Add an experimental basic_static_cstring template in example/

This introduces an alternative to basic_static_string designed for use
in POD types: Trivially copyable, having a sizeof == N + 1, with no
embedded NULs.

Placed in example/ to gather user feedback before committing to a public
API. See issue #23.

.github/workflows/example.yml

@@ -0,0 +1,156 @@
+#
+# Copyright (c) 2025 Gennaro Prota (gennaro dot prota at gmail dot com)
+#
+# Distributed under the Boost Software License, Version 1.0. (See accompanying
+# file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
+#
+# Official repository: https://github.com/boostorg/static_string
+#
+
+name: Example (basic_static_cstring)
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+      - develop
+      - bugfix/**
+      - feature/**
+      - fix/**
+      - github/**
+      - pr/**
+    paths-ignore:
+      - LICENSE
+      - meta/**
+      - README.md
+
+jobs:
+  linux:
+    runs-on: ubuntu-24.04
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - { toolset: gcc-13,   cxxstd: '20,23' }
+          - { toolset: gcc-14,   cxxstd: '20,23,26' }
+          - { toolset: clang-17, cxxstd: '20,23' }
+          - { toolset: clang-18, cxxstd: '20,23,26' }
+
+    steps:
+      - name: Checkout Boost super-project
+        uses: actions/checkout@v4
+        with:
+          repository: boostorg/boost
+          ref: develop
+          fetch-depth: 0
+
+      - name: Checkout this library
+        uses: actions/checkout@v4
+        with:
+          path: libs/static_string
+          fetch-depth: 0
+
+      - name: Initialize Boost submodules
+        run: |
+          git submodule update --init tools/boostdep
+          python tools/boostdep/depinst/depinst.py --git_args '--jobs 4' static_string
+
+      - name: Bootstrap b2
+        run: ./bootstrap.sh
+
+      - name: Generate Boost headers
+        run: ./b2 headers
+
+      - name: Build and run example tests
+        run: |
+          ./b2 libs/static_string/example/static_cstring \
+            toolset=${{ matrix.toolset }} \
+            cxxstd=${{ matrix.cxxstd }} \
+            variant=debug,release \
+            -j$(nproc)
+
+  macos:
+    runs-on: macos-14
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - { toolset: clang, cxxstd: '20,23' }
+
+    steps:
+      - name: Checkout Boost super-project
+        uses: actions/checkout@v4
+        with:
+          repository: boostorg/boost
+          ref: develop
+          fetch-depth: 0
+
+      - name: Checkout this library
+        uses: actions/checkout@v4
+        with:
+          path: libs/static_string
+          fetch-depth: 0
+
+      - name: Initialize Boost submodules
+        run: |
+          git submodule update --init tools/boostdep
+          python3 tools/boostdep/depinst/depinst.py --git_args '--jobs 4' static_string
+
+      - name: Bootstrap b2
+        run: ./bootstrap.sh
+
+      - name: Generate Boost headers
+        run: ./b2 headers
+
+      - name: Build and run example tests
+        run: |
+          ./b2 libs/static_string/example/static_cstring \
+            toolset=${{ matrix.toolset }} \
+            cxxstd=${{ matrix.cxxstd }} \
+            variant=debug,release \
+            -j$(sysctl -n hw.ncpu)
+
+  windows:
+    runs-on: windows-2022
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - { toolset: msvc-14.3, cxxstd: '20,latest' }
+
+    steps:
+      - name: Checkout Boost super-project
+        uses: actions/checkout@v4
+        with:
+          repository: boostorg/boost
+          ref: develop
+          fetch-depth: 0
+
+      - name: Checkout this library
+        uses: actions/checkout@v4
+        with:
+          path: libs/static_string
+          fetch-depth: 0
+
+      - name: Initialize Boost submodules
+        run: |
+          git submodule update --init tools/boostdep
+          python tools/boostdep/depinst/depinst.py --git_args '--jobs 4' static_string
+
+      - name: Bootstrap b2
+        run: .\bootstrap.bat
+        shell: cmd
+
+      - name: Generate Boost headers
+        run: .\b2 headers
+        shell: cmd
+
+      - name: Build and run example tests
+        run: |
+          .\b2 libs/static_string/example/static_cstring ^
+            toolset=${{ matrix.toolset }} ^
+            cxxstd=${{ matrix.cxxstd }} ^
+            variant=debug,release ^
+            address-model=64
+        shell: cmd

example/static_cstring/Jamfile

@@ -0,0 +1,19 @@
+#
+# Copyright (c) 2025 Gennaro Prota (gennaro dot prota at gmail dot com)
+#
+# Distributed under the Boost Software License, Version 1.0. (See accompanying
+# file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
+#
+# Official repository: https://github.com/boostorg/static_string
+#
+
+import testing ;
+
+project
+  : requirements
+    <include>../../include
+    <warnings>extra
+    <cxxstd>20
+  ;
+
+run static_cstring_test.cpp ;

example/static_cstring/README.md

@@ -0,0 +1,13 @@
+This directory contains an experimental implementation of `basic_static_cstring`, which differs from `basic_static_string` in the following ways:
+
+|                     | `basic_static_cstring` | `basic_static_string` |
+|---------------------|------------------------|-----------------------|
+| Layout              | `sizeof == N + 1`      | Has size member       |
+| Embedded NULs       | Not supported          | Supported             |
+| Trivially copyable  | Yes                    | No                    |
+
+Additionally, when `N <= UCHAR_MAX`, `basic_static_cstring` employs an optimization that avoids calling `std::strlen()` to compute the size.
