Add support for C++17.

.github/workflows/build-linux-clang.yml

@@ -23,7 +23,11 @@ permissions:
 jobs:
   build-linux-clang:
     runs-on: ubuntu-latest
-    name: Ubuntu Clang 18
+    name: Ubuntu Clang 18 (C++${{ matrix.std }})
+    strategy:
+      fail-fast: false
+      matrix:
+        std: [17, 20]
 
     steps:
       - name: Checkout repository
@@ -37,9 +41,9 @@ jobs:
           path: |
             build
             ~/.cache/ccache
-          key: ${{ runner.os }}-clang18-ninja-${{ hashFiles('**/CMakeLists.txt', 'include/**') }}
+          key: ${{ runner.os }}-clang18-cxx${{ matrix.std }}-ninja-${{ hashFiles('**/CMakeLists.txt', 'include/**') }}
           restore-keys: |
-            ${{ runner.os }}-clang18-ninja-
+            ${{ runner.os }}-clang18-cxx${{ matrix.std }}-ninja-
 
       - name: Set up build environment
         run: |
@@ -65,6 +69,7 @@ jobs:
           cmake -B build \
                 -G "Ninja" \
                 -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} \
+                -DNFX_STRINGBUILDER_CXX_STANDARD=${{ matrix.std }} \
                 -DNFX_STRINGBUILDER_BUILD_STATIC=ON \
                 -DNFX_STRINGBUILDER_BUILD_SHARED=ON \
                 -DNFX_STRINGBUILDER_BUILD_TESTS=ON \
@@ -83,7 +88,7 @@ jobs:
         if: failure()
         uses: actions/upload-artifact@v4
         with:
-          name: test-results-linux-clang18
+          name: test-results-linux-clang18-cxx${{ matrix.std }}
           path: |
             build/Testing/Temporary/LastTest.log
             build/Testing/Temporary/LastTestsFailed.log

.github/workflows/build-linux-gcc.yml

@@ -23,7 +23,11 @@ permissions:
 jobs:
   build-linux-gcc:
     runs-on: ubuntu-latest
-    name: Ubuntu GCC 14
+    name: Ubuntu GCC 14 (C++${{ matrix.std }})
+    strategy:
+      fail-fast: false
+      matrix:
+        std: [17, 20]
 
     steps:
       - name: Checkout repository
@@ -37,9 +41,9 @@ jobs:
           path: |
             build
             ~/.cache/ccache
-          key: ${{ runner.os }}-gcc14-ninja-${{ hashFiles('**/CMakeLists.txt', 'include/**') }}
+          key: ${{ runner.os }}-gcc14-cxx${{ matrix.std }}-ninja-${{ hashFiles('**/CMakeLists.txt', 'include/**') }}
           restore-keys: |
-            ${{ runner.os }}-gcc14-ninja-
+            ${{ runner.os }}-gcc14-cxx${{ matrix.std }}-ninja-
 
       - name: Set up build environment
         run: |
@@ -65,6 +69,7 @@ jobs:
           cmake -B build \
                 -G "Ninja" \
                 -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} \
+                -DNFX_STRINGBUILDER_CXX_STANDARD=${{ matrix.std }} \
                 -DNFX_STRINGBUILDER_BUILD_STATIC=ON \
                 -DNFX_STRINGBUILDER_BUILD_SHARED=ON \
                 -DNFX_STRINGBUILDER_BUILD_TESTS=ON \
@@ -83,7 +88,7 @@ jobs:
         if: failure()
         uses: actions/upload-artifact@v4
         with:
-          name: test-results-linux-gcc14
+          name: test-results-linux-gcc14-cxx${{ matrix.std }}
           path: |
             build/Testing/Temporary/LastTest.log
             build/Testing/Temporary/LastTestsFailed.log

