I am reading "The C++ standard library - a tutorial and reference" by Nicolai M. Josuttis, Chapter 15: Input/Output Using Stream Classes. At 15.7.6 Floating-Point Notation, there is a table explaining how the general formatting works for some numbers.
General formatting is the format that is applied when neither fixed nor scientific notation are picked by the developer.
| precision() | 421.0 | 0.0123456789 |
|---|---|---|
| 2 | 4.2e+02 | 0.012 |
| 6 | 421 | 0.0123457. |
The table shows how a floating-point number is formatted and printed depending on the precision value assigned to the output stream.
For example,
std::cout << std::setprecision(2) << 421.0 << std::endl;
Will print 4.2e+02
I don't understand why for precision = 2, in the general format, 0.0123456789 prints 0.012 which shows a number of significant digits greater than the precision value.
I was expecting it to print 0.01 instead of 0.012
It works correctly because in this case, precision means the amount of significant digits; it's a mathematical term. E.g. in the number 0.00078 there are 2 significant digits - 78. Not to be confused with the number of digits after decimal point (i.e. number of decimal places).