Best practices for checking valid input in OO design?

[dolle39]

What is the best practices for checking valid input in a Object Oriented design?

Consider the following simple example, where we have a class Person which takes as argument age, in its constructor.

class Person {
public:
    Person(int age) {
      mAge = age;
    }
private:
    int mAge;
}

Now assume that in my main program, I want the user to enter data, such as age, and from that I will instantiate a Person, example:

int main() {
   
   int age;
   cin >> age;
   // if (age < 0 )   write error to user
   Person p(age);

   return 0;
}

The question is then, where should one check for valid input? I see two options.

1) Check after reading input in main() method before creating a Person object.
2) Check inside the constructor and throw an Exception if input is invalid.

class Person {
public:
    Person(int age) {
      if(age < 0)
        throw new InvalidAgeException();
      mAge = age;
    }

What do you think is the best design? I see good benefits from letting the constructor handle all the checking of valid data, however that would force me to use a try/catch every time one instantiates the class.

[andywestken]

Actually, yhe first thing you shoud so is use the right data type.

class Person {
public:
    Person(unsigned int age) {
      mAge = age;
    }
private:
    unsigned int mAge;
}

Now you can't pass a negative age to the object.

Then all you need to worry about is handling bad inputs.

Andy

[MiiNiPaa]

It is a good idea to make constructor check invariants. If constructor cannot create working object, it should throw an exception.

Also it is a good idea to have basic sanity check in user input to dive user chance to fix error before it will be passed further into your program.