example of the need to use a pointer reference

Hello, I wanted to find an example of mandatory use of a pointer reference... I never found it... here, let's say, there is a code:

template<typename T>
void ptr_diff(T*& ptr, size_t length) {
    delete[] ptr;
    ptr = new T[length];
    for(unsigned i=0u; i<length; ++i) {
        ptr[i] = i;
    }
}

int main() {
int* ptr = new int[5];
    for(unsigned i=0u; i<5; ++i) {
        ptr[i] = i*i;
    }
    for(unsigned i=0u; i<5; ++i) {
        std::cout<<ptr[i]<<" ";
    }
    std::cout<<std::endl;
    ptr_diff(ptr, 5);
    for(unsigned i=0u; i<5; ++i) {
        std::cout<<ptr[i]<<" ";
    }
}

Unfortunately, everything worked correctly, the values on the pointer changed... even if there were ptr_diff(T*& ptr, size_t length), even without a reference to the pointer ... please write some implementation of the ptr_diff function such that you definitely need the signature of the ptr_diff function (T*& ptr, size_t length) with a reference to the pointer, otherwise, so that the value of the pointer does not change or something else happens to it

Author: Vlad from Moscow, 2016-11-19
Source

1 answers

Everything worked out correctly for you precisely because you pass a pointer to the function by reference

template<typename T>
void ptr_diff(T*& ptr, size_t length) {
              ^^^^
//...

If you remove the reference from the function declaration

template<typename T>
void ptr_diff(T* ptr, size_t length) {
              ^^^^
//...

Then you will see that everything will work, or rather not work, in a different way.:)

Since you are in the function deleting the memory that you originally allocated in main for the pointer ptr, and this pointer does not get a new value, since the function is dealing with a copy of the pointer, and not with the pointer itself.

Therefore further, after calling the function, this loop has an undefined behavior

ptr_diff(ptr, 5);
for(unsigned i=0u; i<5; ++i) {
    std::cout<<ptr[i]<<" ";
}

And most likely it will crash, because there is an attempt to access memory that has already been released by calling the function.

 1
Author: Vlad from Moscow, 2016-11-19 02:35:47