int hex2int (std::string str)
{
int num;
std::istringstream i(str);
i >> std::hex >> num;
return num;
}

#include <iostream>
#include <sstream>
#include <string>
 
using namespace std;
template<typename Type>
string toHex(const Type& value, bool showBase = true){
	stringstream strm;
	if(showBase)
		strm << showbase;
	strm  <<  hex << value;
	string to_hex;
	if(!(strm >>  to_hex)) throw std::exception("Conversion to hex failed!\n");
	return to_hex;
}
typedef __int64 int64;
 
int64 hexToInt64(const string hexStr){
	stringstream strm;
	strm << hex << hexStr;
	int64 value = 0;
	if(!(strm >> value)) throw std::exception("Conversion to int64 failed!\n");
	return value;
}
int main(){
	cout << hexToInt64(toHex(3145131833)) << endl;	
	cout << toHex( hexToInt64("bb76e739")) << endl;
}

#include <iostream>
#include <sstream>
#include <string>
 
using namespace std;
 
//supports hex, dec and octal
//base can only equal decimal(10),octal(8),or hex(16)
template<typename ReturnType, typename InputType>
void convertBases(const InputType& src, ReturnType& dest,const int base,bool showBase = true)
{
	stringstream stream;
 
	if(showBase) 
		stream << showbase;	
 
	switch(base)
	{
	case 10: stream << dec;  break; /* dec by default but...*/ 
	case 16: stream << hex;  break;
	case 8:  stream << oct; break;
	default: throw std::exception("Base not supported"); break;
	}
 
	stream << src;
 
	if(!(stream >> dest) ){
		string errMsg = "Conversion failed\n";
		errMsg +=  "Tried to convert \"" + string(typeid(src).name());
		errMsg +=  "\" into \"" + string(typeid(dest).name()) + "\"";
		throw std::exception(errMsg.c_str());
	}	
}
int main(){
	const int decValue = 15;
 
	enum{ HEX = 16, DEC = 10, OCT = 8 };
 
	string hexValue;
	string octalValue;
	string deciValue;
 
	try{
		convertBases(decValue,hexValue,HEX);
		convertBases<string>(decValue,octalValue,OCT);
		convertBases<string>(decValue,deciValue,DEC);
	}catch(std::exception& e) { 
		cout << e.what() << endl;
		return -1; 
	}
 
	cout <<"For " << decValue << " : \n";
	cout << "hexValue = " << hexValue << endl;
	cout << "octalValue = " << octalValue << endl;
	cout << "decValue = " << deciValue << endl;
	cout << endl;
 
	return 0;
}