I want to know how to pass an array of pointer objects into my constructor in C++

Question

In my header file for my PCBTable.cpp I have this:

class PCBTable {
private:
PCB* table[];
}

In my PCBTable.cpp I am trying to create a constructor that initializes the table to a blank table of PCB objects, with the size of my parameter and it looks like this

PCBTable::PCBTable(int size) {
    table = new PCB*[size];
}

However I get the error that table must be a modifiable value(E0137), and that array type PCB*[] is not assignable.

I've tried messing around with this but I can't get it to work.

Tip: Forget this new[] stuff and use std::vector. If you're slinging bare pointers around, you may want to wrap those as well. — tadman
– tadman, Commented Feb 6, 2023 at 22:22

Remy Lebeau · Accepted Answer · 2023-02-06 23:01:35Z

new T[...] returns a T* pointer, where T in your case is PCB*, thus you are asking new[] to return a PCB** pointer, which cannot be assigned to a PCB*[] ¹ fixed array, hence the error.

(¹ you are not specifying the size of the array, which is invalid syntax)

You need to change your table declaration to match what new[] actually returns, eg:

class PCBTable {
private:
    PCB** table;
    int table_size;
public:
    PCBTable(int size);
    ~PCBTable();
};

PCBTable::PCBTable(int size) : table_size(size) {
    table = new PCB*[size];
    for (int i = 0; i < size; ++i) {
        table[i] = new PCB;
    }
}

PCBTable::~PCBTable() {
    for (int i = 0; i < table_size; ++i) {
        delete table[i];
    }
    delete[] table;
}

Alternatively, if you just need an array of objects, rather than an array of pointers to objects:

class PCBTable {
private:
    PCB* table;
    int table_size;
public:
    PCBTable(int size);
    ~PCBTable();
};

PCBTable::PCBTable(int size) {
    table_size = size;
    table = new PCB[size];
}

PCBTable::~PCBTable() {
    delete[] table;
}

That being said, you really should be using std::vector instead of new[]/delete[] manually. And if you are using C++11 or later, use std::unique_ptr or std::shared_ptr instead of raw pointers, eg:

#include <vector>
#include <memory>

class PCBTable {
private:
    std::vector<std::unique_ptr<PCB>> table;
public:
    PCBTable(int size);
};

PCBTable::PCBTable(int size) : table(size) {
    for (auto &elem : table) {
        elem = std::make_unique<PCB>();
    }
}

Or:

#include <vector>
#include <memory>

class PCBTable {
private:
    std::vector<PCB> table;
public:
    PCBTable(int size);
};

PCBTable::PCBTable(int size) : table(size) {
}

pizzamonkey · Accepted Answer · 2023-02-06 23:41:32Z

There seems to be some confusion here. Arrays (declared like int foo[12]) are fixed-size at compile time and can be found on the stack. If you want a dynamically sized array, a list whose size is defined at runtime and isn't necessarily known at compile time, that needs to be a pointer. If the elements you want in your array are themselves pointers, it will have to be a pointer to pointers.

class PCBTable {
private:
    PCB** table;
}
PCBTable::PCBTable(int size){
    table = new PCB*[size];
}

Or, better yet, you could use std::vector, which gives you all the best of both worlds.

#include <vector>

class PCBTable {
private:
    std::vector<PCB*> table;
}
PCBTable::PCBTable(int size){
    table.resize(size);
}

You can use std::vector any way you would use a normal array. This also provides the dual benefit of not having to store the array size in an external variable, as table.size() takes care of that for you.

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