Re: Template And Arrays
* Marcelo De Brito:
I have created a template in which there is a generic vector/array
It's not a "generic vector/array", it is a fixed size raw array.
that can hold any type defined by the programmer or built-in types.
Well, this is subtle, but it can't hold just any type. If the type in question
requires construction but does not support default construction, then there's no
way to instantiate the array. std::vector solves this by using, well, advanced
techniques, but std::vector still doesn't support non-copyable element type.
When I define the vector/array using built-in types (int, double,
etc.), I must insert data of the respective type in it.
OK, taken as description of your class' interface.
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!
That's your choice as designer of the class.
It's a bad choice.
Don't expose implementation details.
See the lines of code below for a better explanation.
class B { //Programmer defined type.
public:
B() {};
This leaves the 'id' member with indeterminate value => later UB.
B(int idd) : id(idd) {};
OK.
B(const B&) {};
This leaves the 'id' member with indeterminate value => later UB.
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) {};
OK.
D1(const D1&) {};
This leaves the 'index' member with indeterminate value => later UB.
Depending on the type T it also leaves the 'vt' member with indeterminate values
(which is OK), or default constructed values (which impacts negatively on
efficiency).
virtual ~D1() {};
OK.
T vt[size];
This is a raw array of constant size.
void push(T* i) {
vt[index] = *i;
index++;
}
This is bad design. It requires client code to use the address operator, and it
opens the door for nullpointer argument, cuasing UB (most likely a crash). Make
the formal argument type 'T const&'.
void exvet() {
for(int i = 0; i<index; i++)
cout << "vt[" << i << "] = " << vt[i] << endl;
}
OK, debugging?
void vetsize() {cout << size << endl;}
OK, more debugging?
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!
So?
}
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". BUT, in the "int" case, I had to insert data in the
vector "vt" of the template in order to properly access it.
No, you didn't.
Therefore,
how could I access the members of "od5" if I did not insert any class
"B" object in the respective array "vt"?
You can access the elements because you haven't prevented access.
That's your choice as class designer.
Make the array 'private' (or 'protected') to prevent client code access.
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.
for(int i = 0; i<10; i++)
od5.vt[i].dummy(); //Accessing members of the objects inserted.
How about it??
Well, how about it?
Have you tried actually writing that (and compiling and running)?
Cheers & hth.,
- Alf
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