From d63bbdf9f7c74870dd0794387327999f43bf2063 Mon Sep 17 00:00:00 2001
From: Thomas Willson <thomas.willson@staflsystems.com>
Date: Tue, 7 Jul 2026 22:48:19 +0000
Subject: [PATCH] Add snprintf fallback for float/double formatting on
 toolchains without floating std::to_chars

Integer std::to_chars is universally available, but the floating-point
overloads are not implemented everywhere: IAR's libc++ (bxarm) ships
std::to_chars(float/double) as deleted functions, so StringBuilder.cpp
fails to compile there even when no float/double is ever formatted.

Guard the four floating-point formatters (append/prepend for float and
double) behind NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS, which defaults to 0
under IAR and 1 elsewhere, and can be overridden by the consumer. When
disabled, format via std::snprintf("%.*g", max_digits10, ...) into a
stack buffer and reuse the existing append/prepend(string_view) path.
Integer formatting is untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 include/nfx/detail/string/StringBuilder.inl | 45 +++++++++++++++++++++
 include/nfx/string/StringBuilder.h          | 16 ++++++++
 2 files changed, 61 insertions(+)

diff --git a/include/nfx/detail/string/StringBuilder.inl b/include/nfx/detail/string/StringBuilder.inl
index 74db4dc..cc6b828 100644
--- a/include/nfx/detail/string/StringBuilder.inl
+++ b/include/nfx/detail/string/StringBuilder.inl
@@ -33,6 +33,11 @@
 #include <iterator>
 #include <utility>
 
+#if !NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS
+#    include <cstdio>
+#    include <limits>
+#endif
+
 namespace nfx::string
 {
     //=====================================================================
@@ -298,11 +303,21 @@ namespace nfx::string
         {
             ensureCapacity( m_size + FLOAT_MAX_CHARS );
         }
+#if NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS
         auto [ptr, ec] = std::to_chars( m_buffer + m_size, m_buffer + m_capacity, value );
         if( ec == std::errc() ) NFX_STRINGBUILDER_LIKELY
         {
             m_size = ptr - m_buffer;
         }
+#else
+        char scratch[FLOAT_MAX_CHARS + 1];
+        const int written = std::snprintf( scratch, sizeof( scratch ), "%.*g",
+            std::numeric_limits<float>::max_digits10, static_cast<double>( value ) );
+        if( written > 0 )
+        {
+            return append( std::string_view{ scratch, static_cast<size_t>( written ) } );
+        }
+#endif
         return *this;
     }
 
@@ -312,11 +327,21 @@ namespace nfx::string
         {
             ensureCapacity( m_size + DOUBLE_MAX_CHARS );
         }
+#if NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS
         auto [ptr, ec] = std::to_chars( m_buffer + m_size, m_buffer + m_capacity, value );
         if( ec == std::errc() ) NFX_STRINGBUILDER_LIKELY
         {
             m_size = ptr - m_buffer;
         }
+#else
+        char scratch[DOUBLE_MAX_CHARS + 1];
+        const int written = std::snprintf( scratch, sizeof( scratch ), "%.*g",
+            std::numeric_limits<double>::max_digits10, value );
+        if( written > 0 )
+        {
+            return append( std::string_view{ scratch, static_cast<size_t>( written ) } );
+        }
+#endif
         return *this;
     }
 
@@ -491,23 +516,43 @@ namespace nfx::string
 
     inline StringBuilder& StringBuilder::prepend( float value )
     {
+#if NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS
         char buffer[FLOAT_MAX_CHARS];
         auto [ptr, ec] = std::to_chars( buffer, buffer + FLOAT_MAX_CHARS, value );
         if( ec == std::errc() ) NFX_STRINGBUILDER_LIKELY
         {
             return prepend( std::string_view{ buffer, static_cast<size_t>( ptr - buffer ) } );
         }
+#else
+        char buffer[FLOAT_MAX_CHARS + 1];
+        const int written = std::snprintf( buffer, sizeof( buffer ), "%.*g",
+            std::numeric_limits<float>::max_digits10, static_cast<double>( value ) );
+        if( written > 0 )
+        {
+            return prepend( std::string_view{ buffer, static_cast<size_t>( written ) } );
+        }
+#endif
         return *this;
     }
 
     inline StringBuilder& StringBuilder::prepend( double value )
     {
+#if NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS
         char buffer[DOUBLE_MAX_CHARS];
         auto [ptr, ec] = std::to_chars( buffer, buffer + DOUBLE_MAX_CHARS, value );
         if( ec == std::errc() ) NFX_STRINGBUILDER_LIKELY
         {
             return prepend( std::string_view{ buffer, static_cast<size_t>( ptr - buffer ) } );
         }
+#else
+        char buffer[DOUBLE_MAX_CHARS + 1];
+        const int written = std::snprintf( buffer, sizeof( buffer ), "%.*g",
+            std::numeric_limits<double>::max_digits10, value );
+        if( written > 0 )
+        {
+            return prepend( std::string_view{ buffer, static_cast<size_t>( written ) } );
+        }
+#endif
         return *this;
     }
 
diff --git a/include/nfx/string/StringBuilder.h b/include/nfx/string/StringBuilder.h
index 416d002..2169f6a 100644
--- a/include/nfx/string/StringBuilder.h
+++ b/include/nfx/string/StringBuilder.h
@@ -132,6 +132,22 @@
 #    define NFX_STRINGBUILDER_UNLIKELY
 #endif
 
+/**
+ * @def NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS
+ * @brief Whether std::to_chars(float/double) is usable on this toolchain.
+ * @details Integer std::to_chars is universally available, but the
+ *          floating-point overloads are not: IAR's libc++ ships them as deleted
+ *          functions. When unavailable the float/double formatters fall back to
+ *          std::snprintf. Define this to 0/1 before including to override.
+ */
+#if !defined( NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS )
+#    if defined( __IAR_SYSTEMS_ICC__ )
+#        define NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS 0
+#    else
+#        define NFX_STRINGBUILDER_HAS_FLOAT_TO_CHARS 1
+#    endif
+#endif
+
 namespace nfx::string
 {
     //=====================================================================