Question

I’ve written a custom exception class to deal with OLE automation errors. The what() method looks like this:

const char* OleException::what() const throw() {
    std::string res = std::runtime_error::what();
    LPTSTR errorText = NULL;
    FormatMessage(
        FORMAT_MESSAGE_FROM_SYSTEM
        | FORMAT_MESSAGE_ALLOCATE_BUFFER
        | FORMAT_MESSAGE_IGNORE_INSERTS,
        NULL,
        hresult,
        MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPTSTR)&errorText,
        0,
        NULL);
    if(NULL != errorText) {
        res = res + " - " + errorText;
        LocalFree(errorText);
    }
    //std::cout << res << std::endl;
    return res.c_str();
}

However, when I try to print the string it returns in my main() function, all I get is the letter “I”. The weird part is: when I uncomment the second-to-last line of the method, it works just fine, i.e. I get the same message twice on my terminal. What am I doing wrong?

Answer 1

As others have pointed out, the pointer returned by c_str is invalidated the moment what returns, because ret is then no longer alive.

The solution is to make the error message a member of your exception class of type std::string and do the formatting at construction time, rather than in what. You can then safely return from c_str.

class OleException : std::runtime_error  // just guessing here
{
    std::string msg;

  public:
    OleException()
    {
        // construct msg
    }
    const char* OleException::what() const throw() { return msg.c_str(); }
    // etc.
};