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 0.10000000000000001 (because 0.1 can't be represented). When val is converted using stringstream, strVal is equal to "0.1". However, when using boost::lexical_cast, the resulting strVal is "0.10000000000000001".

Another example is the following:

double val = 12.12305000012;

Under visual studio val appears as 12.123050000119999, and using stringstream and default precision (6) it becomes 12.1231. I don't really understand why it is not 12.12305(...).

Is there a default precision, or does stringstream have a particular algorithm to convert a double value which can't be exactly represented?

Thanks.

Answer 1

You can change the floating-point precision of a stringstream as follows:

double num = 2.25149;
std::stringstream ss(stringstream::in | stringstream::out);
ss << std::setprecision(5) << num << endl;
ss << std::setprecision(4) << num << endl;

Output:

2.2515
2.251

Note how the numbers are also rounded when appropriate.

Answer 2

For anyone who gets "error: ‘setprecision’ is not a member of ‘std’" you must #include <iomanip> else setprecision(17) will not work!