.github/workflows/build-windows-mingw.yml

@@ -23,7 +23,11 @@ permissions:
 jobs:
   build-windows-mingw:
     runs-on: windows-2022
-    name: Windows MinGW
+    name: Windows MinGW (C++${{ matrix.std }})
+    strategy:
+      fail-fast: false
+      matrix:
+        std: [17, 20]
 
     steps:
       - name: Checkout repository
@@ -37,9 +41,9 @@ jobs:
           path: |
             build
             ${{ runner.temp }}/ccache
-          key: ${{ runner.os }}-mingw-ninja-${{ hashFiles('**/CMakeLists.txt', 'include/**') }}
+          key: ${{ runner.os }}-mingw-cxx${{ matrix.std }}-ninja-${{ hashFiles('**/CMakeLists.txt', 'include/**') }}
           restore-keys: |
-            ${{ runner.os }}-mingw-ninja-
+            ${{ runner.os }}-mingw-cxx${{ matrix.std }}-ninja-
 
       - name: Set up MinGW
         uses: msys2/setup-msys2@v2
@@ -72,6 +76,7 @@ jobs:
           cmake -B build \
                 -G "Ninja" \
                 -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} \
+                -DNFX_STRINGBUILDER_CXX_STANDARD=${{ matrix.std }} \
                 -DNFX_STRINGBUILDER_BUILD_STATIC=ON \
                 -DNFX_STRINGBUILDER_BUILD_SHARED=ON \
                 -DNFX_STRINGBUILDER_BUILD_TESTS=ON \
@@ -94,7 +99,7 @@ jobs:
         if: failure()
         uses: actions/upload-artifact@v4
         with:
-          name: test-results-windows-mingw
+          name: test-results-windows-mingw-cxx${{ matrix.std }}
           path: |
             build/Testing/Temporary/LastTest.log
             build/Testing/Temporary/LastTestsFailed.log

.github/workflows/build-windows-msvc.yml

@@ -23,7 +23,11 @@ permissions:
 jobs:
   build-windows-msvc:
     runs-on: windows-2022
-    name: Windows MSVC 2022
+    name: Windows MSVC 2022 (C++${{ matrix.std }})
+    strategy:
+      fail-fast: false
+      matrix:
+        std: [17, 20]
 
     steps:
       - name: Checkout repository
@@ -37,12 +41,12 @@ jobs:
           path: |
             build
             C:\Users\runneradmin\AppData\Local\Microsoft\MSBuild
-          key: ${{ runner.os }}-msvc2022-vs2022-${{ hashFiles('**/CMakeLists.txt', 'include/**') }}
+          key: ${{ runner.os }}-msvc2022-cxx${{ matrix.std }}-vs2022-${{ hashFiles('**/CMakeLists.txt', 'include/**') }}
           restore-keys: |
-            ${{ runner.os }}-msvc2022-vs2022-
+            ${{ runner.os }}-msvc2022-cxx${{ matrix.std }}-vs2022-
 
       - name: Configure CMake
-        run: cmake -B build -G "Visual Studio 17 2022" -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} -DNFX_STRINGBUILDER_BUILD_STATIC=ON -DNFX_STRINGBUILDER_BUILD_SHARED=ON -DNFX_STRINGBUILDER_BUILD_TESTS=ON -DNFX_STRINGBUILDER_BUILD_SAMPLES=ON -DNFX_STRINGBUILDER_BUILD_BENCHMARKS=ON -DNFX_STRINGBUILDER_BUILD_DOCUMENTATION=OFF
+        run: cmake -B build -G "Visual Studio 17 2022" -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} -DNFX_STRINGBUILDER_CXX_STANDARD=${{ matrix.std }} -DNFX_STRINGBUILDER_BUILD_STATIC=ON -DNFX_STRINGBUILDER_BUILD_SHARED=ON -DNFX_STRINGBUILDER_BUILD_TESTS=ON -DNFX_STRINGBUILDER_BUILD_SAMPLES=ON -DNFX_STRINGBUILDER_BUILD_BENCHMARKS=ON -DNFX_STRINGBUILDER_BUILD_DOCUMENTATION=OFF
 