+
+This work stems from [boostorg/static_string#23](https://github.com/boostorg/static_string/issues/23).
+
+If you believe `basic_static_cstring` should become part of the public API, please share your feedback on the Boost mailing list.

example/static_cstring/static_cstring.hpp

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+//
+// Copyright (c) 2025 Gennaro Prota (gennaro dot prota at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/static_string
+//
+
+#ifndef BOOST_STATIC_STRING_STATIC_CSTRING_HPP
+#define BOOST_STATIC_STRING_STATIC_CSTRING_HPP
+
+#include <boost/static_string/config.hpp>
+#include <algorithm>
+#include <climits>
+#include <cstddef>
+#include <ostream>
+#include <string>
+#include <string_view>
+#include <stdexcept>
+#include <type_traits>
+
+namespace boost {
+namespace static_strings {
+
+namespace detail {
+
+// Primary template: No remaining-capacity trick; uses traits::length() for length.
+template<std::size_t N, typename CharT, typename Traits, bool UseRemaining>
+class static_cstring_base
+{
+public:
+    using traits_type = Traits;
+    using value_type = CharT;
+    using size_type = std::size_t;
+
+    value_type data_[N + 1]{};
+
+    constexpr size_type get_size() const noexcept
+    {
+        return traits_type::length(data_);
+    }
+
+    constexpr void set_size(size_type sz) noexcept
+    {
+        data_[sz] = value_type{};
+    }
+
+    // Defaulted comparisons for structural type support.
+    constexpr bool operator==(const static_cstring_base&) const noexcept = default;
+    constexpr auto operator<=>(const static_cstring_base&) const noexcept = default;
+};
+
+// Specialization for N <= UCHAR_MAX: Uses remaining-capacity trick.
+template<std::size_t N, typename CharT, typename Traits>
+class static_cstring_base<N, CharT, Traits, true>
+{
+public:
+    using traits_type = Traits;
+    using value_type = CharT;
+    using size_type = std::size_t;
+
+    value_type data_[N + 1]{};
+
+    constexpr size_type get_size() const noexcept
+    {
+        return N - static_cast<unsigned char>(data_[N]);
+    }
+
+    constexpr void set_size(size_type sz) noexcept
+    {
+        data_[sz] = value_type{};
+        data_[N] = static_cast<value_type>(N - sz);
+    }
+
+    // Defaulted comparisons for structural type support.
+    constexpr bool operator==(const static_cstring_base&) const noexcept = default;
+    constexpr auto operator<=>(const static_cstring_base&) const noexcept = default;
+};
+
+} // namespace detail
+
+template<std::size_t N, typename CharT = char, typename Traits = std::char_traits<CharT>>
+class basic_static_cstring
+    : public detail::static_cstring_base<N, CharT, Traits, (N <= UCHAR_MAX)>
+{
+public:
+    using base = detail::static_cstring_base<N, CharT, Traits, (N <= UCHAR_MAX)>;
+    using base::data_;
+    using base::get_size;
+    using base::set_size;
+
+    // Member types
+    using traits_type     = Traits;
+    using value_type      = CharT;
+    using size_type       = std::size_t;
+    using difference_type = std::ptrdiff_t;
+    using reference       = value_type&;
+    using const_reference = const value_type&;
+    using pointer         = value_type*;
+    using const_pointer   = const value_type*;
+    using iterator        = pointer;
+    using const_iterator  = const_pointer;
+
+    static constexpr size_type npos = static_cast<size_type>(-1);
+    static constexpr size_type static_capacity = N;
+
+    // Constructors.
