Throwing an exception

Question

I have created one exception in a function like so:

void testing(int &X)
{
....
X=...
if (X>5)
throw "X greater than 5!"
}

and then in main.cpp

try
{
int X=0; 
testing(X);
}

catch (const char *msgX)
{
....
}

But now I would like to also introduce Y as X. and the prototype of testing will be :

void testing(int &X, int &Y)

My question, how can I throw two exceptions, in which if X>5, I throw an exception regarding X and if Y>10, I throw another exception about Y and I catch them all at the end in my main program?

Answer 1

In C++ it is impossible to have two exceptions "in flight" at the same time. If that condition ever arises (eg by a destructor throwing during stack unwinding), the program is terminated (with no way to catch the second exception).

What you can do is make a suitable exception class, and throw that. For example:

class my_exception : public std::exception {
public:
    my_exception() : x(0), y(0) {} // assumes 0 is never a bad value
    void set_bad_x(int value) { x = value; }
    void set_bad_y(int value) { y = value; }
    virtual const char* what() {
        text.clear();
        text << "error:";
        if (x)
            text << " bad x=" << x;
        if (y)
            text << " bad y=" << y;
        return text.str().c_str();
    }
private:
    int x;
    int y;
    std::ostringstream text; // ok, this is not nothrow, sue me
};

Then:

void testing(int &X, int &Y)
{
    // ....
    if (X>5 || Y>10) {
        my_exception ex;
        if (X>5)
            ex.set_bad_x(X);
        if (Y>10)
            ex.set_bad_y(Y);
        throw ex;
    }
}

Anyway you should never throw raw strings or integers or the like--only classes derived from std::exception (or perhaps your favorite library's exception classes, which hopefully then derive from there, but may not).

Answer 2

You can throw different exception types or you can through the same exception type with different content.

struct myexception : public std::exception
{
   std::string description;
   myexception(std::string const & ss) : description(ss) {}
   ~myexception() throw () {} // Updated
   const char* what() const throw() { return description.c_str(); }
};

void testing(int &X, int &Y)
{
   if (X>5)
      throw myexception("X greater than 5!")
   if (Y>5)
      throw myexception("Y greater than 5!")
}

try
{
   int X=0; 
   testing(X);
}
catch (myexception const & ex)
{

}

Answer 3

(btw. I did not down-vote...)

Here is a sketch:

class x_out_of_range : public std::exception {
  virtual const char* what() { return "x > 5"; }
};

class y_out_of_range : public std::exception {
  virtual const char* what() { return "y > 10"; }
};

Now in your function:

if (x > 5)
  throw x_out_of_range();

:

if (y > 10)
  throw y_out_of_range();

Now your catch code:

try
{
  :
}
catch (x_out_of_range const& e)
{
}
catch (y_out_of_range const& e)
{
}

NOTE: in any case, you can only throw one exception from the function...