You can store custom state in your stream instance:
See it Live On Coliru
#include <iostream>
static int const index = std::ios_base::xalloc();
std::ostream& verbose(std::ostream& stream) {
stream.iword(index) = 1;
return stream;
}
std::ostream& noverbose(std::ostream& stream) {
stream.iword(index) = 0;
return stream;
}
struct StructA {
std::string mLongName;
std::string mShortName;
inline friend std::ostream& operator << (std::ostream& os, const StructA& rStruct) {
switch (os.iword(index)) {
case 1: return os << rStruct.mLongName;
case 0:
default: return os << rStruct.mShortName;
}
}
};
int main()
{
StructA a;
a.mLongName = "loooooooooooooooooooong names are tedious";
a.mShortName = "succinctness";
std::cout << a << '\n';
std::cout << verbose;
std::cout << a << '\n';
std::cout << noverbose;
std::cout << a << '\n';
}
Credits go to Dietmar Kühl's answer.
If you needed siginificant amounts of state/logic, you would have to look at imbue-ing a custom locale facet. His answer shows the basics of this approach too.