Example 1: variable vs pointer in c++
int a = 5;
cout << a;
int a = 5;
int *ab;
ab = &a;
cout << ab;
Example 2: c pointers vs references
Pointers: A pointer is a variable that holds memory address of another variable. A pointer needs to be dereferenced with * operator to access the memory location it points to.
References : A reference variable is an alias, that is, another name for an already existing variable. A reference, like a pointer, is also implemented by storing the address of an object.
A reference can be thought of as a constant pointer (not to be confused with a pointer to a constant value!) with automatic indirection, i.e the compiler will apply the * operator for you.
*******************************************
*******************************************
A pointer can be re-assigned:
int x = 5;
int y = 6;
int *p;
p = &x;
p = &y;
*p = 10;
assert(x == 5);
assert(y == 10);
-----------------------------------------
A reference cannot, and must be assigned at initialization:
int x = 5;
int y = 6;
int &r = x;
-------------------------------------------
A pointer has its own memory address and size on the stack (4 bytes on x86), whereas a reference shares the same memory address (with the original variable) but also takes up some space on the stack. Since a reference has the same address as the original variable itself, it is safe to think of a reference as another name for the same variable. Note: What a pointer points to can be on the stack or heap. Ditto a reference. My claim in this statement is not that a pointer must point to the stack. A pointer is just a variable that holds a memory address. This variable is on the stack. Since a reference has its own space on the stack, and since the address is the same as the variable it references. More on stack vs heap. This implies that there is a real address of a reference that the compiler will not tell you.
int x = 0;
int &r = x;
int *p = &x;
int *p2 = &r;
assert(p == p2);
-------------------------------------------
You can have pointers to pointers to pointers offering extra levels of indirection. Whereas references only offer one level of indirection.
int x = 0;
int y = 0;
int *p = &x;
int *q = &y;
int **pp = &p;
pp = &q;
**pp = 4;
assert(y == 4);
assert(x == 0);
-------------------------------------------
A pointer can be assigned nullptr directly, whereas reference cannot. If you try hard enough, and you know how, you can make the address of a reference nullptr. Likewise, if you try hard enough, you can have a reference to a pointer, and then that reference can contain nullptr.
int *p = nullptr;
int &r = nullptr; <--- compiling error
int &r = *p; <--- likely no compiling error, especially if the nullptr is hidden behind a function call, yet it refers to a non-existent int at address 0
Pointers can iterate over an array; you can use ++ to go to the next item that a pointer is pointing to, and + 4 to go to the 5th element. This is no matter what size the object is that the pointer points to.
-------------------------------------------
A pointer needs to be dereferenced with * to access the memory location it points to, whereas a reference can be used directly. A pointer to a class/struct uses -> to access it's members whereas a reference uses a ..
-------------------------------------------
References cannot be stuffed into an array, whereas pointers can be!
-------------------------------------------
Const references can be bound to temporaries. Pointers cannot (not without some indirection):
const int &x = int(12);
int *y = &int(12);
This makes const& safer for use in argument lists and so forth.
Example 3: difference between pointer and reference
#include <iostream>
using namespace std;
void swap(int* x, int* y)
{
int z = *x;
*x = *y;
*y = z;
}
int main()
{
int a = 45, b = 35;
cout << "Before Swap\n";
cout << "a = " << a << " b = " << b << "\n";
swap(&a, &b);
cout << "After Swap with pass by pointer\n";
cout << "a = " << a << " b = " << b << "\n";
}
o/p:
Before Swap
a = 45 b = 35
After Swap with pass by pointer
a = 35 b = 45
#include <iostream>
using namespace std;
void swap(int& x, int& y)
{
int z = x;
x = y;
y = z;
}
int main()
{
int a = 45, b = 35;
cout << "Before Swap\n";
cout << "a = " << a << " b = " << b << "\n";
swap(a, b);
cout << "After Swap with pass by reference\n";
cout << "a = " << a << " b = " << b << "\n";
}
o/p:
Before Swap
a = 45 b = 35
After Swap with pass by reference
a = 35 b = 45
References are generally implemented using pointers. A reference is same object, just with a different name and reference must refer to an object. Since references can’t be NULL, they are safer to use.
A pointer can be re-assigned while reference cannot, and must be assigned at initialization only.
Pointer can be assigned NULL directly, whereas reference cannot.
Pointers can iterate over an array, we can use ++ to go to the next item that a pointer is pointing to.
A pointer is a variable that holds a memory address. A reference has the same memory address as the item it references.
A pointer to a class/struct uses ‘->'(arrow operator) to access it’s members whereas a reference uses a ‘.'(dot operator)
A pointer needs to be dereferenced with * to access the memory location it points to, whereas a reference can be used directly.
Example 4: difference between pointer and reference in c++
Pointers:
A pointer is a variable that holds memory address of another variable.
A pointer needs to be dereferenced with * operator to access the
memory location it points to.
References :
A reference variable is an alias, that is,
another name for an already existing variable.
A reference, like a pointer, is also implemented
by storing the address of an object.