指针向量

Question

I am trying to read data in from a file, create a critic from the data and then create a vector of pointers to each critic. After I read the data for the first critic and set it to theCritic, I have

NightHawks *nightHawkPtr = &theCritic; 
criticPointer.push_back(nightHawkPtr);

and then I try to print them out.

 for (int i = 0; i < criticPointer.size(); i++)
    {
        criticPointer[i]->text();
    }

However, each time the pointer is pushed back, all the pointers point to the same critic.

Answer 1

From your code, it looks like you are setting all the pointer values to one instance and just keep changing that instance. You must create new instances everytime the value changes:

NightHawks *nhp = new Critic();

If you are inserting them into a vector, it is strongly advised to use shared_ptrs so you don't have to worry about freeing memory.

Answer 2

They point to the same critic because you assign them the address of the same variable theCritic

NightHawks *nightHawkPtr = &theCritic; 
criticPointer.push_back(nightHawkPtr);

Moreover if variable theCritic is a local variable then the program can have undefined behaviour because a pointer to the variable becames invalid after the local variable will be destroyed.

Each time you build an object of type critic you should build it in the heap that is you should use operator new .

Answer 3

You need to make a new critic when you place it in the vector. Else all the data point to the same memory location.

criticPointer.push_back(new NightHawks(theCritic));

Better Yet don't store pointers just store the critic value in the vector.

Answer 4

As you mentioned in your comment that, I declare the critic like this NightHawks theCritic; and then i use getLine to read the input from the file and then I use setters to set the input to theCritic I declare the critic like this NightHawks theCritic; and then i use getLine to read the input from the file and then I use setters to set the input to theCritic , your problem is that you are changing the contents of the same variable. Create new variables for different critics, assign to them, then push the address into the vector.