Why is this allocation of a vector not accepted?

Question

So I have declared a vector in my class header like this:

 ...
 private:
    vector<Instruction> instructions;
 ...

Then in the .cpp implementation in the constructor, I try to initialize it like this:

 instructions = new vector<Instruction>();

Xcode tells me: No viable overloaded '='

I am basically trying to get this class to behave like I would expect in java, where instances of the class retain this vector. Thats why I wanted to dynamically allocate it using new, so as to make sure that it doesn't get lost on the stack or something. Any help would be appreciated with this, thanks so much.

Answer 1

In order to do what you're trying to do the instructions = new vector<Instruction>() line is entirely unnecessary. Simply remove it. The vector will automatically get default-constructed when an instance of your class gets constructed.

An alternative is to make instructions into a pointer, but there doesn't appear to be any reason to do this here.

Answer 2

when you write

vector<Instruction> instructions;

you already have instantiated instructions to whatever memory model the user of your class is using e.g.

class YourClass
{
vector<Instruction> instructions;
};

...
int main()
{
   YourClass class1; // stack
   std::unique_ptr<YourClass> class2(new YourClass); // heap
...
}

Answer 3

In your class, you declare a std::vector<Instruction>. new vector<Instruction>(); returns you a std::vector<Instruction>*.

Answer 4

operator new returns a pointer, so you have a type mismatch.

The real issue is the fact that you are doing it at all. Do you have a good reason for dynamically allocating that vector? I doubt it, just omit that entirely as it will be allocated along with instances of your type.

Answer 5

You have a member value but you try to initialize it from a vector<Instruction>*. Initialize it from vector<Instruction> or change the declaration to a pointer. If you go down the second route, you need to observe the rule of three.

You might also want to get a decent C++ book from this list.

Also, I think you have a using namespace std; in your header which is bad.

Answer 6

Do not use new in C++ unless you know what you are doing. (Which you do not, currently.)

Instead use automatic objects. You already defined instructions to be an automatic object. You just need to init it as if it were one:

class wrgxl {
 public:
   wrgxl()
    : instructions() // this initializes the vector using its default constructor
   {
     // nothing needed here
   }
 ...
 private:
    vector<Instruction> instructions;
 ...
};

The initialization of instructions in the constructor's initialization list is optional, though, if you only want to call the default constructor anyway. So in this case, this would be enough:

wrgxl()
{
}

---

If you wanted to dynamically allocate a vector, you would need to make instructions a pointer to a vector. But this rarely ever make sense, since the vector already allocates its data dynamically, but wraps this, so you do not have to deal with the ugly details resulting from this.
One of those details is that, if you have a dynamically allocated object in a class, you will then have to worry about destruction, copy construction, and copy assignment for that class.

As Kerrek already pointed out, you will need to have a good C++ book in order to properly learn C++. Make your pick.

Answer 7

I think you are confusing C++'s with C#'s syntax.

First, unlike in many languages, variables allocated on the stack (such as yours), are initialized by calling the default constructor, so I suspect that what you are doing is unnecessary.

Second, in order to do what you are trying to do, you use the following syntax:

instructions = vector<Instruction>();

however, as I said, this is likely redundant (and wasteful on a non-optimizing compiler as it might call both the constructor and the assignment operator). A much better way to do this is found in sbi's answer.

Third, unlike in C#, the new operator allocates memory on the heap and returns a pointer to the newly allocated data. Your variable instructions is not a pointer, thus the error.