如何在 c++ 中不使用 new 运算符的情况下在堆中创建 object？

Question

new operator in C++ is used for creating new objects in heap memory. I have no idea if how to create object in heap memory without using new operator. Is it possible, and how?

Also can anyone also suggest me creating object in stack using new opeartor in C++?

Answer 1

In my limited experience with C++, Using new operator is the only way to create common object in heap. HeapAlloc can allocate memory in heap, but how can you create a class in heap without using new operator?

Answer 2

In modern C++ it is common to avoid using new directly by using the functions, std::make_unique and std::make_shared which return smart pointers .

Eg.

class Object
{
public:
    Object(int i): i(i) {}
    // ...
    void do_stuff() { /*...*/ }

private:
    int i;
};

auto object = std::make_unique<Object>(4);

object->do_stuff();

Now you don't have to worry about deleting object .

Answer 3

To answer one of your questions, you can use placement new to allocate objects on the stack. For example:

int main()
{
    using namespace std;

    char buf[sizeof(string)];                // allocate char array on stack
    string *str = new (buf) string{ "I'm a placement new string object" };

    cout << *str << endl;

    str->~string();                          // delete string object
                                             // character array object will be
                                             // automatically deallocated

    return 0;
}

Answer 4

you can use c language in c++ to create objects without using the new keyword.You can create a pointer to allocate to an address in heap memory using malloc:

// make sure to include iostream to access the c library so you can use malloc
#include <iostream>
class Rectangle
{
private:
    int length;
    int breadth;


public:

    Rectangle(int l, int b) :length(l), breadth(b)
{
    std::cout << "overloaded constructor called!" << std::endl;
}

void set_length(int length) 
{
    length = length;
}

void set_breadth(int breadth)
{
    breadth = breadth;
}

int area() {
    return length * breadth;
}

int get_length() 
{
    return length;
}

int get_breadth()
{
    return breadth;
}



};

int main(int argc, char const *argv[])
{
// create a pointer object
    Rectangle *ptr;
// allocate object to heap memory using malloc
    ptr = (Rectangle *) malloc(sizeof(Rectangle));
    ptr->set_length(10);
    ptr->set_breadth(15);

    printf("length %d\n", ptr->get_length());
    printf("breadth %d\n", ptr->get_breadth());

// free up heap memory when done using delete keyword
    delete ptr;

    return 0;
}