C++: Printing with conditional format depending on the base of an int value

问题

In c++, Is there any format specifier to print an unsigned in different base, depending on its value? A format specifier expressing something like this:

using namespace std; 
if(x > 0xF000) 
   cout << hex << "0x" << x;
else 
   cout << dec << x ;

Because I will have to do this a lot of times in my current project, I would like to know if c++ provides such a format specifier.

回答1:

There is no such functionality built-in to C++. You can use a simple wrapper to accomplish this, though:

struct large_hex {
    unsigned int x;
};

ostream& operator <<(ostream& os, const large_hex& lh) {
    if (lh.x > 0xF000) {
        return os << "0x" << hex << lh.x << dec;
    } else {
        return os << lh.x;
    }
}

Use as cout << large_hex{x}.

If you want to make the threshold configurable you could make it a second field of large_hex or a template parameter (exercise for the reader).

来源：https://stackoverflow.com/questions/58975980/c-printing-with-conditional-format-depending-on-the-base-of-an-int-value