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negative precision values in ostream
原标题:

This is more of a question of curiosity but does anyone know how negative precision values are handled in C++? For example:

double pi = 3.14159265;

cout.precision(-10);
cout.setf(ios::fixed, ios::floatfield);

cout << pi << endl;

I ve tried this out and using GCC and it seems that the precision value is ignored but I was curious if there is some official line on what happens in this situation.

最佳回答

Strangely (and wrongly, IMHO) the C++ Standard specifies a signed type (streamsize) as the parameter for precision, so it won t be converted to a large number. However, the standard is silent on what a negative number might mean, if anything.

问题回答

I can t find any specification of this behaviour neither in C++03 nor in draft of C++0x (N3092). However, the C89 standard says

7.19.6.1 The fprintf function

A negative precision argument is taken as if the precision were omitted.

I would expect that C++ std::ostream behaviour is consistent with the C I/O printf. fprintf and related functions.

A quick test with Visual C++ 10.0 and GCC 4.4.1 suggests it to be this way and negative precision means precision omitted. And, precision omitted means a default value which is 6 places as specified in 27.5.4.1 basic_ios constructors in Table 125





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