C++ inheritance: string construction in constructor

I'm learning C++ at the moment and implementing a sample program for exception handling right now.
The main program implements a number system with arbitrary bases.
I made a class nums_exception : public std::exception with function virtual const char *what();

#include <stdexcept>
#include <sstream>
class Nums
{
public:
Nums() {}
unsigned int getBase() { return 2; }
};
class nums_exception: public std::exception
{
public:
nums_exception(Nums* obj, std::string errortext);
virtual ~nums_exception();
virtual const char *what() const noexcept;
const Nums *failObj();
private:
std::string errortext;
Nums *obj;
};
nums_exception::nums_exception(Nums *obj, std::string errortext)
: errortext(errortext)
, obj(obj)
{
}
nums_exception::~nums_exception() = default;
const char *nums_exception::what() const noexcept
{
if (this->errortext.size() == 0)
{
return "Nums exception!";
}
else
{
std::stringstream ret;
ret << "Nums exception at "
<< obj
<< " :"
<< this->errortext;
return ret.str().c_str();
}
}
// Now i derived  nums_bad_digit, which should look like this:
class nums_bad_digit: public nums_exception
{
public:
nums_bad_digit(Nums* obj, uint base, char digit);
virtual ~nums_bad_digit() override;
static std::string ERRORTEXT(Nums *obj, char digit);
private:
std::string errortext;
const uint base;
Nums *obj;
};
inline std::string nums_bad_digit::ERRORTEXT(Nums *obj, char digit)
{
return std::string(std::to_string(digit) + " not in alphabet for base " +
std::to_string(obj->getBase()) + '.');
}
nums_bad_digit::nums_bad_digit(Nums *obj, uint base, char digit)
: nums_exception(obj, ERRORTEXT(obj, digit))
, base(base)
{
}
nums_bad_digit::~nums_bad_digit() = default;
int main()
{
Nums n;
throw nums_bad_digit(&n, 42, 'x');
}

Here I tried to reach my goal using a static method.

I want to construct a ready-to-show errormessage, which says why exactly I threw this exception, and then pass it to the nums_exception constructor. As example, nums_bad_digit::what() should return
Nums_expection at 0x777fff: 'Q' not in alphabet for base 16.

I also tried a compiler macro... But whatever I try - writing it as normal code works fine. But when I want to pass the string to the nums_exception constructor, it will always get an empty string.

There are several problems here.

1. std::to_string does not take a char.

   The argument would presumably get promoted to int, with the value being the ASCII code (or whatever you're using) of the input. So 'x' becomes "120". Not what you wanted.

   You should use std::string(1, digit) instead.

   Read the documentation for the functions that you use! At the very least, test the units of your program in isolation.

    

2. what() returns a const char*, but the data it points to is dead.

   The stringstream was local, and the string was temporary, so this pointer is immediately dangling. I'd generate the full string in the constructor instead, and store it as a member so that it survives for as long as the exception object does.

    

So, this:

nums_exception::nums_exception(Nums *obj, std::string errortext)
: obj(obj)
{
if (errortext.size() == 0)
{
this->errortext = "Nums exception!";
}
else
{
std::stringstream ret;
ret << "Nums exception at "
<< obj
<< " :"
<< errortext;
this->errortext = ret.str();
}
}

and this:

const char *nums_exception::what() const noexcept
{
return errortext.c_str();
}

and this:

inline std::string nums_bad_digit::ERRORTEXT(Nums *obj, char digit)
{
return std::string(1, digit) + " not in alphabet for base " +
std::to_string(obj->getBase()) + '.';
}

(live demo)