verifying that input is hex

[grcunning]

I need a way to make sure that input (4 digits) is in hex format. As long as you input hex, everything works fine, but if you enter a q or s or something it blows up.
The input is stored as an unsigned short int (required), so I can't store them as a string and then loop through and use isxdigit().
Thanks

[firedraco]

How exactly are you storing the input as an unsigned short if it is supposed to be hex? As far as I know 0xffff isn't valid for unsigned short ints.

[grcunning]

I Just used:
cin >> hex >> variable_name
to input the hex digits into an unsigned short.
It will end up being an interactive disassembler for a little simulator called H1 that came with our book(Assembly language and computer architecture using C++ and Java). It has a limited instruction set.
All we have to do in this first assignment is prompt for a machine instruction and then display the disassembled instruction, like this:
?4000
Op Code:4 Address field:000

?1e25
Op Code:1 Address field:e25

?ffff
Op Code:ffff Address field:

Most H1 instructions use a 4bit(single hex digit)op-code and a 12 bit(3 hex digit)address field. Next week we will probably have to add the ones that use 8 and 12 bit instructions, but for this assignment all we had to concern ourselves with one exception (ffff-HALT).
Here is the whole thing. Using structs and a union were a requirement. It works perfectly as long as the input is a 4 digit valid hex number.

int main( )
{
	
	//structures for both one hex character
	//and two hex character instructions
	struct single_byte_inst
	{
		unsigned short int addr:12;
		unsigned short int inst:4;
	};
	struct doub_byte_inst
	{
		unsigned short int addr:8;
		unsigned short int inst:8;
	};
	
	union machine_code
	{
		unsigned short int input;
		single_byte_inst single;
		doub_byte_inst doub;
	
	};
	machine_code x;
	system("cls");
	cout <<"Enter machine code(in hex format), -1 to quit:"<<endl;
	cout << endl << "?: ";
	cin >> hex >> x.input;
	//loop until input is -1
	while (x.input!=-1)
	{
		//check for halt instruction
		if (x.input == 0xffff)
			cout << "Op Code: ffff" << "    Address field: " << endl;
		else
		{
			cout << endl;
			cout << "Op Code: " << hex << setfill('0');
			cout << x.single.inst << "  " ;
			cout << "Address field: " << hex << setw(3)<< x.single.addr << endl;
		}	
		cout << endl;
		cout << "? ";
		cin >> hex >> x.input ;
	}
	cout << "Good bye!"<<endl;

[firedraco]

It's probably the same problem as if you cin >> an int and you input characters. Use:

if(!(cin >> /*...*/)) to check if it worked.