Effective use of C++ iomanip library

Question

I created a `Vector` class in C++ and it works great for my problems. I am now cleaning it up, and I ran into the following piece of code: ``` std::ostream& operator<<(std::ostream &output, const Vector &v){ output<<"[" <<std::setiosflags(std::ios::right | std::ios::scientific) <<std::setw(23) <<std::setprecision(16) <<v._x<<", " <<std::setiosflags(std::ios::right | std::ios::scientific) <<std::setw(23) <<std::setprecision(16) <<v._y<<", " <<std::setiosflags(std::ios::right | std::ios::scientific) <<std::setw(23) <<std::setprecision(16) <<v._z<<"]"; return output; } ``` The code allows to print a vector as `std::cout<<v<<std::endl;`. Each number has 23 spaces, of which 16 are the decimals. The text is right-aligned so that it will print: ``` 1.123456123456e+01 -1.123456123456e+01 ``` Instead of ``` 1.123456123456e+01 -1.123456123456e+01 ``` The code seems awfully repetitive. How can you "store" the format (all the `setiosflags`, `setw` and `setprecision` statements) such that you can…

Accepted Answer

Only `std::setw()` is temporary. The other two calls, `setiosflags`, and `setprecision` have a permanent effect.

So, you could change your code to :

```
std::ostream& operator<<(std::ostream &output, const Vector &v){
  output<<"["
    <<std::setiosflags(std::ios::right | std::ios::scientific)
    <<std::setw(23)
    <<std::setprecision(16)
    <<v._x<<", "
    <<std::setw(23)
    <<v._y<<", "
    <<std::setw(23)
    <<v._z<<"]";
  return output;
} 
```

But now you've borked the flags and precision for the next guy. Try this instead:

```
std::ostream& operator<<(std::ostream &output, const Vector &v){
  std::ios_base::fmtflags f = output.flags(std::ios::right | std::ios::scientific);
  std::streamsize p = output.precision(16);
  output<<"["
    <<std::setw(23)
    <<v._x<<", "
    <<std::setw(23)
    <<v._y<<", "
    <<std::setw(23)
    <<v._z<<"]";
  output.flags(f);
  output.precision(p);
  return output;
} 
```

Finally, if you absolutely have to get rid of the duplication of the constant `23`, you could do something like this (but I wouldn't recommend it):

```
struct width {
  int w;
  width(int w) : w(w) {}
  friend std::ostream& operator<<(std::ostream&os, const width& w) {
    return os << std::setw(width.w);
  }
};

std::ostream& operator<<(std::ostream &output, const Vector &v){
  std::ios_base::fmtflags f = output.flags(std::ios::right | std::ios::scientific);
  std::streamsize p = output.precision(16);
  width w(23);
  output<<"["
    <<w
    <<v._x<<", "
    <<w
    <<v._y<<", "
    <<w
    <<v._z<<"]";
  output.flags(f);
  output.precision(p);
  return output;
} 
```

See also this other question, where they decided that you can't make width permanent.

Effective use of C++ iomanip library

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