+    constexpr basic_static_cstring() noexcept
+    {
+        set_size(0);
+    }
+
+    constexpr basic_static_cstring(const CharT* s)
+    {
+        assign(s);
+    }
+
+    constexpr basic_static_cstring(const CharT* s, size_type count)
+    {
+        assign(s, count);
+    }
+
+    constexpr basic_static_cstring(size_type count, CharT ch)
+    {
+        assign(count, ch);
+    }
+
+    template<std::size_t M>
+    constexpr basic_static_cstring(const CharT (&arr)[M])
+    {
+        static_assert(M <= N + 1, "String literal too long for static_cstring");
+        assign(arr, M - 1);
+    }
+
+    constexpr size_type size() const noexcept
+    {
+        return get_size();
+    }
+
+    constexpr size_type length() const noexcept
+    {
+        return size();
+    }
+
+    constexpr bool empty() const noexcept
+    {
+        return data_[0] == value_type{};
+    }
+
+    static constexpr size_type max_size() noexcept
+    {
+        return N;
+    }
+
+    static constexpr size_type capacity() noexcept
+    {
+        return N;
+    }
+
+    // Element access.
+    constexpr reference operator[](size_type pos) noexcept
+    {
+        return data_[pos];
+    }
+
+    constexpr const_reference operator[](size_type pos) const noexcept
+    {
+        return data_[pos];
+    }
+
+    constexpr reference at(size_type pos)
+    {
+        if (pos >= size())
+        {
+            throw std::out_of_range("static_cstring::at");
+        }
+        return data_[pos];
+    }
+
+    constexpr const_reference at(size_type pos) const
+    {
+        if (pos >= size())
+        {
+            throw std::out_of_range("static_cstring::at");
+        }
+        return data_[pos];
+    }
+
+    constexpr reference front() noexcept
+    {
+        BOOST_STATIC_STRING_ASSERT(!empty());
+        return data_[0];
+    }
+
+    constexpr const_reference front() const noexcept
+    {
+        BOOST_STATIC_STRING_ASSERT(!empty());
+        return data_[0];
+    }
+
+    constexpr reference back() noexcept
+    {
+        BOOST_STATIC_STRING_ASSERT(!empty());
+        return data_[size() - 1];
+    }
+
+    constexpr const_reference back() const noexcept
+    {
+        BOOST_STATIC_STRING_ASSERT(!empty());
+        return data_[size() - 1];
+    }
+
+    constexpr pointer data() noexcept
+    {
+        return data_;
+    }
+
+    constexpr const_pointer data() const noexcept
+    {
+        return data_;
+    }
+
+    constexpr const_pointer c_str() const noexcept
+    {
+        return data_;
+    }
+
+    // Iterators.
+    constexpr iterator begin() noexcept
+    {
+        return data_;
+    }
+
+    constexpr const_iterator begin() const noexcept
+    {
+        return data_;
+    }
+
+    constexpr const_iterator cbegin() const noexcept
+    {
+        return data_;
+    }
+
+    constexpr iterator end() noexcept
+    {
+        return data_ + size();
+    }
+
+    constexpr const_iterator end() const noexcept
+    {
+        return data_ + size();
+    }
+
+    constexpr const_iterator cend() const noexcept
+    {
+        return data_ + size();
+    }
+
+    // Modifiers.
+    constexpr void clear() noexcept
+    {
+        set_size(0);
+    }
+
+    constexpr basic_static_cstring& assign(const CharT* s)
+    {
+        return assign(s, traits_type::length(s));
+    }
+
+    constexpr basic_static_cstring& assign(const CharT* s, size_type count)
+    {
+        if (count > N)
+        {
+            throw std::length_error("static_cstring::assign");
+        }
+        traits_type::copy(data_, s, count);
+        set_size(count);
+        return *this;
+    }
+
+    constexpr basic_static_cstring& assign(size_type count, CharT ch)
+    {
+        if (count > N)
+        {
+            throw std::length_error("static_cstring::assign");
+        }
+        traits_type::assign(data_, count, ch);
+        set_size(count);
+        return *this;
+    }
+
+    constexpr basic_static_cstring& operator=(const CharT* s)
+    {
+        return assign(s);
+    }
+
+    constexpr void push_back(CharT ch)
+    {
+        const size_type sz = size();
+        if (sz >= N)
+        {
+            throw std::length_error("static_cstring::push_back");
+        }
+        data_[sz] = ch;
+        set_size(sz + 1);
+    }
+
+    constexpr void pop_back() noexcept
+    {
+        BOOST_STATIC_STRING_ASSERT(!empty());
+        set_size(size() - 1);
+    }
+
+    constexpr basic_static_cstring& append(const CharT* s)
+    {
+        return append(s, traits_type::length(s));
+    }
+
+    constexpr basic_static_cstring& append(const CharT* s, size_type count)
+    {
+        const size_type sz = size();
+        if (sz + count > N)
+        {
+            throw std::length_error("static_cstring::append");
+        }
+        traits_type::copy(data_ + sz, s, count);
+        set_size(sz + count);
+        return *this;
+    }
+
+    constexpr basic_static_cstring& append(size_type count, CharT ch)
+    {
+        const size_type sz = size();
+        if (sz + count > N)
+        {
+            throw std::length_error("static_cstring::append");
+        }
+        traits_type::assign(data_ + sz, count, ch);
+        set_size(sz + count);
+        return *this;
+    }
+
+    constexpr basic_static_cstring& operator+=(const CharT* s)
+    {
+        return append(s);
+    }
+
+    constexpr basic_static_cstring& operator+=(CharT ch)
+    {
+        push_back(ch);
+        return *this;
+    }
+
+    // Comparisons.
