easiest way to get started with Flux is to download the latest automatically generated single header file and #include it in your sources like any other header.

Compiler support

Flux requires a recent compiler with good support for C++20. It is tested with:

• GCC 11.3 and newer
• MSVC 2022
• Clang 16

Note that older compilers are unlikely to work due to missing language and/or standard library support.

Why Flux?

Flux provides a broadly equivalent feature set to C++20 Ranges, but uses a slightly different model based around cursors rather than iterators. Flux aims to offer:

• Much improved safety by default
• Improved ease of use in common cases, particularly for defining your own sequences and adaptors
• Improved run-time efficiency for some common operations
• Compatibility with existing ranges algorithms

The Flux iteration model

The Flux iteration model is based around cursors, which are a generalisation of array indices (in much the same way that STL iterators are a generalisation of array pointers). A Flux sequence provides four basis operations:

• flux::first(seq) returns an object called a cursor, which represents a position in a sequence. For a sequence with N elements there are N+1 possible cursor positions, including the past-the-end (terminal) position.
• flux::is_last(seq, cursor) returns a boolean value indicating whether the cursor is in the terminal position
• flux::inc(seq, cursor) increments the given cursor, so that it points to the next element in the sequence
• flux::read_at(seq, cursor) returns the sequence element at the given cursor position

These basis operations are equivalent to the basis operations on STL iterators (begin(), iter == end(), ++iter and *iter respectively). The crucial difference is that in the Flux model, you need to provide both the sequence and the cursor to each function call, whereas in the STL model the iterator knows how to increment and dereference itself.

STL iterators are "smart", but Flux cursors are not!

This seemingly small change as some far-reaching consequences. In particular:

• Because we have access to the sequence object during increment and dereference operations, we can provide inexpensive bounds checking for sequences
• Because we need the sequence object in order to do anything useful with a cursor, "dangling" cursors are not possible by design: if the sequence object is no longer around, the cursor can't be used
• Because a cursor only represents a position in a sequence (like an integer index for an array), modifying the underlying sequence is less likely to invalidate a cursor -- if the element at the given position no longer exists, this will be caught by the bounds check at the next attempted read.
• Because element access requires the original sequence, we don't need to make a distinction between mutable iterators and const_iterators -- the same cursor type is used for both const and non-const access, making cursors considerably simpler to implement than STL iterators.

Like STL input ranges, basic Flux sequences are assumed to be single-pass by default. Flux also provides various for more powerful sequences, closely modeled on their STL counterparts:

• multipass_sequences allow multiple cursors to iterate over the sequence independently, potentially passing over each position multiple times
• bidirectional_sequences are multipass sequences whose cursors can be decremented as well as incremented
• random_access_sequences are bidirectional sequences whose cursors can be incremented or decremented an arbitrary number of places in constant time
• contiguous_sequences are random-access sequences which are backed by a contiguous, in-memory array, which potentially allow low-level operations like memcpy() to be used as an optimisation

Reference documentation

Incomplete, work-in-progress documentation can be found at tristanbrindle.com/flux