Can you use std::hex outside of a cin/cout <<

[Brainspell]

instead of

#include <iostream>
int g = 70;
cout << std::hex << g << "\n" ;

//instead like this

int h = 30;
int hex_num = std::hex(h);
cout << hex_num << "\n";

[seeplus]

No. std::hex is a stream manipulator and is only used within the context of a stream.

What you can do is this:

#include <iostream>
#include <iomanip>
#include <sstream>

int main()
{
	const int h {30};
	std::ostringstream oss;

	oss << std::hex << h;
	std::cout << oss.str() << '\n';
}

[jonnin]

int hex_num = std::hex(h);

This implies something more important: that you do not know what an 'int' actually IS.
an int is a group of bits, forming bytes, not a 'base 10 decimal text string' (which is what you see when you cout it). A number is a number: 0011 3 0x03 are all the same value, if you had 3 quarters, you still have 3 quarters, no matter how you write it. Cout is 'how you write it'. INT is how you work with it internally, without looking at it until you need to do so.