C++: Creating an uninitialized placeholder variable rather than a default object

Question

I'm moving from Java to C++ right now and I'm having some difficulties whenever a commonly used concept in Java doesn't map directly into C++. For instance, in Java I would do something like:

Fruit GetFruit(String fruitName) {
    Fruit fruit;
    if(fruitName == "apple") fruit = new Fruit("apple");
    else if(fruitName == "banana") fruit = new Fruit("banana");
    else fruit = new Fruit("kumquat"); //'cause who really wants to eat a kumquat?

    return fruit;
}

Of course, in C++ the Fruit fruit; statement actually creates a fruit. Does this mean I have to have a default constructor? This seems unsafe! What if my default fruit escaped?

Answer 1

C++ gives you much more headache when it comes to creating fruits. Depending on your needs, you can choose one of the following options:

1) create a Fruit on a stack and return a copy (you need a copy constructor) then and must provide some default fruit in case the name does not match:

Fruit GetFruit(const std::string &name)
{
   if ( name == "banana" ) return Fruit("banana");
   if ( name == "apple" )  return Fruit("apple");
   return Fruit("default");
}

2) create a Fruit on a heap and take care, that there could be null-pointer returned and also remember to delete this fruit somewhere and take care that it is deleted only once and only by its owner (and take care that noone holds a pointer to the deleted fruit):

Fruit* GetFruit(const std::string &name)
{
   if ( name == "banana" ) return new Fruit("banana");
   if ( name == "apple" )  return new Fruit("apple");
   return NULL;
}

3) and finally, use a smart pointer to avoid many possible pointer problems (but take care of null pointers). This option is the closest to your Java programming experience:

typedef boost::shared_ptr<Fruit> FruitRef;

FruitRef GetFruit(const std::string &name)
{
   if ( name == "banana" ) return new Fruit("banana");
   if ( name == "apple" )  return new Fruit("apple");
   return FruitRef();
}

Answer 2

Objects in Java are represented by pointers. Since this is pervasive, there is no special notation for pointers. In C++, objects can be represented by themselves or by pointers, so it's necessary to specify pointers when they occur.

The C++ version of your code is:

Fruit * GetFruit(std::string fruitName) {
    Fruit * fruit = 0;
    if (fruitname == "apple") fruit = new Fruit("apple");
    else if (fruitname == "banana") fruit = new Fruit("banana");
    else fruit = new Fruit("kumquat");

    return fruit;
}

This returns a pointer to Fruit. You would access members like fruit->color(), not fruit.color(). You should delete that pointer when you're through with it.

Answer 3

The most straightforward way would be:

Fruit GetFruit(String fruitName) {
    if(fruitName == "apple") return Fruit("apple");
    else if(fruitName == "banana") return Fruit("banana");
    else fruit = return Fruit("kumquat"); //'cause who really wants to eat a kumquat?
}

... and a direct mapping would be to use a (preferably "smart") pointer:

auto_ptr<Fruit> GetFruit(String fruitName) {
    auto_ptr<Fruit> fruit;
    if(fruitName == "apple") fruit = new Fruit("apple");
    else if(fruitName == "banana") fruit = new Fruit("banana");
    else fruit = new Fruit("kumquat"); //'cause who really wants to eat a kumquat?
    return fruit;
}