double-conversion provides efficient binary-decimal and decimal-binary
conversion routines for IEEE doubles.

The library consists of efficient conversion routines that have been 
extracted from the V8 JavaScript engine. The code has been refactored 
and improved so that it can be used more easily in other projects.

There is extensive documentation in 
  double-conversion/string-to-double.h
  double-conversion/double-to-string.h

Other examples can be found in 
  test/cctest/test-conversions.cc