       - name: Build with MSVC
         run: |
@@ -61,7 +65,7 @@ jobs:
         if: failure()
         uses: actions/upload-artifact@v4
         with:
-          name: test-results-windows-msvc2022
+          name: test-results-windows-msvc2022-cxx${{ matrix.std }}
           path: |
             build/Testing/Temporary/LastTest.log
             build/Testing/Temporary/LastTestsFailed.log

CMakeLists.txt

@@ -38,6 +38,12 @@ enable_testing()
 option(NFX_STRINGBUILDER_BUILD_STATIC                 "Build static library"               ON )
 option(NFX_STRINGBUILDER_BUILD_SHARED                 "Build shared library"               OFF)
 
+# --- Language standard ---
+# Minimum required standard is C++17. C++20 additionally enables the
+# std::format()-based formatting API and the std::formatter specialization.
+set(NFX_STRINGBUILDER_CXX_STANDARD "20" CACHE STRING "C++ standard used to build nfx-stringbuilder (17 or 20)")
+set_property(CACHE NFX_STRINGBUILDER_CXX_STANDARD PROPERTY STRINGS "17" "20")
+
 # --- Build components ---
 option(NFX_STRINGBUILDER_BUILD_TESTS                  "Build tests"                        OFF)
 option(NFX_STRINGBUILDER_BUILD_SAMPLES                "Build samples"                      OFF)

README.md

@@ -4,17 +4,17 @@
 
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg?style=flat-square)](https://github.com/nfx-libs/nfx-stringbuilder/blob/main/LICENSE) [![GitHub release (latest by date)](https://img.shields.io/github/v/release/nfx-libs/nfx-stringbuilder?style=flat-square)](https://github.com/nfx-libs/nfx-stringbuilder/releases) [![GitHub tag (latest by date)](https://img.shields.io/github/tag/nfx-libs/nfx-stringbuilder?style=flat-square)](https://github.com/nfx-libs/nfx-stringbuilder/tags)<br/>
 
-![C++20](https://img.shields.io/badge/C%2B%2B-20-blue?style=flat-square) ![CMake](https://img.shields.io/badge/CMake-3.20%2B-green.svg?style=flat-square) ![Cross Platform](https://img.shields.io/badge/Platform-Linux_Windows-lightgrey?style=flat-square)
+![C++17/20](https://img.shields.io/badge/C%2B%2B-17%2F20-blue?style=flat-square) ![CMake](https://img.shields.io/badge/CMake-3.20%2B-green.svg?style=flat-square) ![Cross Platform](https://img.shields.io/badge/Platform-Linux_Windows-lightgrey?style=flat-square)
 
 <!-- CI/CD Status -->
 
 [![Linux GCC](https://img.shields.io/github/actions/workflow/status/nfx-libs/nfx-stringbuilder/build-linux-gcc.yml?branch=main&label=Linux%20GCC&style=flat-square)](https://github.com/nfx-libs/nfx-stringbuilder/actions/workflows/build-linux-gcc.yml) [![Linux Clang](https://img.shields.io/github/actions/workflow/status/nfx-libs/nfx-stringbuilder/build-linux-clang.yml?branch=main&label=Linux%20Clang&style=flat-square)](https://github.com/nfx-libs/nfx-stringbuilder/actions/workflows/build-linux-clang.yml) [![Windows MinGW](https://img.shields.io/github/actions/workflow/status/nfx-libs/nfx-stringbuilder/build-windows-mingw.yml?branch=main&label=Windows%20MinGW&style=flat-square)](https://github.com/nfx-libs/nfx-stringbuilder/actions/workflows/build-windows-mingw.yml) [![Windows MSVC](https://img.shields.io/github/actions/workflow/status/nfx-libs/nfx-stringbuilder/build-windows-msvc.yml?branch=main&label=Windows%20MSVC&style=flat-square)](https://github.com/nfx-libs/nfx-stringbuilder/actions/workflows/build-windows-msvc.yml) [![CodeQL](https://img.shields.io/github/actions/workflow/status/nfx-libs/nfx-stringbuilder/codeql.yml?branch=main&label=CodeQL&style=flat-square)](https://github.com/nfx-libs/nfx-stringbuilder/actions/workflows/codeql.yml)
 
