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

@@ -25,14 +25,132 @@ on:
       - meta/**
       - README.md
 
-env:
-  B2_TARGETS: libs/$SELF/example
-
 jobs:
-  call-boost-ci:
-    name: Run Boost.CI
-    uses: boostorg/boost-ci/.github/workflows/reusable.yml@master
-    with:
-      exclude_cxxstd: '98,03,0x,11,14,17'
-      enable_pr_coverage: false
-      enable_multiarch: false
+  linux:
+    runs-on: ubuntu-24.04
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - { toolset: gcc-13,   cxxstd: '20,23' }
+          - { toolset: gcc-14,   cxxstd: '20,23,26' }
+          - { toolset: clang-17, cxxstd: '20,23' }
+          - { toolset: clang-18, cxxstd: '20,23,26' }
+
+    steps:
+      - name: Checkout Boost super-project
+        uses: actions/checkout@v4
+        with:
+          repository: boostorg/boost
+          ref: develop
+          fetch-depth: 0
+
+      - name: Checkout this library
+        uses: actions/checkout@v4
+        with:
+          path: libs/static_string
+          fetch-depth: 0
+
+      - name: Initialize Boost submodules
+        run: |
+          git submodule update --init tools/boostdep
+          python tools/boostdep/depinst/depinst.py --git_args '--jobs 4' static_string
+
+      - name: Bootstrap b2
+        run: ./bootstrap.sh
+
+      - name: Generate Boost headers
+        run: ./b2 headers
+
+      - name: Build and run example tests
+        run: |
+          ./b2 libs/static_string/example/static_cstring \
+            toolset=${{ 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/static_cstring.hpp

@@ -25,7 +25,7 @@ namespace static_strings {
 
 namespace detail {
 
-// Primary template: No remaining-capacity trick; uses traits::length() for size.
+// 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
 {
@@ -46,17 +46,12 @@ public:
         data_[sz] = value_type{};
     }
 
-    constexpr void init_empty() noexcept
-    {
-        data_[0] = 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: Use remaining-capacity trick.
+// 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>
 {
@@ -78,12 +73,6 @@ public:
         data_[N] = static_cast<value_type>(N - sz);
     }
 
-    constexpr void init_empty() noexcept
-    {
-        data_[0] = value_type{};
-        data_[N] = static_cast<value_type>(N);
-    }
-
     // 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;
@@ -100,7 +89,6 @@ public:
     using base::data_;
     using base::get_size;
     using base::set_size;
-    using base::init_empty;
 
     // Member types
     using traits_type     = Traits;
@@ -120,7 +108,7 @@ public:
     // Constructors.
     constexpr basic_static_cstring() noexcept
     {
-        init_empty();
+        set_size(0);
     }
 
     constexpr basic_static_cstring(const CharT* s)
@@ -201,21 +189,25 @@ public:
 
     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];
     }
 
@@ -268,7 +260,7 @@ public:
     // Modifiers.
     constexpr void clear() noexcept
     {
-        init_empty();
+        set_size(0);
     }
 
     constexpr basic_static_cstring& assign(const CharT* s)

example/static_cstring/static_cstring_test.cpp

@@ -75,41 +75,39 @@ testCStringRemainingCapacityTrick()
         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]) == (large.capacity() - large.size()));
+        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()
 {
-    // static_cstring should be trivially copyable for use in POD structs.
     {
         using S = static_cstring<0>;
-        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, "");
+        CStringTypeTraits< S > check;
+        static_cast<void>(check);
     }
     {
         using S = static_cstring<63>;
-        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, "");
+        CStringTypeTraits< S > check;
+        static_cast<void>(check);
     }
     {
         using S = static_cstring<300>;
-        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, "");
+        CStringTypeTraits< S > check;
+        static_cast<void>(check);
     }
 }
 

test/constexpr_tests.hpp

@@ -72,9 +72,8 @@ struct cxper_char_traits
     // This implementation does not handle overlapping ranges where
     // dest > src. A correct implementation would need to detect this
     // case and copy backwards, but detecting overlap requires pointer
-    // comparisons that are not allowed in constant expressions when
-    // the pointers point to unrelated objects (e.g., a string literal
-    // and the static_string's internal buffer).
+    // comparisons that many of the tested compiles (incorrectly) refuse
+    // in constant expressions.
     //
     // Since cxper_char_traits is only used for testing constexpr
     // functionality and the tests do not exercise overlapping moves