&gt; <i>If you want round-trip conversion for all `double` numbers, i.e. from `double` to text and back</i><p>I don&#x27;t. Rather, I want round-trip conversion of all 15-digit-precision decimal numbers to the machine and back.  If the machine calculates some numbers which are different from some of these but <i>alias</i> to them textually, I don&#x27;t care.<p>Someone else might want the decimal text representation to provide bit-exact storage semantics for arbitrary doubles.<p>For that reason it might be a better design choice to obtain the default printing precision from a special variable, which itself defaults to 15, rather than a hard-coded default.