Merge pull request #735 from boostorg/issue734

Fixes #734 in docs and code

doc/reference_backend_requirements.qbk

@@ -45,7 +45,7 @@ and `ff` is a variable of type `std::ios_base::fmtflags`.
 [[`b = s`][`B&`][Assignment from a string.][Throws a `std::runtime_error` if the string could not be interpreted as a valid number.]]
 [[`b.swap(b)`][`void`][Swaps the contents of its arguments.][`noexcept`]]
 [[`cb.str(ss, ff)`][`std::string`][Returns the string representation of `b` with `ss` digits and formatted according to the flags set in `ff`.
-                  If `ss` is zero, then returns as many digits as are required to reconstruct the original value.][[space]]]
+                  If `ss` is zero and `std::ios_base::fixed` is not set in `ff`, then returns as many digits as are required to reconstruct the original value.][[space]]]
 [[`b.negate()`][`void`][Negates `b`.][[space]]]
 [[`cb.compare(cb2)`][`int`][Compares `cb` and `cb2`, returns a value less than zero if `cb < cb2`, a value greater than zero if `cb > cb2` and zero
                    if `cb == cb2`.][`noexcept`]]

doc/reference_number.qbk

@@ -58,7 +58,7 @@
       bool is_zero()const;
       int sign()const;
       // string conversion:
-      std::string str()const;
+      std::string str(std::streamsize digits = 0, std::ios_base::fmtflags f = std::ios_base::fmtflags(0))const;
       // Generic conversion mechanism
       template <class T>
       T convert_to()const;
@@ -416,10 +416,10 @@ Returns `true` is `*this` is zero, otherwise `false`.
 Returns a value less than zero if `*this` is negative, a value greater than zero if `*this` is positive, and zero
 if `*this` is zero.
 
-      std::string str(unsigned precision, bool scientific = true)const;
+      std::string str(std::streamsize digits = 0, std::ios_base::fmtflags f = std::ios_base::fmtflags(0))const;
 
-Returns the number formatted as a string, with at least /precision/ digits, and in scientific format
-if /scientific/ is true.
+Returns the number formatted as a string, with at least /precision/ digits, and adhering to the format
+flags in /f/ if provided.
 
       template <class T>
       T convert_to()const;

include/boost/multiprecision/cpp_dec_float.hpp

@@ -1932,8 +1932,17 @@ std::string cpp_dec_float<Digits10, ExponentType, Allocator>::str(std::intmax_t
    }
    else if (f & std::ios_base::scientific)
       ++number_of_digits;
-   // Determine the number of elements needed to provide the requested digits from cpp_dec_float<Digits10, ExponentType, Allocator>.
-   const std::size_t number_of_elements = (std::min)(static_cast<std::size_t>(static_cast<std::size_t>(number_of_digits / static_cast<std::intmax_t>(cpp_dec_float_elem_digits10)) + 2u),
+
+   // Determine the number of elements needed to provide the requested
+   // digits from cpp_dec_float<Digits10, ExponentType, Allocator>.
+   const std::intmax_t
+      number_of_elements_signed
+      {
+         (std::max)(static_cast<std::intmax_t>(number_of_digits / static_cast<std::intmax_t>(cpp_dec_float_elem_digits10) + 2),
+                    std::intmax_t { INT8_C(0) })
+      };
+
+   const std::size_t number_of_elements = (std::min)(static_cast<std::size_t>(number_of_elements_signed),
                                                      static_cast<std::size_t>(cpp_dec_float_elem_number));
 
    // Extract the remaining digits from cpp_dec_float<Digits10, ExponentType, Allocator> after the decimal point.

include/boost/multiprecision/cpp_double_fp.hpp

@@ -892,11 +892,6 @@ class cpp_double_fp_backend
 
    auto str(std::streamsize number_of_digits, const std::ios::fmtflags format_flags) const -> std::string
    {
-      if (number_of_digits == 0)
-      {
-         number_of_digits = cpp_double_fp_backend::my_digits10;
-      }
-
       // Use cpp_bin_float when writing to string. This is similar
       // to the use of cpp_bin_float when reading from string.
 

