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