Re: Template And Arrays
On May 27, 12:52 am, Marcelo De Brito <Nosopho...@gmail.com> wrote:
I have created a template in which there is a generic
vector/array that can hold any type defined by the programmer
or built-in types. When I define the vector/array using
built-in types (int, double, etc.), I must insert data of the
respective type in it. But when I define the array using a
programmer defined type (e.g., a class), I can access the
class' members even though I did not insert any object in it!
See the lines of code below for a better explanation.
class B { //Programmer defined type.
public:
B() {};
B(int idd) : id(idd) {};
B(const B&) {};
virtual ~B() {};
void setid(int idd) {id = idd;}
void dummy() {cout << "B::dummy()" << " " << "id = " << id <<
endl;}
private:
int id;
};
template <class T, int size = 10>
class D1 {
public:
D1() : index(0) {};
D1(const D1&) {};
virtual ~D1() {};
T vt[size];
void push(T* i) {
vt[index] = *i;
index++;
}
void exvet() {
for(int i = 0; i<index; i++)
cout << "vt[" << i << "] = " << vt[i] << endl;
}
void vetsize() {cout << size << endl;}
private:
int index;
};
int main()
{
D1<int, 5> od4; //First with int.
for(int i = 0; i<5; i++)
od4.push(&i); //<- Inserting ints in the array. Fine.
od4.exvet(); //<- Shows the ints int the array, Fine too.
D1<B, 10> od5; //Now with the programmer defined type.
for(int i = 0; i<10; i++)
od5.vt[i].dummy(); //<- !!!!!!!HERE!!!!!! It compiles and run!
Why shouldn't it? You've created an array of 10 B in od5.vt.
}
In my mind, when I declared "D1<B, 10> od5", I allocated
memory for 10 elements of the type "B" -- as I did when I
defined for the int type "D1<int, 5> od4".
In both cases, you've created an array of N elements. In the
case of int, the elements are not initialized (since int has a
trivial constructor), and reading the values before writing them
would be undefined behavior (in practice, you'll normally just
get a random value). In the case of B, the object has a user
defined constructor, and that will be used to initialize them.
(Given that the user defined constructor doesn't initialize B::id,
accessing B::id before setting it will still result in undefined
behavior.)
BUT, in the "int" case, I had to insert data in the vector
"vt" of the template in order to properly access it.
No. No more so than in the case of B.
Therefore, how could I access the members of "od5" if I did
not insert any class "B" object in the respective array "vt"?
The same way you could have accessed od4[ 3 ] before inserting
any data.
Wouldn't a more suited way for this case be:
D1<B, 10> od5;
for(int i = 0; i<10; i++)
od5.push(new B); //Inserting class B objects.
Why the new? (For that matter, why does push take a pointer?)
for(int i = 0; i<10; i++)
od5.vt[i].dummy(); //Accessing members of the objects inserted.
When you define an array, all of the elements are created. If
the array is of a non-POD class type, then its constructors are
called; otherwise, it has an indeterminate value; reading this
value is undefined behavior (unless the object has character
type).
It's possible to separate allocation from initialization, but it
requires a bit more work. The array itself must be declared as
unsigned char, and steps must be taken to ensure that it is
properly aligned. And placement new must be used for
initialization.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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