How Can You Create a Custom C Stream Manipulator to Modify Subsequent Stream Items?

Custom C Stream Manipulator to Alter Subsequent Stream Items

In C , the hex manipulator can be used to convert a number to its hexadecimal representation. However, what if you need to modify not only the current item but also subsequent items on the stream?

Creating the plusone Manipulator

To create a manipulator that increments the next numeric value on the stream, follow these steps:

1. Store State on Each Stream:

Use iword and geti() to store state on each stream.

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

2. Define Manipulator Functions:

Create functions to set and unset the stored state.

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

3. Intercept Numeric Output:

Create a my_num_put facet to intercept numeric output and apply the stored increment.

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

4. Test the Manipulator:

Use the manipulators to increment the next numeric value and display the results.

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

Reset Single-Use Increment:

If you want to increment only the next number, reset the stored state after each call to do_put.

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