How do I declare an array of objects whose class has no default constructor?

Question

If a class has only one constructor with one parameter, how to declare an array? I know that vector is recommended in this case. For example, if I have a class

Answer 1

When you declare an object that has no default constructor, you must initialize it in the declaration.

Foo a; // not allowed
Foo b(0); // OK

The same goes for arrays of such types:

Foo c[2]; // not allowed
Foo d[2] = { 0, 1 }; // OK
Foo e[] = { Foo(0), Foo(1), Foo(2) }; // also OK

In your case, you'll probably find it impractical to initialize all 10,000 elements like that, so you might wish to rethink whether the class really shouldn't have a default constructor.

Answer 2

In straight C, using int foo[10000] = {1}; will initialize the first array item to 1 and the remainder of the array to zero. Does C++ not auto-initialize unspecified array members, or does this require a default constructor?

Answer 3

You have to use the aggregate initializer, with 10000 of inidividual initializers between the {}

Foo array[10000] = { 1, 2, 3, ..., 10000 };

Of course, specifying 10000 initializers is something from the realm of impossible, but you asked for it yourself. You wanted to declare an array of 10000 objects with no default constructor.

Answer 4

sbi had the best answer for plain arrays, but didn't give an example. So...

You should use placement new:

char *place = new char [sizeof(Foo) * 10000];
Foo *fooArray = reinterpret_cast<Foo *>(place);
for (unsigned int i = 0; i < 10000; ++i) {
    new (fooArray + i) Foo(i); // Call non-default constructor
}

Keep in mind that when using placement new, you are responsible for calling the objects' destructors -- the compiler won't do it for you:

// In some cleanup code somewhere ...
for (unsigned int i = 0; i < 10000; ++i) {
    fooArray[i].~Foo();
}

// Don't forget to delete the "place"
delete [] reinterpret_cast<char *>(fooArray);

This is about the only time you ever see a legitimate explicit call to a destructor.

NOTE: The first version of this had a subtle bug when deleting the "place". It's important to cast the "place" back to the same type that was newed. In other words, fooArray must be cast back to char * when deleting it. See the comments below for an explanation.

Answer 5

Try this.

Foo **ppInstances=0;
    size_t total_instances = 10000;

    for(int parent=0;parent < total_instances;parent++){
        ppInstances[parent]=new Foo( parent ); 
        ppInstances++;
    }
    for(int parent=0;parent < total_instances;parent++){
        delete *ppInstances;
        ppInstances--;
    }

Answer 6

The proper way is use to std::aligned_storage. You will have to manually construct and destruct items as well as reintrepret_cast when you want to access an item. I recommend you write a small wrapper class around storage_t to take care of this. Someone mentioned using boost::optional which uses a bool and a storage_t under the hood. This method saves you a bool.

template<typename T>
using storage_t = typename std::aligned_storage<sizeof(T), alignof(T)>::type;

struct Foo;

size_t i = 55;
storage_t<Foo> items[1000]; // array of suitable storage for 1000 T's
new (reintrepret_cast<Foo*>(items + i)) Foo(42); // construct new Foo using placement new
*reintrepret_cast<Foo*>(items + i) = Foo(27);    // assign Foo
reintrepret_cast<Foo*>(items + i)->~Foo()        // call destructor