“Permanent” std::setw

Question

Is there any way how to set std::setw manipulator (or its function width) permanently? Look at this:

#include 
#inc         


        

Answer 1

Since setw and width do not result in a persistent setting, one solution is to define a type that overrides operator<<, applying setw before the value. This would allow an ostream_iterator for that type to function with std::copy as below.

int fieldWidth = 4;
std::copy(v.begin(), v.end(),
    std::ostream_iterator< FixedWidthVal >(std::cout, ","));

You could define: (1) FixedWidthVal as a template class with parameters for data type (typename) and width (value), and (2) an operator<< for an ostream and a FixedWidthVal that applies setw for each insertion.

// FixedWidthVal.hpp
#include 

template 
struct FixedWidthVal
{
    FixedWidthVal(T v_) : v(v_) {}
    T v;
};

template 
std::ostream& operator<< (std::ostream& ostr, const FixedWidthVal &fwv)
{
    return ostr << std::setw(W) << fwv.v;
}

Then it could be applied with std::copy (or a for loop):

// fixedWidthTest.cpp
#include 
#include 
#include 
#include "FixedWidthVal.hpp"

int main () {
    // output array of values
    int array[] = { 1, 2, 4, 8, 16, 32, 64, 128, 256 };

    std::copy(array,array+sizeof(array)/sizeof(int), 
        std::ostream_iterator< FixedWidthVal >(std::cout, ","));

    std::cout << std::endl;

    // output values computed in loop
    std::ostream_iterator > osi(std::cout, ",");
    for (int i=1; i<4097; i*=2)
        osi = i; // * and ++ not necessary

    std::cout << std::endl;

    return 0;
}

Output (demo)

   1,   2,   4,   8,  16,  32,  64, 128, 256,
   1,   2,   4,   8,  16,  32,  64, 128, 256, 512,1024,2048,4096,