C++ custom stream manipulator that changes next item on stream
c++, stream
Solution
First, you have to store some state into each stream. You can do that with the function `iword` and an index you pass to it, given by `xalloc`:
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; }
Having that in place, you can already retrieve some state in all streams. Now, you just have to hook into the respective output operation. Numeric output is done by a facet, because it potentially is locale dependent. So you can do
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()));
}
};
Now, you can test the stuff.
int main() {
// outputs: 11121011
cout.imbue(locale(locale(),new my_num_put));
cout << add_one << 10 << 11
<< add_none << 10 << 11;
}
If you want that only the next number is incremented, just set the word to `0` again after each call to `do_put`.
Problem
In C++, to print a number in hexadecimal you do this: ``` int num = 10; std::cout << std::hex << num; // => 'a' ``` I know I can create a manipulator that just adds stuff to the stream like so: ``` std::ostream& windows_feed(std::ostream& out) { out << "\r\n"; return out; } std::cout << "Hello" << windows_feed; // => "Hello\r\n" ``` However, how can I create a manipulator that, like 'hex', modifies items to come on the stream? As a simple example, how would I create the plusone manipulator here?: ``` int num2 = 1; std::cout << "1 + 1 = " << plusone << num2; // => "1 + 1 = 2" // note that the value stored in num2 does not change, just its display above. std::cout << num2; // => "1" ```