Cursor NULL in C // C ++: Cursor NULL in C // C ++

It is always a good practice to assign a NULL pointer to a pointer variable in case you don't know the exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer .

The NULL pointer is a constant with a value of 0 defined in several standard libraries, including iostream . You consider the example below:

 #include using namespace std ; int main () { int * contro = NULL ; cout << "Gia tri cua contro la " << ptr ; return 0 ; } 

When the above code is compiled and executed, it gives the following result:

 Gia tri cua contro la 0 

On most operating systems, programs are not allowed to access memory at address 0, because that memory is reserved by the operating system. However, memory address 0 has special meaning; it signals that the pointer cannot point to an accessible memory location. But by convention, if a pointer contains a value of 0, it is treated as not pointing to anything.

To test a null pointer, you can use an if statement as follows:

 if ( contro ) // true neu contro khong la NULL if (! contro ) // true neu contro la NULL 

Next lesson: Arithmetic pointer in C / C ++