What is C++ version of realloc(), to allocate the new buffer and copy the contents from the old one?

Question

In C we used malloc(), free(), but in C++ youare using new, delete, but in C we also have realloc, which will alloc the new block and

Answer 1

There's no new/delete equivalent of realloc in C++.

From Bjarne Stroustrup's FAQ :

Why doesn't C++ have an equivalent to realloc()?

If you want to, you can of course use realloc(). However, realloc() is only guaranteed to work on arrays allocated by malloc() (and similar functions) containing objects without user-defined copy constructors. Also, please remember that contrary to naive expectations, realloc() occasionally does copy its argument array. In C++, a better way of dealing with reallocation is to use a standard library container, such as vector, and let it grow naturally.

If you want a resizeable container, just use std::vector, otherwise stay with malloc, realloc and free.

And, to answer your last question, the C++ version of your code would be :

main() {
    std::vector x(3);
    x[0] = 10;
    x[1] = 20;
    x[2] = 30;
    x.resize(4);
    x[3] = 40;
    for (int i = 0; i < 4; i++) std::cout << x[i] << std::endl;
}