C++ convert string to enum?

I'm trying to convert a string to an enumerated type on Linux. I found this code on stackoverflow for doing this very thing, but am not sure I'm using function templates correctly. It compiles fine.

template <typename T>
typename boost::enable_if< boost::is_enum<T>, bool>::type
convert_string(const std::string& theString, T& theResult)
{
    typedef typename std::underlying_type<T>::type safe_type;

    std::istringstream iss(theString);
    safe_type temp; 
    const bool isValid = !(iss >> temp).fail();
    theResult = static_cast<T>(temp);

    return isValid;
} 

And here I'm trying to make proper use of it.

enum TestType {
    A1,
    B2,
    C3
};

string s = "A1";
TestType tt;

convert_string(s, tt);

The problem is that convert_string fails and tt is always 0 after the call. Also, the enum is located within an IDL for a middleware, but I'm testing with this.