Merge pull request #562 from cppalliance/charconv_limits

Add buffer sizing for charconv

Author: mborland

doc/decimal/charconv.adoc

@@ -67,3 +67,44 @@ NOTE: `BOOST_DECIMAL_CONSTEXPR` is defined if:
  - `\\__GNUC__` >= 9
  - Compiler has: `__builtin_is_constant_evaluated()`
  - C++20 support with: `std::is_constant_evaluated()`
+
+The library offers an additional feature for sizing buffers without specified precision and in general format
+[source, c++]
+----
+namespace boost {
+namespace decimal {
+
+template <typename T>
+struct limits
+{
+    static constexpr int max_chars;
+}
+
+} //namespace decimal
+} //namespace boost
+----
+
+The member can then be used to size buffers such as:
+
+[source, c++]
+----
+#include <boost/decimal.hpp>
+#include <iostream>
+
+int main()
+{
+    using namespace boost::decimal;
+
+    decimal32 val {5, -1};
+
+    char buffer[limits<T>::max_chars];
+
+    auto r_to = to_chars(buffer, buffer + sizeof(buffer), val);
+    *r_to.ptr = '\0';
+
+    std::cout << buffer << std::endl;
+
+    return 0;
+}
+
+----

include/boost/decimal/charconv.hpp

@@ -812,6 +812,18 @@ BOOST_DECIMAL_EXPORT BOOST_DECIMAL_CONSTEXPR auto to_chars(char* first, char* la
     return detail::to_chars_impl(first, last, value, fmt, precision);
 }
 
+template <typename T>
+struct limits
+{
+    BOOST_DECIMAL_ATTRIBUTE_UNUSED static constexpr int max_chars = boost::decimal::detail::max_string_length_v<T>;
+};
+
+#if !(defined(__cpp_inline_variables) && __cpp_inline_variables >= 201606L) && (!defined(_MSC_VER) || _MSC_VER != 1900)
+
+template <typename T> BOOST_DECIMAL_ATTRIBUTE_UNUSED constexpr int limits<T>::max_chars;
+
+#endif
+
 } //namespace decimal
 } //namespace boost
 

include/boost/decimal/detail/attributes.hpp

@@ -78,6 +78,7 @@ template <>
 BOOST_DECIMAL_CONSTEXPR_VARIABLE_SPECIALIZATION auto max_significand_v<decimal128> =
         uint128{UINT64_C(0b1111111111'1111111111'1111111111'1111111111'111111), UINT64_MAX};
 
+// sign + decimal digits + '.' + 'e' + '+/-' + max digits of exponent + null term
 template <typename Dec, std::enable_if_t<detail::is_decimal_floating_point_v<Dec>, bool> = true>
 BOOST_DECIMAL_ATTRIBUTE_UNUSED BOOST_DECIMAL_CONSTEXPR_VARIABLE auto max_string_length_v = std::is_same<Dec, decimal32>::value ? 15 : 25;
 

test/test_to_chars.cpp

@@ -23,7 +23,7 @@ static constexpr auto N = static_cast<std::size_t>(1024U >> 4U); // Number of tr
 #if !defined(BOOST_DECIMAL_DISABLE_CLIB) && !(defined(__GNUC__) && __GNUC__ >= 13 && !defined(__aarch64__))
 
 template <typename T>
-void test_value(T val, const char* result, chars_format fmt = boost::decimal::chars_format::general, int precision = -1)
+void test_value(T val, const char* result, chars_format fmt, int precision = -1)
 {
     char buffer[256] {};
     auto r = to_chars(buffer, buffer + sizeof(buffer), val, fmt, precision);
@@ -32,6 +32,16 @@ void test_value(T val, const char* result, chars_format fmt = boost::decimal::ch
     BOOST_TEST_CSTR_EQ(result, buffer);
 }
 
+template <typename T>
+void test_value(T val, const char* result)
+{
+    char buffer[boost::decimal::limits<T>::max_chars] {};
+    auto r = to_chars(buffer, buffer + sizeof(buffer), val, chars_format::general);
+    *r.ptr = '\0';
+    BOOST_TEST(r);
+    BOOST_TEST_CSTR_EQ(result, buffer);
+}
+
 template <typename T>
 void test_non_finite_values()
 {