+    constexpr int compare(const basic_static_cstring& other) const noexcept
+    {
+        const size_type lhs_sz = size();
+        const size_type rhs_sz = other.size();
+        const int result = traits_type::compare(data_, other.data_, (std::min)(lhs_sz, rhs_sz));
+
+        return result != 0
+                 ? result
+                 : lhs_sz < rhs_sz
+                 ? -1
+                 : lhs_sz > rhs_sz
+                 ? 1
+                 : 0;
+    }
+
+    constexpr int compare(const CharT* s) const noexcept
+    {
+        return compare(basic_static_cstring(s));
+    }
+
+    // Conversions.
+    constexpr operator std::basic_string_view<CharT, Traits>() const noexcept
+    {
+        return {data_, size()};
+    }
+
+    std::basic_string<CharT, Traits> str() const
+    {
+        return {data_, size()};
+    }
+
+    // Swap.
+    constexpr void swap(basic_static_cstring& other) noexcept
+    {
+        basic_static_cstring tmp = *this;
+        *this = other;
+        other = tmp;
+    }
+
+    // Defaulted comparisons for structural type (C++20).
+    constexpr bool operator==(const basic_static_cstring&) const noexcept = default;
+    constexpr auto operator<=>(const basic_static_cstring&) const noexcept = default;
+};
+
+#if defined(BOOST_STATIC_STRING_USE_DEDUCT)
+
+// Deduction guide.
+template<std::size_t N, typename CharT>
+basic_static_cstring(const CharT(&)[N]) -> basic_static_cstring<N - 1, CharT>;
+
+#endif
+
+// Comparison with const CharT*.
+template<std::size_t N, typename CharT, typename Traits>
+constexpr bool operator==(const basic_static_cstring<N, CharT, Traits>& lhs,
+                          const CharT* rhs) noexcept
+{
+    return lhs.compare(rhs) == 0;
+}
+
+template<std::size_t N, typename CharT, typename Traits>
+constexpr bool operator==(const CharT* lhs,
+                          const basic_static_cstring<N, CharT, Traits>& rhs) noexcept
+{
+    return rhs.compare(lhs) == 0;
+}
+
+// Stream output.
+template<std::size_t N, typename CharT, typename Traits>
+std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>& os,
+                                              const basic_static_cstring<N, CharT, Traits>& str)
+{
+    return os << str.c_str();
+}
+
+// Alias templates.
+template<std::size_t N>
+using static_cstring = basic_static_cstring<N, char>;
+
+template<std::size_t N>
+using static_wcstring = basic_static_cstring<N, wchar_t>;
+
+}
+}
+
+#endif

example/static_cstring/static_cstring_test.cpp

@@ -0,0 +1,671 @@
+//
+// Copyright (c) 2025 Gennaro Prota (gennaro dot prota at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/static_string
+//
+
+#include "static_cstring.hpp"
+
+#include <boost/core/lightweight_test.hpp>
+#include <climits>
+#include <cstring>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+namespace boost {
+namespace static_strings {
+
+static
+void
+testCStringSizeGuarantee()
+{
+    // Core guarantee: sizeof is exactly N + 1 (no size member).
+    static_assert(sizeof(static_cstring<0>) == 1, "");
+    static_assert(sizeof(static_cstring<1>) == 2, "");
+    static_assert(sizeof(static_cstring<10>) == 11, "");
+    static_assert(sizeof(static_cstring<63>) == 64, "");
+    static_assert(sizeof(static_cstring<64>) == 65, "");
+    static_assert(sizeof(static_cstring<127>) == 128, "");
+    static_assert(sizeof(static_cstring<UCHAR_MAX>) == (UCHAR_MAX + 1), "");
+    static_assert(sizeof(static_cstring<UCHAR_MAX + 1>) == (UCHAR_MAX + 2), "");
+    static_assert(sizeof(static_cstring<1000>) == 1001, "");
+}
+
+static
+void
+testCStringRemainingCapacityTrick()
+{
+    // Test the remaining-capacity optimization for N <= UCHAR_MAX.
+
+    // Full capacity: Last byte is both null terminator AND remaining == 0.
+    {
+        static_cstring<5> full("12345");
+        BOOST_TEST(full.size() == 5);
+        BOOST_TEST(full.capacity() == 5);
+        BOOST_TEST(full.data()[5] == '\0');
+        BOOST_TEST(std::strcmp(full.c_str(), "12345") == 0);
+    }
+
+    // Partial fill: Null terminator at position size, remaining at position N.
