double and stringstream formatting

Question

double val = 0.1;
std::stringstream ss;
ss << val;
std::string strVal= ss.str();

In the Visual Studio debugger, val has the value 0.

Answer 1

The problem occurs at the stream insertion ss << 0.1; rather than at the conversion to string. If you want non-default precision you need to specify this prior to inserting the double:

ss << std::setprecision(17) << val;

On my computer, if I just use setprecision(16) I still get "0.1" rather than "0.10000000000000001". I need a (slightly bogus) precision of 17 to see that final 1.

Addendum
A better demonstration arises with a value of 1.0/3.0. With the default precision you get a string representation of "0.333333". This is not the string equivalent of a double precision 1/3. Using setprecision(16) makes the string "0.3333333333333333"; a precision of 17 yields "0.33333333333333331".