C++ output with prefix padding

Question

How can I print an string with padding in C++? Specifically what I want is:

cout << "something in one line (no prefix padding)"
cout << "something in another line (no prefix padding)"
set_padding(4)
    cout << "something with padding"
    cout << "something with padding"
set_padding(8)
        cout << "something with padding"
        cout << "something with padding"

That is, I'll call cout many times and I don't want to call setw(len) << "" all the time.

What is the wider context of the program? That might help us to better advise you. — Saladin Akara, Aug 11 '12 at 18:48
possible duplicate of http://stackoverflow.com/questions/1532640/which-iomanip-manipulators-are-sticky — TemplateRex, Aug 11 '12 at 18:51
I want to print a tree where all nodes on the same level with have the same padding. — JohnTortugo, Aug 11 '12 at 18:52
Possible duplicate with http://stackoverflow.com/questions/524641/how-do-i-create-my-own-ostream-streambuf — Mahmoud Fayez, Aug 11 '12 at 19:05
My question can be stated in another way as: How can I tell "cout" what column it should start printing the text from further calls? — JohnTortugo, Aug 11 '12 at 19:13
Just to clarify - by "padding" you actually mean "indentation", right? — Asaf, Aug 11 '12 at 19:24

Nathan Andrew Mullenax · Accepted Answer · 2012-08-11T19:54:31.263

4

I suppose you could just make the pre-processor type it for you:

#include <iostream>
#define mout std::cout << std::string(width,' ')
#define mndl "\n" << std::string(width,' ')

int width=0;

int main()
{
    mout << "Hello" << std::endl; 

    width = 8;

    mout << "World." << mndl << "Next line--still indented";
    // more indented lines...
}

edited Aug 11 '12 at 19:54

answered Aug 11 '12 at 19:14

Nathan Andrew Mullenax

814
8
13

Although it wasn't what I was expecting, it is a nice approach. thanks. – JohnTortugo Aug 11 '12 at 19:19
The unique problem is that I can't use endl inside a statement like this: mout << "line1" << endl << "line2"; Anyway, good answer. – JohnTortugo Aug 11 '12 at 19:35
Unless you do `#define endl "\n" << std::string(width,' ')` :) – Nathan Andrew Mullenax Aug 11 '12 at 19:44
rsrsrrs the interesting thing of your sugestion is that I can continue using cout whenever I want. I "redefine" endl all goes for nothing. But I could define a "pendl" =D – JohnTortugo Aug 11 '12 at 19:49

score 3 · Answer 2 · answered Aug 11 '12 at 19:53

How about something like:

class IndentedOutput
{
public:
    IndentedOutput()
    {
        m_indent = 0;
    }

    void setIndent(unsigned int indent)
    {
        m_indent = indent;
    }

    template <typename T>
    std::ostream& operator<< (const T& val)
    {
        return (std::cout << std::string(m_indent,' ') << val);
    }

private:
    unsigned int m_indent;
};

And the you can use it like this:

IndentedOutput ind;

int i =0;
ind << "number is " << i++ << '\n';
ind << "number is " << i++ << '\n';
ind.setIndent(4);
ind << "number is " << i++ << '\n';
ind << "number is " << i++ << '\n';
ind.setIndent(6);
ind << "number is " << i++ << '\n';
ind << "number is " << i++ << '\n';
ind.setIndent(0);
ind << "number is " << i++ << '\n';

dans3itz · Answer 3 · 2012-08-11T20:31:11.790

http://www.cplusplus.com/reference/iostream/manipulators/

EDIT: Sorry, I was a little short handed. What I meant was to research how iomanip works, I know you are aware of sets(n) << ""

Using iomanip as a baseline for a quick and dirty implementation, this is what I came up with:

#include <iostream>
#include <iomanip>
#include <string>

class setpw_ {
  int n_;
  std::string s_;
public:
  explicit setpw_(int n) : n_(n), s_("") {}
  setpw_(int n, const std::string& s) : n_(n), s_(s) {}

  template<typename classT, typename traitsT>
  friend std::basic_ostream<classT, traitsT>&
  operator<<(std::basic_ostream<classT, traitsT>& os_, const setpw_& x) {
    os_.width(x.n_);
    os_ << "";
    if ( x.s_.length()){
      os_ << x.s_;
    }
    return os_;
  }
};

setpw_ setpw(int n) { return setpw_(n); }
setpw_ indent(int n, const std::string& s) { return setpw_(n, s); }

int
main(int argc, char** argv) {
  std::cout << setpw(8) << "Hello, World" << std::endl;
  std::cout << indent(8,   "----^BYE^---") << std::endl;
  return 0;
}

If you think I can accomplish what I want using manipulators, please complete your answer appropriately. — JohnTortugo, Aug 11 '12 at 19:04

C++ output with prefix padding

3 Answers3