+    {
+        static_cstring<10> partial("Hi");
+        BOOST_TEST(partial.size() == 2);
+        BOOST_TEST(partial.capacity() == 10);
+        BOOST_TEST(partial.data()[2] == '\0');
+        BOOST_TEST(static_cast<unsigned char>(partial.data()[10]) == 8);
+    }
+
+    // Empty string.
+    {
+        static_cstring<10> empty;
+        BOOST_TEST(empty.size() == 0);
+        BOOST_TEST(empty.capacity() == 10);
+        BOOST_TEST(empty.data()[0] == '\0');
+        BOOST_TEST(static_cast<unsigned char>(empty.data()[10]) == 10);
+    }
+
+    // Edge case: N == UCHAR_MAX (max for our trick).
+    {
+        static_cstring<UCHAR_MAX> large;
+        large.assign(100, 'x');
+        BOOST_TEST(large.size() == 100);
+        BOOST_TEST(large.capacity() == UCHAR_MAX);
+        BOOST_TEST(static_cast<unsigned char>(large.data()[UCHAR_MAX]) == (UCHAR_MAX - large.size()));
+    }
+}
+
+template<typename S>
+struct CStringTypeTraits
+{
+    static_assert(std::is_trivially_copyable<S>::value);
+    static_assert(std::is_trivially_copy_constructible<S>::value);
+    static_assert(std::is_trivially_move_constructible<S>::value);
+    static_assert(std::is_trivially_copy_assignable<S>::value);
+    static_assert(std::is_trivially_move_assignable<S>::value);
+    static_assert(std::is_trivially_destructible<S>::value);
+};
+
+static
+void
+testCStringTypeTraits()
+{
+    {
+        using S = static_cstring<0>;
+        CStringTypeTraits< S > check;
+        static_cast<void>(check);
+    }
+    {
+        using S = static_cstring<63>;
+        CStringTypeTraits< S > check;
+        static_cast<void>(check);
+    }
+    {
+        using S = static_cstring<300>;
+        CStringTypeTraits< S > check;
+        static_cast<void>(check);
+    }
+}
+
+static
+void
+testCStringConstruct()
+{
+    // Default construction.
+    {
+        static_cstring<1> s;
+        BOOST_TEST(s.empty());
+        BOOST_TEST(s.size() == 0);
+        BOOST_TEST(s == "");
+        BOOST_TEST(*s.end() == 0);
+    }
+
+    // Construct with count and char.
+    {
+        static_cstring<4> s1(3, 'x');
+        BOOST_TEST(!s1.empty());
+        BOOST_TEST(s1.size() == 3);
+        BOOST_TEST(s1 == "xxx");
+        BOOST_TEST(*s1.end() == 0);
+        BOOST_TEST_THROWS(
+            (static_cstring<2>(3, 'x')),
+            std::length_error);
+    }
+
+    // Construct from a C string.
+    {
+        static_cstring<5> s1("12345");
+        BOOST_TEST(s1.size() == 5);
+        BOOST_TEST(s1 == "12345");
+        BOOST_TEST(*s1.end() == 0);
+        BOOST_TEST_THROWS(
+            (static_cstring<4>("12345")),
+            std::length_error);
+    }
+
+    // Construct from a C string with count.
+    {
+        static_cstring<5> s1("UVXYZ", 3);
+        BOOST_TEST(s1 == "UVX");
+        BOOST_TEST(*s1.end() == 0);
+    }
+
+    // Copy construction.
+    {
+        static_cstring<5> s1("12345");
+        static_cstring<5> s2(s1);
+        BOOST_TEST(s2 == "12345");
+        BOOST_TEST(*s2.end() == 0);
+    }
+}
+
+static
+void
+testCStringAssignment()
+{
+    // assign(size_type count, CharT ch).
+    BOOST_TEST(static_cstring<3>{}.assign(1, '*') == "*");
+    BOOST_TEST(static_cstring<3>{}.assign(3, '*') == "***");
+    BOOST_TEST(static_cstring<3>{"abc"}.assign(3, '*') == "***");
+    BOOST_TEST_THROWS(static_cstring<1>{"a"}.assign(2, '*'), std::length_error);
+
+    // assign(CharT const* s, size_type count).
+    BOOST_TEST(static_cstring<3>{}.assign("abc", 3) == "abc");
+    BOOST_TEST(static_cstring<3>{"*"}.assign("abc", 3) == "abc");
+    BOOST_TEST_THROWS(static_cstring<1>{}.assign("abc", 3), std::length_error);
+
+    // assign(CharT const* s).
+    BOOST_TEST(static_cstring<3>{}.assign("abc") == "abc");
+    BOOST_TEST(static_cstring<3>{"*"}.assign("abc") == "abc");
+    BOOST_TEST_THROWS(static_cstring<1>{}.assign("abc"), std::length_error);
+
+    // operator=(const CharT* s).
