is there a good way to combine stream manipulators? - c++

Question

If I wanted to output a fixed width hex number with 4 digits on a stream, I would need to do something like this:
cout << "0x" << hex << setw(4) << setfill('0') << 0xABC;
which seems a bit long winded. Using a macro helps:
#define HEX(n) "0x" << hex << setw(n) << setfill('0')
cout << HEX(4) << 0xABC;
Is there a better way to combine the manipulators?

Answer 1

Avoid the macros when you can! They hide code, making things hard to debug, don't respect scope, etc.
You can use a simple function as KenE provided. If you want to get all fancy and flexible, then you can write your own manipulator:
#include <iostream>
#include <iomanip>
using namespace std;
ostream& hex4(ostream& out)
{
return out << "0x" << hex << setw(4) << setfill('0');
}
int main()
{
cout << hex4 << 123 << endl;
}
This makes it a little more general. The reason the function above can be used is because operator<< is already overloaded like this: ostream& operator<<(ostream&, ostream& (*funtion_ptr)(ostream&)). endl and some other manipulators are also implemented like this.
If you want to allow the number of digits to be specified at runtime, we can use a class:
#include <iostream>
#include <iomanip>
using namespace std;
struct formatted_hex
{
unsigned int n;
explicit formatted_hex(unsigned int in): n(in) {}
};
ostream& operator<<(ostream& out, const formatted_hex& fh)
{
return out << "0x" << hex << setw(fh.n) << setfill('0');
}
int main()
{
cout << formatted_hex(4) << 123 << endl;
}
If the size can be determined at compile-time, however, might as well just use a function template [thanks to Jon Purdy for this suggestion]:
template <unsigned int N>
ostream& formatted_hex(ostream& out)
{
return out << "0x" << hex << setw(N) << setfill('0');
}
int main()
{
cout << formatted_hex<4> << 123 << endl;
}

Answer 2

Why a macro - can't you use a function instead?
void write4dhex(ostream& strm, int n)
{
strm << "0x" << hex << setw(4) << setfill('0') << n;
}

Answer 3

In C++20 you'll be able to use std::format to make this much less verbose:
std::cout << std::format("0x{:04x}", 0xABC);
Output:
0x0abc
You can also easily reuse the format string by storing it in a constant.
In the meantime you can use the {fmt} library, std::format is based on. {fmt} also provides the print function that makes this even easier and more efficient (godbolt):
fmt::print("0x{:04x}", 0xABC);
Disclaimer: I'm the author of {fmt} and C++20 std::format.