How to Create Custom Stream Manipulators that Affect Subsequent Outputs in C ?

Custom Stream Manipulators for Modifying Subsequent Output in C

In C , stream manipulators are powerful tools for customizing and enhancing input and output operations. One common manipulator is std::hex, which modifies the format of subsequent numeric output to hexadecimal.

However, what if you need a manipulator that not only adds additional elements to the stream but also modifies the values of items that come after it? This article explores how to create such a manipulator and demonstrates its use case with a simple example.

Custom Manipulators with State Storage

To modify subsequent items on the stream, you need to store state information associated with each stream. This can be achieved using the ios_base::iword function and an index generated by xalloc.

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

With this mechanism in place, you can define two manipulators: add_one and add_none. add_one sets the stored state to 1, while add_none sets it to 0.

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

Customizing Numeric Output

Numeric output in C is handled by facets. To modify the behavior of numeric output, you can define a custom facet. Here, we create a custom facet called my_num_put that increments each numeric value by the stored state value.

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()));
    }
};

Testing the Manipulator

To test the custom manipulator, you can imbue the output stream with the new facet and use the manipulators to modify numeric output.

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

In this example, the output stream will print the numbers 10, 11, 10, and 11 with the value 1 added to each of the first two numbers, resulting in 11, 12, 10, and 11.

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