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ntbs: null-terminated byte strings

cat and cut: concatenate and slice
functions for splicing 'constexpr C strings'

Copyright © 2019 Will Wray. Distributed under the Boost Software License, V1.0

Boost Software License - Version 1.0 - August 17th, 2003

Permission is hereby granted, free of charge, to any person or organization
obtaining a copy of the software and accompanying documentation covered by
this license (the "Software") to use, reproduce, display, distribute,
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must be included in all copies of the Software, in whole or in part, and
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SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
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License

Also at boost.org and accompanying file LICENSE_1_0.txt

C++17. Targets GCC and Clang -std=c++17, MSVC /std:c++17
Namespace ltl::ntbs

Hello-world example: cat

 #include "ntbs.hpp"
 using ltl::ntbs;

 constexpr auto hello_world = cat(cat<',',' '>("Hello","world"),'!');

 static_assert( hello_world == "Hello, world!" );

Hello-world example continued: cut

 constexpr auto hello_comma = cut<0,6>(hello_world);
 constexpr auto world_exclaim = cut<-7>(hello_world);

 #include <cstdio>
 int main() { puts( cat<' '>(hello_comma, world_exclaim) ); }

Outputs "Hello, world!", spliced back together from the sliced words.

Note that cut indices B,E are signed integers specifying range [B,E):

  • Positive values index forward from begin index 0 as usual
  • Negative values index backward from the end of the char array

Here, -1 serves as end index (actually the null-terminator index for NTBS)
so cut<-7>(hello_world) is equivalent to cut<-7,-1>(hello_world)

Results are returned in a constexpr C string type; a char[N] array wrapped in a class
and understood to contain N-1 characters followed by a terminating zero character.
This class acts like a copyable / returnable char[N], call it Char<N> (the actual type,
ltl::ntbs::array<N>, is an implementation detail, not meant for direct use).

Generic access is provided by free-function 'size' and 'data' overloads:

  • size(Char<N>{}) returns N, the array size (including null terminator)
  • data(a) returns a const char* (or array ref) for the array begin

Char<N> has implicit conversion to const char(&)[N], decaying to const char*.

operator== and != are provided for comparisons, up to full static size
including the terminating character and any embedded null characters.

ntbs::cmp is a constexpr version of strcmp for lexicographic comparison.

Design notes

Constexpr std::string, as proposed for C++2a, will likely replace many use cases.
C++2a constexpr string splicing will also benefit from more constexpr algorithms.
C++17 std::string_view is a good companion but constexpr size is lost in passing
to a constexpr function.

  • ntbs is not intended for run-time so compile-time is the appropriate metric
  • Originally implemented with index_sequence then switched to looping
    (experimented with Clang 8 constexpr builtin memcpy but saw no benefit).
  • Originally held non-null-terminated data, then switched to null termination -
    having a null terminator is convenient in more use cases.
  • ntbs size is limited by the constexpr loop iteration count limit
  • Compile times increase to seconds for cats around 256K chars
  • Run-time use cases are constrained by the need for static size,
    for example 3-char currency-pair stitching "USD"+"CAD" = "USDCAD"
Linux Travis Windows Appveyor
gcc-9, clang-7
-std=c++17		 MSVC 19.21.27702
/std:c++17
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