+    {
+        static_cstring<3> s1;
+        s1 = "123";
+        BOOST_TEST(s1 == "123");
+        BOOST_TEST(*s1.end() == 0);
+        static_cstring<1> s2;
+        BOOST_TEST_THROWS(
+            s2 = "123",
+            std::length_error);
+    }
+
+    // Copy assignment.
+    {
+        static_cstring<3> s1("123");
+        static_cstring<3> s2;
+        s2 = s1;
+        BOOST_TEST(s2 == "123");
+        BOOST_TEST(*s2.end() == 0);
+    }
+}
+
+static
+void
+testCStringElements()
+{
+    using ccs3 = static_cstring<3> const;
+
+    // at(size_type pos).
+    BOOST_TEST(static_cstring<3>{"abc"}.at(0) == 'a');
+    BOOST_TEST(static_cstring<3>{"abc"}.at(2) == 'c');
+    BOOST_TEST_THROWS(static_cstring<3>{""}.at(0), std::out_of_range);
+    BOOST_TEST_THROWS(static_cstring<3>{"abc"}.at(4), std::out_of_range);
+
+    // at(size_type pos) const.
+    BOOST_TEST(ccs3{"abc"}.at(0) == 'a');
+    BOOST_TEST(ccs3{"abc"}.at(2) == 'c');
+    BOOST_TEST_THROWS(ccs3{""}.at(0), std::out_of_range);
+
+    // operator[](size_type pos).
+    BOOST_TEST(static_cstring<3>{"abc"}[0] == 'a');
+    BOOST_TEST(static_cstring<3>{"abc"}[2] == 'c');
+    BOOST_TEST(static_cstring<3>{"abc"}[3] == 0);
+    BOOST_TEST(static_cstring<3>{""}[0] == 0);
+
+    // front() / back().
+    BOOST_TEST(static_cstring<3>{"abc"}.front() == 'a');
+    BOOST_TEST(static_cstring<3>{"abc"}.back() == 'c');
+
+    // data() / c_str().
+    {
+        static_cstring<3> s("123");
+        BOOST_TEST(std::memcmp(s.data(), "123", 3) == 0);
+        BOOST_TEST(std::memcmp(s.c_str(), "123\0", 4) == 0);
+    }
+
+    // Modification through element access.
+    {
+        static_cstring<5> s("12345");
+        s[1] = '_';
+        BOOST_TEST(s == "1_345");
+        s.front() = 'A';
+        BOOST_TEST(s == "A_345");
+        s.back() = 'Z';
+        BOOST_TEST(s == "A_34Z");
+    }
+}
+
+static
+void
+testCStringIterators()
+{
+    {
+        static_cstring<3> s;
+        BOOST_TEST(std::distance(s.begin(), s.end()) == 0);
+        s = "123";
+        BOOST_TEST(std::distance(s.begin(), s.end()) == 3);
+    }
+    {
+        static_cstring<3> const s("123");
+        BOOST_TEST(std::distance(s.begin(), s.end()) == 3);
+        BOOST_TEST(std::distance(s.cbegin(), s.cend()) == 3);
+    }
+
+    // Iteration.
+    {
+        static_cstring<5> s("hello");
+        std::string result;
+        for (const char c : s)
+        {
+            result += c;
+        }
+        BOOST_TEST(result == "hello");
+    }
+}
+
+static
+void
+testCStringCapacity()
+{
+    // empty().
+    BOOST_TEST(static_cstring<0>{}.empty());
+    BOOST_TEST(static_cstring<1>{}.empty());
+    BOOST_TEST(!static_cstring<1>{"a"}.empty());
+
+    // size().
+    BOOST_TEST(static_cstring<0>{}.size() == 0);
+    BOOST_TEST(static_cstring<1>{"a"}.size() == 1);
+    BOOST_TEST(static_cstring<5>{"abc"}.size() == 3);
+
+    // length().
+    BOOST_TEST(static_cstring<0>{}.length() == 0);
+    BOOST_TEST(static_cstring<3>{"abc"}.length() == 3);
+
+    // max_size().
+    BOOST_TEST(static_cstring<0>{}.max_size() == 0);
+    BOOST_TEST(static_cstring<5>{"abc"}.max_size() == 5);
+
+    // capacity().
+    BOOST_TEST(static_cstring<0>{}.capacity() == 0);
+    BOOST_TEST(static_cstring<5>{"abc"}.capacity() == 5);
+}
+
+static
+void
+testCStringClear()
+{
+    static_cstring<3> s("123");
+    BOOST_TEST(!s.empty());
+    s.clear();
+    BOOST_TEST(s.empty());
+    BOOST_TEST(s.size() == 0);
+    BOOST_TEST(*s.end() == 0);
+    BOOST_TEST(s == "");
+}
+
+static
+void
+testCStringPushPop()
+{
+    // push_back().