-> A cross-platform C++20 high-performance string builder with Small Buffer Optimization and efficient memory management
+> A cross-platform C++17/C++20 high-performance string builder with Small Buffer Optimization and efficient memory management
 
 ## Overview
 
-**nfx-stringbuilder** is a modern C++20 library providing efficient string building capabilities with zero heap allocations for small strings (≤256 bytes). Designed for applications requiring high-performance string concatenation with minimal allocations, it features Small Buffer Optimization (SBO), comprehensive type support, and C++20 std::format integration.
+**nfx-stringbuilder** is a modern C++ library providing efficient string building capabilities with zero heap allocations for small strings (≤256 bytes). Designed for applications requiring high-performance string concatenation with minimal allocations, it features Small Buffer Optimization (SBO) and comprehensive type support. It builds with **C++17 or C++20**: the core API is fully available under C++17, while C++20 additionally enables the `std::format`-based `format()` method and the `std::formatter` specialization.
 
 ## Key Features
 
@@ -23,8 +23,8 @@
 - **Fluent API**: Method chaining and stream operators (`<<`) for natural concatenation
 - **Variadic append()**: Batch multiple arguments in a single call for optimal performance
 - **Type Support**: Strings, string_view, C-strings, characters, and numeric types (int8/16/32/64, uint8/16/32/64, float, double)
-- **C++20 std::format Integration**: Template `format()` method for modern formatting
-- **std::formatter Specializations**: Zero-copy integration with `std::format` for StringBuilder
+- **C++20 std::format Integration**: Template `format()` method for modern formatting *(requires C++20)*
+- **std::formatter Specializations**: Zero-copy integration with `std::format` for StringBuilder *(requires C++20)*
 - **Capacity Hints**: Pre-allocate buffers with constructor parameter for optimal performance
 - **Direct Buffer Access**: High-performance operations without wrappers
 - **Iterator Support**: Range-based for loops and STL algorithms
@@ -57,19 +57,31 @@
 
 ### Requirements
 
-- C++20 compatible compiler:
+- C++17 or C++20 compatible compiler:
   - **GCC 14+** (14.2.0 tested)
   - **Clang 18+** (19.1.7 tested)
   - **MSVC 2022+** (19.44+ tested)
 - CMake 3.20 or higher
 
