How can we modify the size of array in C++?

No, you can't... realloc() can only be used with malloc()/free()

Best call for a new[] allocated array is to create a new one and then memcpy() the data from one to another.

Better way - use an std::vector or std::string instead of array if you know you'll need resizing. Internally they're pretty much the same array.

In C++ it is best to use the STL std::vector class for this kind of thing. Either that, or a std::string.

I have an array of characters allocated with new and i want to modify the size of the array.

You can't resize an array, you can only allocate a new, larger one, move the contents to the new array, and delete the old one.

Can i use realloc function for that?

If you used malloc to allocate the original array, yes. But that's usually a bad idea in C++, where you usually want to deal with arrays of non-trivial objects not raw memory.

What is the best way to do so?

Use std::string (or perhaps std::vector<char>) to manage a dynamic array of characters automatically. These also have the advantage of using RAII to reduce the risk of memory leaks and other memory management errors.

You can use realloc(), if your array is allocated dynamically(via malloc/calloc/realloc). If you have static array, you can't resize it.If you have allocated with new:

int* Copy = new int[newSize];
std::copy(oldCopy,oldCopy+size,Copy);

But the best way would be to use std::vector<type> from c++ standard library

As was said by others, you cannot resize the array that was allocated per se, but you can create a larger one, copy the content of the first array to the second and delete the first one.

Here's an example, using std::copy().

int main(int argc, char** argv) {
    int* arr = new int[5];
    arr[0] = 1;
    arr[1] = 2;
    arr[2] = 3;
    arr[3] = 4;
    arr[4] = 5;
    int* tmp = new int[10];
    std::copy(arr, arr + 5, tmp);
    std::copy(arr, arr, tmp + 5);
    delete[] arr;
    arr = tmp;

    for(int i = 0; i < 10; i++) {
        std::cout << arr[i] << "\n";
    }
    delete[] arr;
    std::cin.get();
    return 0;
}

This first creates an integer array and fills it. It then creates a larger array and fills it with the content of the first array twice and display that new larger array.

The principle is the same for an array of characters.

As the others have mentionned, your best bet in C++ is to use the standard library. For a resizable array of characters, you should probably use std::string or a vector of strings, but it's a bit overkill in some situations.