Whats the use of std::cin >> setprecision(n)?

Question

cppreference says:

When used in an expression out << setprecision(n) or in >> setprecision(n), sets the precision parameter of the stream out or in to exactly n.

The example on the bottom of the same page demonstrates the use with std::cout.

Also for inputstreams the page explains that str >> setprecision(n) is effectively the same as calling a funciton that calls str.precision(n). But what for? What is the effect of std::cin >> setprecision(n) ?

This code

#include <iostream>
#include <iomanip>

int main() {
    double d;
    std::cin >> std::setprecision(1) >> d;
    std::cout << d << "\n";
    std::cin >> std::fixed >> std::setprecision(2) >> d;    
    std::cout << d << "\n";
}

With this input:

3.12
5.1234

Produces this output:

3.12
5.1234

So it looks like it has no effect at all. And reading about std::ios_base::precision it also seems like precision only makes sense for out streams:

Manages the precision (i.e. how many digits are generated) of floating point output performed by std::num_put::do_put.

What is the reason std::setprecision can be passed to std::istream::operator>>, when, according to cppreference, it effectively calls str.precision(n) which only affects output?

Icosahedron

Probably easier to let the code compile for this one case than to try and fix it such that the std::setprecision stream modifier was an exception. – Grainger

@RichardCritten this was one of my first ideas, but only on a second thought I actually understand it ;). iomanips are just functions and there is only one >> overload for all manips.... – Icosahedron

Answer 1

Using setprecision with either << on a std::basic_ostream or >> on a std::basic_istream has the exact same effect, affecting only how output on the stream will be handled. The precision attribute is stored in the virtual std::ios_base base class, which is unified among input and output streams, which is why it still can work on an input stream.

But on a pure input stream I don't see any application for it. On a bidirectional stream however, this allows using both << or >> to set the output precision.

Whether there is a specific benefit to allowing setprecision with >> isn't clear to me. This originally wasn't in the first C++ standard draft, but was added with a CD ballot comment to C++98, see comment CD2-27-015 in N1064. Unfortunately the comment doesn't give a rationale.

Symbology