+> **C++ standard:** The library defaults to building with C++20. Select the
+> standard explicitly with `-DNFX_STRINGBUILDER_CXX_STANDARD=17` or `=20`.
+> Under C++17 the entire API is available except the `std::format`-based
+> `format()` method and the `std::formatter<StringBuilder>` specialization,
+> which require C++20. The exported `nfx-stringbuilder::static` and
+> `nfx-stringbuilder::nfx-stringbuilder` targets propagate a minimum
+> requirement of C++17 to consumers, who may compile at a higher standard than
+> the library was built with.
+
 ### CMake Integration
 
 ```cmake
 # --- Library build types ---
 option(NFX_STRINGBUILDER_BUILD_STATIC                 "Build static library"               ON )
 option(NFX_STRINGBUILDER_BUILD_SHARED                 "Build shared library"               OFF)
 
+# --- Language standard (17 or 20; C++20 enables std::format APIs) ---
+set(NFX_STRINGBUILDER_CXX_STANDARD "20" CACHE STRING "C++ standard used to build nfx-stringbuilder (17 or 20)")
+
 # --- Build components ---
 option(NFX_STRINGBUILDER_BUILD_TESTS                  "Build tests"                        OFF)
 option(NFX_STRINGBUILDER_BUILD_SAMPLES                "Build samples"                      OFF)

benchmark/BM_Comparative.cpp

@@ -42,7 +42,9 @@
 #    include <absl/strings/str_join.h>
 #endif
 
-#include <format>
+#if defined( NFX_STRINGBUILDER_HAS_STD_FORMAT )
+#    include <format>
+#endif
 #include <sstream>
 
 namespace nfx::string::benchmark
@@ -545,9 +547,11 @@ namespace nfx::string::benchmark
         for( auto _ : state )
         {
             (void)_;
+#if defined( NFX_STRINGBUILDER_HAS_STD_FORMAT )
             std::string result =
                 std::format( "User {} (ID: {}) scored {:.2f} points at {}", "Alice", 42, 95.75, "2024-01-15 10:30:00" );
             ::benchmark::DoNotOptimize( result );
+#endif
         }
     }
 
@@ -582,11 +586,13 @@ namespace nfx::string::benchmark
         for( auto _ : state )
         {
             (void)_;
+#if defined( NFX_STRINGBUILDER_HAS_STD_FORMAT )
             StringBuilder builder;
             builder.append( std::format(
                 "User {} (ID: {}) scored {:.2f} points at {}", "Alice", 42, 95.75, "2024-01-15 10:30:00" ) );
             std::string result = builder.toString();
             ::benchmark::DoNotOptimize( result );
+#endif
         }
     }
 

benchmark/CMakeLists.txt

@@ -211,7 +211,7 @@ foreach(benchmark_source ${benchmark_sources})
 
         set_target_properties(${benchmark_target_name}
             PROPERTIES
-                CXX_STANDARD 20
+                CXX_STANDARD ${NFX_STRINGBUILDER_CXX_STANDARD}
                 CXX_STANDARD_REQUIRED ON
                 CXX_EXTENSIONS OFF
                 POSITION_INDEPENDENT_CODE ON

cmake/nfxStringBuilderBuildConfig.cmake

@@ -30,6 +30,19 @@ if(NOT NFX_STRINGBUILDER_BUILD_STATIC AND NOT NFX_STRINGBUILDER_BUILD_SHARED)
     endif()
 endif()
 
+# --- Validate the requested C++ standard ---
+if(NOT NFX_STRINGBUILDER_CXX_STANDARD MATCHES "^(17|20)$")
+    message(FATAL_ERROR
+        "NFX_STRINGBUILDER_CXX_STANDARD must be either 17 or 20 "
+        "(got '${NFX_STRINGBUILDER_CXX_STANDARD}').")
+endif()
+
+if(NFX_STRINGBUILDER_CXX_STANDARD STREQUAL "17")
+    message(STATUS "nfx-stringbuilder: Building with C++17 (std::format API disabled)")
+else()
+    message(STATUS "nfx-stringbuilder: Building with C++${NFX_STRINGBUILDER_CXX_STANDARD}")
+endif()
+
 #----------------------------------------------
 # Multi-config generator setup
 #----------------------------------------------

cmake/nfxStringBuilderTargets.cmake

@@ -65,7 +65,7 @@ function(configure_target target_name)
     # --- Properties ---
     set_target_properties(${target_name}
         PROPERTIES
-            CXX_STANDARD 20
+            CXX_STANDARD ${NFX_STRINGBUILDER_CXX_STANDARD}
             CXX_STANDARD_REQUIRED ON
             CXX_EXTENSIONS OFF
             DEBUG_POSTFIX "-d"
@@ -74,6 +74,12 @@ function(configure_target target_name)
             SOVERSION ${PROJECT_VERSION_MAJOR}
     )
 
+    # --- Public language requirement ---
+    # Propagate a minimum of C++17 to consumers of the static/shared targets.
+    # The headers compile under both C++17 and C++20; consumers may build at a
+    # higher standard than the library was compiled with.
+    target_compile_features(${target_name} PUBLIC cxx_std_17)
+
     # --- Enable specific CPU features ---
     if(NFX_STRINGBUILDER_ENABLE_SIMD)
         include(CheckCXXSourceCompiles)