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I have 4 building, each building has ten apartments. I created a class for each unit. class unit

to create an object, I need to parameters unit myunit(int building, int apartments)

at the beginning of the program, I need to create an object for each unit and add them to vector, which way is better to do that

the first way, I got Error operand type are: unit = unit *

int main()
{
    int building[4] = {1,2,3,4 };
    int apts[10]  = {1,2,3,4,5,6,7,8,9,10 };
    vector<unit > units(40);
    int vectorCounter = 0;
    for (int i = 0; i < 4; i++) {
        for (int j = 0; j < 10; j++) {
            // I got error in the line below.operand type are:unit = unit*
            units[vectorCounter++] = new unit (building[i], apts [j]);  
        }
    }

second way: create the same object name but different parameters

int main()
{
    int building = {1,2,3,4 };
    int apts  = {1,2,3,4,5,6,7,8,9,10 };
    vector<unit > units(40);
    int vectorCounter = 0;
    for (int i = 0; i < 4; i++) {
        for (int j = 0; j < 10; j++) {
        unit myunit(building[i], apts [j])   ;
            units[vectorCounter++] = myunit ;
        }
    }

third way : same as second way just add the destructor

int main()
{
    int building = {1,2,3,4 };
    int apts  = {1,2,3,4,5,6,7,8,9,10 };
    vector<unit > units(40);
    int vectorCounter = 0;
    for (int i = 0; i < 4; i++) {
        for (int j = 0; j < 10; j++) {
            unit myunit(building[i], apts [j])     ;
            units[vectorCounter++] = myunit ;
        ~unit() ;   
        }
    }

which way is better and why I got an error in the first way

junsal
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    `units[vectorCounter++] = new unit (building[i], apts [j]);` -- This could never work, since the vector is not one that holds pointers to `unit`. I would suggest stop guessing and get some good C++ reading material. – PaulMcKenzie Mar 28 '19 at 05:10
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    please show code that compiles. – Captain Giraffe Mar 28 '19 at 05:11
  • second way and the third are working but my question is this the best way to do that – junsal Mar 28 '19 at 05:13
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    Consider `reserve`ing space in the `vector` and then adding `unit`s with `emplace_back` eg. `units.emplace_back((building[i], apts [j]);` – user4581301 Mar 28 '19 at 05:13
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    Manually calling the destructor is unnecessary, `myunit` will go out of scope and destroy itself. You almost never want to manually call a destructor. Look up Placement New for one of the rare cases where you do want to call a destructor. – user4581301 Mar 28 '19 at 05:18
  • ok i will remove the destructor but is the second way is best or there is a better way – junsal Mar 28 '19 at 05:29

1 Answers1

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First of all, your are not really creating an array when you wrote int building = {1,2,3,4 };

It should be int building[] = {1,2,3,4 };

The compile error is due to your vector is holding on to unit object but you are trying to assign a pointer to it. In your first method, you might want to let the vector store unit *

Your second way is okay, but the third way is wrong, destructors are not called this way and you should not manually call destructor, the compiler will do it for you when it leave your for loop scope.

In my opinion, Best is really dependent on what way do you want to use your vector, the fastest and safest method would be the second method, it use stack allocation and it is extremely fast and safe (no need to manually call delete). It is also cache friendly since everything is sequential in the memory, traversing thru all the units is fast.

Why should C++ programmers minimize the use of new

Zhou Zhi Hua
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