Configuration Macros

User Configurable Macros

The following configuration macros are available:

  • BOOST_DECIMAL_DISABLE_CASSERT: Disables the use of <cassert> and all run-time assertions.

  • BOOST_DECIMAL_DISABLE_IOSTREAM: Disables the use of I/O streaming and removes all associated headers (e.g. <iostream>, <iosfwd>, <cwchar>, etc.)

  • BOOST_DECIMAL_DISABLE_CLIB: Defines both of the above macros. In testing this reduces ROM usage by ~50% and can be useful if running on embedded platforms.

  • BOOST_DECIMAL_DISABLE_EXCEPTIONS: This allows the user to disable any calls to throw. This macro will also be set automatically in the presence of -fno-exceptions or various /EH flags on MSVC.

  • BOOST_DECIMAL_ALLOW_IMPLICIT_CONVERSIONS: Allows a binary floating-point type (e.g. double) to be implicitly converted to a decimal floating point type. This option is not recommended, but can be useful if you want to use specific functionality from the standard library with internal conversions such as:

#define BOOST_DECIMAL_ALLOW_IMPLICIT_CONVERSIONS
#include <boost/decimal.hpp>
#include <complex>

constexpr decimal64_t half {5, -1};
std::complex<decimal64_t> test_val {half, half};
const auto res = std::acos(test_val);

  • BOOST_DECIMAL_FAST_MATH performs optimizations similar to that of the -ffast-math compiler flag such as removing all checks for non-finite values. This flag increases the performance of the basis operations (e.g. add, sub, mul, div, and comparisons) by up to 20%. Again, it must be defined before inclusion of decimal headers like so:

#define BOOST_DECIMAL_FAST_MATH
#include <boost/decimal.hpp>

  • BOOST_DECIMAL_DEC_EVAL_METHOD: See <cfloat> section for explanation

Automatic Configuration Macros

  • BOOST_DECIMAL_CXX20_CONSTEXPR: This is defined to constexpr when compiling with C++20 or greater, otherwise it expands to nothing.

  • BOOST_DECIMAL_CONSTEXPR: This is defined to constexpr when any of the following are met:

    • _MSC_FULL_VER >= 192528326

    • GNUC >= 9

    • Compiler has: __builtin_is_constant_evaluated()

    • C++20 support with: std::is_constant_evaluated()

  • BOOST_DECIMAL_HAS_STD_CHARCONV: This macro is defined if header <charconv> exists and the language standard used is >= C++17

    • We only need the structs and enums out of the header so we are not concerned with being overly restrictive about the feature test macros.

      • Known compilers that support this lighter requirement are: GCC >= 10, Clang >= 13, and MSVC >= 14.2

  • BOOST_DECIMAL_HAS_STD_STRING_VIEW: This macro is defined if header <string_view> exists and the langauge standard used is >= C++17

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