<vector> and new

Hi I'm trying to do this (and it seem works):

Code:

---

std::vector<Light*> light; // it's a vector of pointer to Light
Light* l;
l = new Light[10];
light.push_back(l);
delete [] l; //delete is necessary

---

I I understand fine, this is a vector of Light with only one element(a pointer) that point to an array of 10 Light. Is it this?
Otherwise:

Code:

---

std::vector<Light*> light; // it's a vector of pointer to Light
Light* l;
l = new Light;
for (int i=0; < 10< i++)
     light.push_back(l);
delete [] l; //delete is necessary

---

Here I've a vecotr of 10 elements (pointer) that each one point to a Light;

Code:

---

std::vector<Light> light;
for (int i=0; <10; i++) 
    light.push_back(Light());

---

I prefer the last....
Wich is better? Thanks

But these are different. The first one stores arrays of objects and the last one stores objects -- the first one is twodimensional whereas the last one is onedimensional.

Quote:

---

Originally Posted by mickey

Hi I'm trying to do this (and it seem works):

Code:

---

std::vector<Light*> light; // it's a vector of pointer to Light
Light* l;
l = new Light[10];
light.push_back(l);
delete [] l; //delete is necessary

---

---

Here you use that vector as if it was a vector of arrays, but there is one problem --- if you delete the "l" pointer, the pointer you have appended to the vector will be invalid, because it will point to a deleted array.

It should be:

Code:

---

std::vector<Light*> light;
Light* l;
l = new Light[10];
light.push_back(l);
 
// ... use light ...
 
// delete the contents of light vector
for( std::vector<Light*>::iterator i = light.begin(); i != light.end(); ++i ) {
   delete [] *i;
}
lisht.clear();

---

Quote:

---

Originally Posted by mickey

Otherwise:

Code:

---

std::vector<Light*> light; // it's a vector of pointer to Light
Light* l;
l = new Light;
for (int i=0; < 10< i++)
     light.push_back(l);
delete [] l; //delete is necessary

---

---

Here you use the same variable as if it was a vector of pointers to Light instance, but again there is a problem --- all elements point to the same instance, if you delete it, all pointers in the vector will be invalid.

Code:

---

std::vector<Light*> light; // it's a vector of pointer to Light
 
for (int i=0; < 10< i++) {
     Light* l = new Light();
     light.push_back(l);
     // or simply:
     // light.push_back( new Light() );
}
 
// ... use light ...
 
// delete the contents of light vector
for( std::vector<Light*>::iterator i = light.begin(); i != light.end(); ++i ) {
   delete *i; // note no []
}
lisht.clear();

---

Another problem is that you can't use delete[] to free memory allocated with new --- always use the same kind of operator: delete after new and delete [] after new[].

Quote:

---

Originally Posted by mickey

Here I've a vecotr of 10 elements (pointer) that each one point to a Light;

Code:

---

std::vector<Light> light;
for (int i=0; <10; i++) 
    light.push_back(Light());

---

I prefer the last....
Wich is better?

---

This is the safest way to use a vector.

Note that you can achieve the same with this:

Code:

---

std::vector<Light> light;
light.resize( 10 );
// or even
// std::vector<Light> light( 10 );

---

It will fill the vector with 10 copies of Light().

Anther question: the vector <Light> is member of a Class A; I putted only delete [] l inside the destructor of A; Is it stiil necessary the vector destructor as you write and clear()? If I destruct the class that contatin vector....
Thanks

Furthermore in the first case: coding delete [] *i; Are you deleting the same space of memory more time? There's only one array of Light[10]....

Quote:

---

Originally Posted by mickey

Anther question: the vector <Light> is member of a Class A; I putted only delete [] l inside the destructor of A; Is it stiil necessary the vector destructor as you write and clear()? If I destruct the class that contatin vector....

---

No, you don't have to --- the vector will clear itself when you destroy it.

Quote:

---

Originally Posted by mickey

Furthermore in the first case: coding delete [] *i; Are you deleting the same space of memory more time? There's only one array of Light[10]....

---

If there is only one array, you don't need the vector.

Hence, if I code as below is clear() necessary? Thanks

Code:

---

class Scene {
  std::vector<Light> light;
};
Scene::Scene() {
  for (int i=0; <10; i++) 
    light.push_back(Light()); 
}
 
Scene::~Scene() {
    light.clear(); // necessary?
}

---

The vector will clear itself when it's destructed so an explicit clear() is not necessary (and remember clear() doesn't free the memory occupied by items stored in the vector).

Sorry I've still compile problem; the first compile (and seem works too), the 2th no;Why? What is happening?

Code:

---

class Scene {
  std::vector<Light> light;
};
 
Scene::Scene() {
    light.push_back(Light()); 
}
 
Scene::~Scene() {
    light.clear(); 
}

---

Code:

---

class Scene {
  std::vector<Light> light(1); //with (1) don't compile
};
 
Scene::Scene() {
    light.push_back(Light()); 
}
 
Scene::~Scene() {
    light.clear(); 
}

---

Code:

---

qsignalslotimp.h(66): fatal error C1903: unable to recover from previous error(s); stopping compilation
mainform.ui.h(75): error C2109: subscript requires array or pointer type
mainform.ui.h(75): error C2228: left of '.getPosition' must have class/struct/union type
mainform.ui.h(75): error C2475: 'Scene::light' : forming a pointer-to-member requires explicit use of the address-of operator ('&') and a qualified name
mainform.ui.h(75): error C2475: 'Scene::light' : forming a pointer-to-member requires explicit use of the address-of operator ('&') and a qualified name
mainform.ui.h(959): error C2109: subscript requires array or pointer type
mainform.ui.h(959): error C2228: left of '.setXPosition' must have class/struct/union type
..........................................

---

It should be:

Code:

---

class Scene 
{
  public:
     Scene();
     // ...
  private:
     std::vector<Light> light;
};
 
Scene::Scene() : light(1) 
{
   // ...
}

---

OK It works! but I don't understand where's the problem in my solutions.. Can't I give dimension of vector inside class? thanks

Quote:

---

Originally Posted by mickey

Can't I give dimension of vector inside class?

---

No, you can't. You can only declare member variables there.