Formating support
Boost.Decimal supports formatting with both <format> (when C++20 and header are both available) and <fmt/format.h>.
<format>
Format is supported when using C++20 and a compiler with appropriate support: GCC >= 13, Clang >= 18, MSVC >= 19.40
Type Modifiers
The following type modifiers are the same as those used by built-in floating point values:
Modifier |
Format |
"g" or "G" |
General |
"e" or "E" |
Scientific |
"f" |
Fixed |
"a" or "A" |
Hex |
Example usage for scientific format would be: {:e}
| The uppercase format will return with all applicable values in uppercase (e.g. 3.14E+02 vs 3.14e+02) |
Precision Modifiers
Precision can be specified in the same way as built-in floating point values.
For example a scientific format with 3 digits or precision would be: {:.3e}
Padding Modifiers
If you want all values to be printed with a fixed width padding is allowed before the precision modifier.
For example with {:10.3e}:
-
3.14 → " 3.140e+00"
-
3.141 → " 3.141e+00"
Note the space at the front of these string to keep with width at 10 characters
Examples
The example is padding modifiers can be done like so
#include <boost/decimal.hpp> // or <boost/decimal/format.hpp>
#include <format>
#include <iostream>
int main()
{
constexpr boost::decimal::decimal64_t val1 {314, -2};
constexpr boost::decimal::decimal32_t val2 {3141, -3};
std::cout << std::format("{:10.3e}", val1) << '\n';
std::cout << std::format("{:10.3e}", val2) << std::endl;
return 0;
}<fmt/format.h>
Support for {fmt} is available as long as <fmt/format.h> is present.
All the above information on modifiers is the same for fmtlib, just in a different namespace (i.e. fmt:: instead of std::).
#include <fmt/format.h>
#include <boost/decimal.hpp> // or <boost/decimal/fmt_format.hpp>
#include <iostream>
int main()
{
constexpr boost::decimal::decimal64_t val1 {314, -2};
constexpr boost::decimal::decimal32_t val2 {3141, -3};
std::cout << fmt::format("{:10.3e}", val1) << '\n';
std::cout << fmt::format("{:10.3e}", val2) << std::endl;
return 0;
}