+    {
+        static_cstring<5> s("abc");
+        s.push_back('d');
+        BOOST_TEST(s == "abcd");
+        BOOST_TEST(s.size() == 4);
+        BOOST_TEST(*s.end() == 0);
+
+        s.push_back('e');
+        BOOST_TEST(s == "abcde");
+        BOOST_TEST(s.size() == 5);
+
+        BOOST_TEST_THROWS(s.push_back('f'), std::length_error);
+    }
+
+    // pop_back().
+    {
+        static_cstring<5> s("abcde");
+        s.pop_back();
+        BOOST_TEST(s == "abcd");
+        BOOST_TEST(s.size() == 4);
+        BOOST_TEST(*s.end() == 0);
+
+        s.pop_back();
+        s.pop_back();
+        s.pop_back();
+        s.pop_back();
+        BOOST_TEST(s.empty());
+    }
+}
+
+static
+void
+testCStringAppend()
+{
+    // append(const CharT* s).
+    {
+        static_cstring<12> s("Hello");
+        s.append(", World");
+        BOOST_TEST(s == "Hello, World");
+        BOOST_TEST(*s.end() == 0);
+    }
+    {
+        static_cstring<5> s("abc");
+        BOOST_TEST_THROWS(s.append("def"), std::length_error);
+    }
+
+    // append(const CharT* s, size_type count)
+    {
+        static_cstring<10> s("abc");
+        s.append("defgh", 3);
+        BOOST_TEST(s == "abcdef");
+        BOOST_TEST(*s.end() == 0);
+    }
+
+    // append(size_type count, CharT ch)
+    {
+        static_cstring<10> s("abc");
+        s.append(3, 'x');
+        BOOST_TEST(s == "abcxxx");
+        BOOST_TEST(*s.end() == 0);
+        BOOST_TEST_THROWS(s.append(5, 'y'), std::length_error);
+    }
+
+    // operator+=()
+    {
+        static_cstring<10> s("abc");
+        s += "def";
+        BOOST_TEST(s == "abcdef");
+        s += 'g';
+        BOOST_TEST(s == "abcdefg");
+        BOOST_TEST(*s.end() == 0);
+    }
+}
+
+static
+void
+testCStringComparison()
+{
+    // operator==() / operator!=().
+    {
+        static_cstring<10> a("abc");
+        static_cstring<10> b("abc");
+        static_cstring<10> c("abd");
+
+        BOOST_TEST(a == b);
+        BOOST_TEST(!(a == c));
+        BOOST_TEST(a != c);
+        BOOST_TEST(!(a != b));
+    }
+
+    // operator<(), <=(), >(), >=().
+    {
+        static_cstring<10> a("abc");
+        static_cstring<10> b("abd");
+
+        BOOST_TEST(a < b);
+        BOOST_TEST(a <= b);
+        BOOST_TEST(a <= a);
+        BOOST_TEST(b > a);
+        BOOST_TEST(b >= a);
+        BOOST_TEST(b >= b);
+    }
+
+    // Comparison with CharT const*.
+    {
+        static_cstring<10> s("hello");
+        BOOST_TEST(s == "hello");
+        BOOST_TEST("hello" == s);
+        BOOST_TEST(s != "world");
+        BOOST_TEST("world" != s);
+    }
+
+    // compare().
+    {
+        static_cstring<10> s("abc");
+        BOOST_TEST(s.compare("abc") == 0);
+        BOOST_TEST(s.compare("abd") < 0);
+        BOOST_TEST(s.compare("abb") > 0);
+    }
+}
+
+static
+void
+testCStringConversion()
+{
+    // operator string_view().
+    {
+        static_cstring<10> s("hello");
+        std::string_view sv = s;
+        BOOST_TEST(sv == "hello");
+        BOOST_TEST(sv.size() == 5);
+    }
+
+    // str().
+    {
+        static_cstring<10> s("hello");
+        std::string str = s.str();
+        BOOST_TEST(str == "hello");
+        BOOST_TEST(str.size() == 5);
+    }
+}
+
+static
+void
+testCStringStream()
+{
+    static_cstring<10> s("hello");
+    std::ostringstream oss;
+    oss << s;
+    BOOST_TEST(oss.str() == "hello");
+}
+
+static
+void
+testCStringSwap()
+{
+    static_cstring<10> a("hello");
+    static_cstring<10> b("world");
+
+    a.swap(b);
+
+    BOOST_TEST(a == "world");
+    BOOST_TEST(b == "hello");
+}
+
+static
+void
+testCStringConstexpr()
+{
+    // constexpr construction and operations.
