Need manipulator to set maximum I/O width

[Johanus Dagius]

I've written a lot of C programs but only a few in C++. In C you can set both the minimum and maximum width of a formatted I/O output by using standard printf() formats. For example, if I have a very large string and I only want to print out the first 10 chars I can do this in C:

printf("s=%.10s\n", very_long_string).

How do I do this with C++ manipulators? I've tried std::setw(width), but it seems to control only the *minimum* width, not the max. I'm looking for something like in C++:

std::cout << std::maxw(10) << very_long_string << std:endl;

I know there are many "work-arounds" such as using substr(), but that only works with std::string. I often work with plain old c-strings.

I've been told that manipulators can mimic anything that printf() does, but it seems they can't even constrain fields to some specified size! Correct me if I'm wrong.

Tnx, Johanus

[weaknessforcats]

Then write your own manipulator. Like endl. It's just a function with this prototype:

ostream& MyFunction(ostream&);
 

For example suppose you want five asterisks to appear:

ostream& FiveStars(ostream& arg)
{
    arg << "*****";
    return arg;
}
 

Then you call it by address:

cout << FiveStars << " Important! " << FiveStars << endl;
 

The ostream::operator<<(ostream&, ostream& (*fp)(ostream&) ) just calls the function by address passing in the ostream itself as the argument.

Now you can do anything you want.

[Johanus Dagius]

>>> Then write your own manipulator ...
OK, I understand your simple manipulator, which merely inserts and returns its own stuff into the output stream.

But I don't see how it can truncate output(s) further down the stream. I suspect this is not trivial thing to do. Would appreciate if someone could give me some pointers on how to do this (or even better: write the maxw(n) code for me :-)

I must say that I am surprised that something like maxw(n) is not part of "std"

Tnx,
Johanus

[weaknessforcats]

I have a partial solution. The problem with a max manipulator is that it requires fiddling with the private data of ios_base. setw() calls ios_base::width(). You would have to derive from ios_base, or more likely, from basic_ostream to add these features.

I went another way.

What I did was go to the C/C++ Articles section and copied out the code for a simple Singleton object. This object contains the max field width. I added methods to set/get this value.

Next, I wrote an operator<<(ostream&, const string&). But this one is not in the std namespace so it is differentiable from std::operator<<(ostream&, const string&). Further, is occurs after the #include <iostream> so the compiler will prefer it to the one in std. My operator<< for string uses the singleton to determine how many characters of the string to display.

It's not a manipulator but it does get your desired output.

#include <iostream>
using namespace std;
 
class SysParms
{   
   private:
        streamsize maxwidth;
        SysParms(const SysParms&);  //Prevents making a copy
 
   protected:
        SysParms();       //Only a SysParms member can call this
                     //Prevents a user from creating singleton objects
        virtual ~SysParms();  //Prevents just anyone from deleting the singleton
 
   public:
 
 
       //The "official" access point.
       static SysParms* Instance();
       void MaxWidth(streamsize arg);
       streamsize GetMaxWidth();
 
};
 
 
namespace
{
    SysParms* instance = 0;        //Address of the singleton
 
}
 
SysParms::SysParms()
{
 
}
SysParms::~SysParms()
{
    delete instance;
              instance = 0;
}
 
//The "official" access point
SysParms* SysParms::Instance()
{
   //"Lazy" initialization. Singleton not created until it's needed
   if (!instance)
   {
      instance = new SysParms;
   }
   return instance;
}
void SysParms::MaxWidth(std::streamsize arg)
{
    this->maxwidth = arg;
}
streamsize SysParms::GetMaxWidth()
{
    return this->maxwidth;
}    
 
ostream& operator<<(ostream& os, const string& str)
{
   streamsize max = SysParms::Instance()->GetMaxWidth();
   streamsize length = str.size();
 
   for (int i = 0; (i < max) && ( i < length); ++i)
   {
      os << str[i];
   }
   return os;
}
 
int main()
{
 
    string str("abcdefghijklmnop");
    SysParms::Instance()->MaxWidth(10);
    cout <<  str << endl;
    SysParms::Instance()->MaxWidth(5);
    cout <<  str << endl;
 
    return 0;
}
 
 

[Johanus Dagius]

weaknessforcats wrote:
>>> The problem with a max manipulator is that it requires fiddling with the
>>> private data of ios_base. setw() calls ios_base::width(). You would have to
>>> derive from ios_base, or more likely, from basic_ostream to add these features ....

Hey, thanks for the partial solution. (It works!) I really appreciate your efforts here to help me, but I think I'll just use fprintf() in my C++ code to format my outputs. The fstream manipulators are just too clunky.

Thanks,
Johanus