Error/Exception catching within a class - c++

Question

I have written a class that returns a random string after you provide a seed, a sequence length, and a character set as shown below.
I would like to know how to catch any bad input, such as setting the seed to 0 or to a negative number.
The examples that I have seen have just used cout to send an error message and then exit(1); I am thinking about putting a bool flag as a private variable and setting it to false when invalid input is detected. Then after you attempt to generate a random string with this class you would just check that flag through an accessor function.
Is there a better and/or standard way of handling this that will not just exit the program like exit(1)? Any general input about the class is welcomed also. Thanks for the assistance.
RandomString.h
// This is the header file randomstring.h. This is the interface for the class RandomString.
// Values of this type are a random string of the specified length from the specified string character set.
// The values that are needed for input are a positive integer seed, an integer desired length, and a string character set.
// Uses the mt19937 random number engine with a uniform_int_distribution.
#ifndef RANDOMSTRING_H
#define RANDOMSTRING_H
#include <string>
#include <random>
using namespace std;
namespace RandomString
{
class RandomString
{
public:
RandomString(double newSeed, unsigned int newLength, string newCharacterSet); // Initializes the RandomString object with the provided arguments.
RandomString(); // Initializes the seed to 1, the length to 0, and the character set to '0'.
double getSeed();
unsigned int getLength();
string getCharacterSet();
string getSequence();
void setSeed(double newSeed); // Sets the new seed but does not produce a new random sequence.
void setLength(unsigned int newLength); // This is the length of randomSequence.
void setCharacterSet(string newCharacterSet);
void generateNext(); // Generates the next random sequence.
private:
double seed;
unsigned int length;
string characterSet;
string randomSequence;
mt19937 engine;
};
} // RandomString namespace
#endif
RandomString.cpp
// This is the implementation file randomstring.cpp. This is the implementation for the class RandomString.
// The interface for the class RandomString is in the header file randomstring.h.
#include "stdafx.h"
#include <string>
#include <random>
#include "randomstring.h"
using std::string;
using std::uniform_int_distribution;
namespace RandomString
{
RandomString::RandomString(double newSeed, unsigned int newLength, string newCharacterSet)
{
setSeed(newSeed);
setLength(newLength);
setCharacterSet(newCharacterSet);
}
RandomString::RandomString()
{
seed = 1;
length = 0;
characterSet = '0';
}
double RandomString::getSeed()
{
return seed;
}
unsigned int RandomString::getLength()
{
return length;
}
string RandomString::getCharacterSet()
{
return characterSet;
}
string RandomString::getSequence()
{
return randomSequence;
}
void RandomString::setSeed(double newSeed)
{
seed = newSeed;
engine.seed(seed);
}
void RandomString::setLength(unsigned int newLength)
{
length = newLength;
}
void RandomString::setCharacterSet(string newCharacterSet)
{
characterSet = newCharacterSet;
}
void RandomString::generateNext()
{
randomSequence.resize(length);
uniform_int_distribution<> distribution(0,characterSet.length() - 1);
for (int i = 0; i < length; i++)
{
randomSequence[i] = characterSet[distribution(engine)];
}
}
} // RandomString namespace
Finally, here is the test program that I am using.
// test.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
#include "randomstring.h"
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
do
{
double seed = 0;
unsigned int length = 0;
cout << "Enter seed: ";
cin >> seed;
cout << "Enter length of string: ";
cin >> length;
cout << endl;
RandomString::RandomString randomTest(seed,length,"ABCDEFGHJKLMNPQRSTUVWXYZ1234567890");
cout << "class RandomString" << endl;
cout << "seed: " << randomTest.getSeed() << endl;
cout << "length: " << randomTest.getLength() << endl;
randomTest.generateNext();
cout << "random sequence: " << randomTest.getSequence() << endl;
randomTest.generateNext();
cout << "next random sequence: " << randomTest.getSequence() << endl << endl;
}while(true);
}

Answer 1

Throw an exception
void RandomString::setSeed(double newSeed)
{
if (newSeed <= 0) {
throw std::runtime_error("seed should be positive");
}
seed = newSeed;
engine.seed(seed);
}
User would have to use try catch to handle error, else std::terminate is called.

Answer 2

How about letting the user have another chance at giving a good input?
What is the next thing the user will do after they find out they have put in a bad input? There is a high chance they will try again with a good input so just ask for another one.
while(true)
{
cout << "Enter seed: ";
cin >> seed;
if(seed > 0)
{
cout << "ERROR: Seed must be greater than 0." << endl;
break;
}
}
To answer your question about the bool error flag, I wouldn't do it that way. I have no need for bad data that was generated from a bad seed. No data is better than bad data. I generally try to "fail early, fail often", meaning I should stop and report the error as soon as I realize it exists.
If you were set on storing it the error flag, I'd have generateNext() or setSeed return false rather than nothing if there was a problem. Exceptions also work, but are slower and some projects don't like them in C++.
If you plan to have others use RandomString, I'd change the method signature from a double to uint (unsigned int). This would catch negative numbers being used at compile time rather than run time...again helping to fail even earlier. Sounds like this might not work in your classroom scenario, but is a good principle in the real world.