Convert hexa to decimal

hex2dec.cpp

#include <iostream>
#include <string.h>

using namespace std;
int hex2dec(string str);
int power(int exponent, int base);
int count(string s);
int main ()
{
	string hexa;
	cout << "Enter HEXA number to convert into decimal \t";
	cin >> hexa;
	cout << "Decimal is => " << hex2dec(hexa);
	return 0;
}
int hex2dec(string hexa)
{
	int len,i,decimal = 0,exp = 0;
	len = count(hexa) - 1;
	for (i = len; i >= 0; i--){
		if (hexa[i] <= '9' && hexa[i] >= '0'){
			decimal += (hexa[i] - 48) * (power(16,exp));
		}
		if (hexa[i] <= 'F' && hexa[i] >= 'A') {
			decimal += (hexa[i] - 55) * power(16,exp);
		}
		exp++;
	}
	return decimal;
}
int count (string s)
{
	int count = 0;
    while (s[count] != '\0') {
    	count ++;
	}
	return count;
}
int power(int base, int exponent)
{
	int result = 1;
	while (exponent != 0){
		result *= base;
		--exponent;
	}
	return result;
}