+    constexpr static_cstring<10> ce("test");
+    static_assert(ce.size() == 4, "");
+    static_assert(ce[0] == 't', "");
+    static_assert(ce[1] == 'e', "");
+    static_assert(ce[2] == 's', "");
+    static_assert(ce[3] == 't', "");
+    static_assert(!ce.empty(), "");
+    static_assert(ce.max_size() == 10, "");
+    static_assert(ce.capacity() == 10, "");
+
+    constexpr static_cstring<5> ce_empty;
+    static_assert(ce_empty.empty(), "");
+    static_assert(ce_empty.size() == 0, "");
+}
+
+// Helper struct to test NTTP.
+template<basic_static_cstring S>
+struct NTTPHelper
+{
+    static constexpr std::size_t size() noexcept
+    {
+        return S.size();
+    }
+
+    static constexpr const char* c_str() noexcept
+    {
+        return S.c_str();
+    }
+};
+
+static
+void
+testCStringNTTP()
+{
+    // Test non-type template parameter usage (C++20).
+
+    // Test size deduction from string literals.
+    BOOST_TEST(NTTPHelper<"hello">::size() == 5);
+    BOOST_TEST(NTTPHelper<"">::size() == 0);
+    BOOST_TEST(NTTPHelper<"test">::size() == 4);
+
+    // Test that different strings create different types
+    constexpr bool same_type = std::is_same_v<
+        NTTPHelper<"hello">,
+        NTTPHelper<"hello">>;
+    BOOST_TEST(same_type);
+
+    constexpr bool different_type = !std::is_same_v<
+        NTTPHelper<"hello">,
+        NTTPHelper<"world">>;
+    BOOST_TEST(different_type);
+
+    // Test constexpr access.
+    constexpr std::size_t len = NTTPHelper<"constexpr">::size();
+    static_assert(len == 9, "");
+}
+
+static
+void
+testCStringPODUsage()
+{
+    // Test usage in POD types (the original motivation).
+    struct UserRecord
+    {
+        int id;
+        static_cstring<63> name;
+        unsigned int flags;
+    };
+
+    static_assert(sizeof(static_cstring<63>) == 64, "");
+    static_assert(std::is_trivially_copyable<UserRecord>::value, "");
+
+    UserRecord user{};
+    user.id = 42;
+    user.name = "Alice";
+    user.flags = 0xFF;
+
+    BOOST_TEST(user.id == 42);
+    BOOST_TEST(user.name.size() == 5);
+    BOOST_TEST(user.name == "Alice");
+    BOOST_TEST(user.flags == 0xFF);
+
+    // Copy the struct.
+    UserRecord copy = user;
+    BOOST_TEST(copy.name == "Alice");
+
+    // memcpy() should work.
+    UserRecord memcpy_dest;
+    std::memcpy(&memcpy_dest, &user, sizeof(UserRecord));
+    BOOST_TEST(memcpy_dest.name == "Alice");
+}
+
+static
+void
+testCStringLargeCapacity()
+{
+    // Test strings with N > UCHAR_MAX (no remaining-capacity trick).
+    {
+        static_cstring<300> s;
+        BOOST_TEST(s.empty());
+        BOOST_TEST(s.size() == 0);
+
+        s = "This is a test string";
+        BOOST_TEST(s.size() == 21);
+        BOOST_TEST(s == "This is a test string");
+
+        s.clear();
+        BOOST_TEST(s.empty());
+    }
+
+    // Large string operations.
+    {
+        static_cstring<500> s;
+        s.assign(400, 'x');
+        BOOST_TEST(s.size() == 400);
+        BOOST_TEST(s[0] == 'x');
+        BOOST_TEST(s[399] == 'x');
+        BOOST_TEST(*s.end() == 0);
+    }
+}
+
+static
+void
+testCStringWideChar()
+{
+    // Test with wchar_t.
+    {
+        static_wcstring<10> ws;
+        BOOST_TEST(ws.empty());
+        BOOST_TEST(ws.size() == 0);
+    }
+
+    {
+        static_wcstring<10> ws(L"hello");
+        BOOST_TEST(ws.size() == 5);
+        BOOST_TEST(ws[0] == L'h');
+        BOOST_TEST(ws[4] == L'o');
+        BOOST_TEST(*ws.end() == 0);
+    }
+}
+
+int
+runTests()
+{
+  testCStringSizeGuarantee();
+  testCStringRemainingCapacityTrick();
+  testCStringTypeTraits();
+  testCStringConstruct();
+  testCStringAssignment();
+  testCStringElements();
+  testCStringIterators();
+  testCStringCapacity();
+  testCStringClear();
+  testCStringPushPop();
+  testCStringAppend();
+  testCStringComparison();
+  testCStringConversion();
+  testCStringStream();
+  testCStringSwap();
+  testCStringConstexpr();
+  testCStringNTTP();
+  testCStringPODUsage();
+  testCStringLargeCapacity();
+  testCStringWideChar();
+
+  return report_errors();
+}
+}
+}
+
+int
+main()
+{
+  return boost::static_strings::runTests();
+}