Units To Strings

All the class in the units library can be given as an argument to a to_string function. This function converts the units or value into a std::string that is representative of the unit or measurement. In all cases the primary aim of the to_string is to generate a string that the corresponding *_from_string function will recognize and convert back to the original unit. The secondary aim is to generate string that is human readable in standard notation. While this is achieved for many common units there is some work left to do to make it better.

For example

measurement density=10.0*kg/m.pow(3);
measurement meas2=2.7*puMW;

auto str1=to_string(density);
auto str2=to_string(meas2);

// from google tests
EXPECT_EQ(str1, "10 kg/m^3");
EXPECT_EQ(str2, "2.7 puMW");
uncertain_measurement um1(10.0, 0.4, m);
auto str = to_string(um1);
EXPECT_EQ(str, "10+/-0.4 m");

Uncertain measurement string conversions make some attempt to honor significant digits based on the uncertainty.

Advanced Usage

The to_string function also takes a second argument which is a std::uint64_t match_flags in all cases this default to 0, it is currently unused though will be used in the future to allow some fine tuning of the output in specific cases. In the near future a flag to allow utf 8 output strings will convert certain units to more common utf8 symbols such as unit Powers and degree symbols, and a few others. The output string would default to ascii only characters.

Stream Operators

Output stream operators are NOT included in the library. It was debatable to include them or not but there would be a lot of additional overloads that would add quite a bit of code to the header files, that in most cases is not necessary so the decision was made to exclude them. The to_string operations provide most of the capabilities with some additional flexibility, and if needed for a particular use case can be added to the user code in a simple fashion

namespace units{
   std::ostream& operator<<(std::ostream& os, const precise_unit& u)
       os << to_string(u);
       return os;
} // namespace units

Any of the types in the units library with a to_string operation can be handled in the same way. Depending on the compiler, placing the operator in the namespace may or may not be necessary.

Underlying Conversion Map Access

The underlying conversion maps may be accessed by users if desired. To access them a compile time definition needs to be added to the build ENABLE_UNIT_MAP_ACCESS

namespace detail {
    const std::unordered_map<std::string, precise_unit>& getUnitStringMap();
    const std::unordered_map<unit, const char*>& getUnitNameMap();

These may be useful for building a GUI or smart lookup or some other operations. getUnitStringMap() returns a map of known unit strings, and getUnitNameMap() is a mapping of common units back to strings as a building block for the to_string operation.