How do you safely clear an object from memory (with attributes) which was created using the new keyword?

Question

As far as I know on this topic, every "new" call needs a corresponding "delete" call to that object. So is this really correct?:

using namespace std;

class Box {
   public:
      double length;
      char letters_in_box[80];
};

int main() {
   Box *b = new Box;
   b->length = 2.0;
   b->letters_in_box = "Hello world";

   //Some code with b

   delete b;

   return 0;
}

Is the memory associated with the "length" double and "letters_in_box" array cleared with this?

Answer 1

Yes. When you delete b it deletes also letters_in_box array.

But, for your b->letters_in_box = "Hello world"; you will get a compile error: "error C3863: array type 'char [80]' is not assignable"

#include <memory> // For 'memcpy_s' (since C11)

class Box
{
public:
    double length;
    char letters_in_box[80];
};

int main()
{
    Box* b = new Box;

    b->length = 2.0;
    // b->letters_in_box = "Hello world"; ** Compile Error C3863: array type 'char [80]' is not assignable **
    memcpy_s(b->letters_in_box, sizeof(b->letters_in_box), "Hello world", sizeof("Hello world"));

    // Some code with b

    delete b;
}

MODERN C++

A better practice than new is smart pointers, than for example you don't have to bother with delete in case of exception and at all:

#include <memory> // For 'std::unique_ptr' and for 'memcpy_s'

class Box
{
public:
    double length;
    char letters_in_box[80];
};

constexpr char my_text[] = "Hello world"; 

int main()
{
    auto b = std::make_unique<Box>(); // No need to delete

    b->length = 2.0;
    memcpy_s(b->letters_in_box, sizeof(b->letters_in_box), my_text, sizeof(my_text));

    // Some code with b
}

Also, (instead of C array) I prefer to use C++ array:

#include <array>  // For 'std::array'
#include <memory> // For 'std::unique_ptr' and for 'memcpy_s' 

class Box
{
public:
    double length;
    std::array<char, 80> letters_in_box;
};
   
constexpr char my_text[] = "Hello world";

int main()
{
    auto b = std::make_unique<Box>(); // No need to delete

    b->length = 2.0;
    memcpy_s(&b->letters_in_box, b->letters_in_box.size(), my_text, sizeof(my_text));

    //Some code with b
}

--

One last comment: Without a constraint to use char[], I would use std::string instead:

#include <string> // For 'std::string'
#include <memory> // For 'std::unique_ptr' 

class Box
{
public:
    double length;
    std::string letters_in_box;
};

int main()
{
    auto b = std::make_unique<Box>(); // No need to delete

    b->length = 2.0;
    b->letters_in_box = "Hello world";

    //Some code with b
}