How to Create a Custom Stream Manipulator that Increments the Next Outputted Integer in C ?

Creating a Custom Stream Manipulator that Modifies Next Item on the Stream

In C , the hex stream manipulator provides a convenient way to print integers in hexadecimal format. This article explores how to create a custom stream manipulator that modifies the next item on the stream.

Specifically, we aim to create a plusone manipulator that increments the value of the next integer printed by 1. To achieve this, we need to store some state in each stream. We can use the iword function and an index allocated by xalloc for this purpose:

inline int geti() { 
    static int i = ios_base::xalloc();
    return i;
}

ostream& add_one(ostream& os) { os.iword(geti()) = 1; return os; } 
ostream& add_none(ostream& os) { os.iword(geti()) = 0; return os; }

With this state in place, we can retrieve it in all streams. To hook into the output operation that performs numeric formatting, we define a custom facet:

struct my_num_put : num_put<char> {
    iter_type 
    do_put(iter_type s, ios_base& f, char_type fill, long v) const { 
        return num_put<char>::do_put(s, f, fill, v + f.iword(geti())); 
    } 

    iter_type 
    do_put(iter_type s, ios_base& f, char_type fill, unsigned long v) const { 
        return num_put<char>::do_put(s, f, fill, v + f.iword(geti())); 
    } 
}; 

This facet adds the value stored in the stream state to the number being printed.

Now, we can test the plusone manipulator:

int main() {
    // outputs: 11121011
    cout.imbue(locale(locale(),new my_num_put));
    cout << add_one << 10 << 11 
         << add_none << 10 << 11;
}

This code demonstrates how to define a custom stream manipulator that modifies the next item on the stream. To ensure that only the next item is incremented, we can reset the stream state to 0 after each do_put call.

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