test/Jamfile.v2

@@ -1276,6 +1276,7 @@ test-suite misc :
       [ run git_issue_626.cpp ]
       [ run git_issue_636.cpp : : : [ check-target-builds ../config//has_float128 : <source>quadmath : <build>no ] ]
       [ run git_issue_652.cpp ]
+      [ run git_issue_734.cpp ]
       [ compile git_issue_98.cpp :
          [ check-target-builds ../config//has_float128 : <define>TEST_FLOAT128 <source>quadmath : ]
          [ check-target-builds ../config//has_gmp : <define>TEST_GMP <source>gmp : ]

test/git_issue_734.cpp

@@ -0,0 +1,86 @@
+// Copyright 2025 Christopher Kormanyos
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/core/lightweight_test.hpp>
+#include <boost/multiprecision/cpp_bin_float.hpp>
+#include <boost/multiprecision/cpp_dec_float.hpp>
+#include <boost/multiprecision/cpp_double_fp.hpp>
+
+#include <iomanip>
+#include <iostream>
+#include <sstream>
+#include <typeinfo>
+
+namespace local {
+
+template<typename BackendType>
+auto my_stringify_via_top_level_number(const std::streamsize strm_size) -> std::string
+{
+  using float_type = boost::multiprecision::number<BackendType, boost::multiprecision::et_off>;
+
+  #if defined(BOOST_MSVC)
+  using streamsize_type = std::streamsize;
+  #else
+  using streamsize_type = int;
+  #endif
+
+  float_type x("0.0000000000000000222222222222222");
+
+  std::stringstream strm { };
+
+  strm << std::setprecision(static_cast<streamsize_type>(strm_size)) << std::fixed;
+  strm << x;
+
+  return strm.str();
+}
+
+template<typename BackendType>
+auto my_stringify_via_str_and_backend_str(const std::streamsize strm_size) -> std::string
+{
+  using float_type = boost::multiprecision::number<BackendType, boost::multiprecision::et_off>;
+
+  #if defined(BOOST_MSVC)
+  using streamsize_type = std::streamsize;
+  #else
+  using streamsize_type = int;
+  #endif
+
+  float_type x("0.0000000000000000222222222222222");
+
+  std::stringstream strm { };
+
+  strm << std::setprecision(static_cast<streamsize_type>(strm_size)) << std::fixed;
+
+  return x.str(strm_size, strm.flags());
+}
+
+template<typename BackendType>
+auto test() -> void
+{
+  std::cout << "Testing type of test: " << typeid(BackendType).name() << std::endl;
+
+  BOOST_TEST(my_stringify_via_top_level_number<BackendType>(std::streamsize { INT8_C(0) })  == "0");
+  BOOST_TEST(my_stringify_via_top_level_number<BackendType>(std::streamsize { INT8_C(8) })  == "0.00000000");
+  BOOST_TEST(my_stringify_via_top_level_number<BackendType>(std::streamsize { INT8_C(12) }) == "0.000000000000");
+  BOOST_TEST(my_stringify_via_top_level_number<BackendType>(std::streamsize { INT8_C(31) }) == "0.0000000000000000222222222222222");
+
+  BOOST_TEST(my_stringify_via_str_and_backend_str<BackendType>(std::streamsize { INT8_C(0) })  == "0");
+  BOOST_TEST(my_stringify_via_str_and_backend_str<BackendType>(std::streamsize { INT8_C(8) })  == "0.00000000");
+  BOOST_TEST(my_stringify_via_str_and_backend_str<BackendType>(std::streamsize { INT8_C(12) }) == "0.000000000000");
+  BOOST_TEST(my_stringify_via_str_and_backend_str<BackendType>(std::streamsize { INT8_C(31) }) == "0.0000000000000000222222222222222");
+}
+
+} // namespace local
+
+auto main() -> int
+{
+  local::test<boost::multiprecision::cpp_bin_float<32>>();
+  local::test<boost::multiprecision::cpp_dec_float<32>>();
+  BOOST_IF_CONSTEXPR(std::numeric_limits<double>::digits >= 53)
+  {
+    local::test<boost::multiprecision::cpp_double_fp_backend<double>>();
+  }
+
+  return boost::report_errors();
+}