Using std::hex to convert hex to decimal - c++

Question

I'm working with VS 2010 on Windows. I have a function which takes a char pointer. Now, inside the function, I am calling std::hex to convert it to decimal, but for some reason it is not working. It is outputting a large value which makes me think that it is converting the address instead.
void convertHexToDec(char* hex, char * dec)
{
long long decimal;
std::stringstream ss;
ss << hex;
ss >> std::hex >> decimal;
sprintf (dec, "%llu", decimal);
}
So, if the pass in a char pointer containing "58", the output decimal value is something like 1D34E78xxxxxxxxx. Looks like it is converting the address of the hex.
I tried these ways too:
ss << *hex;
ss << (char*)hex[0];
ss << (int *)&hex[0];
None of the above worked.
Any idea how I can make this function work?

Answer 1

The reason for your error is, probably, the wrong printf specifier. Also, sprintf is not safe: it assumes the destination buffer (dec) is large enough.
A possible solution using your function signature - not recommended since you do not know the size of the destination:
void convertHexToDec( char* hex, char * dec )
{
std::sprintf( dec, "%lld", std::strtoll( hex, 0, 16 ) );
}
A safe solution:
std::string convertHexToDec( const char* h )
{
return std::to_string( std::strtoll( h, 0, 16 ) );
}
A safe solution using streams:
std::string convertHexToDec( const char* h )
{
long long lld;
std::istringstream( h ) >> std::hex >> lld;
std::ostringstream os;
os << lld;
return os.str();
}

Answer 2

Apart from you not using std::string and refrences, I tried the following code:
#include <iostream>
#include <sstream>
void convertHexToDec(char* hex, char* dec)
{
long long decimal;
std::stringstream ss;
ss << hex;
ss >> std::hex >> decimal;
std::cout << "Decimal: " << decimal << "\n";
sprintf (dec, "%llu", decimal);
}
int main()
{
char hex[] = "58";
char dec[4];
convertHexToDec(hex, dec);
std::cout << "Output string: " << dec << "\n";
}
Output:
Decimal: 88
Output string: 88
live example
So what's your problem?