Some pointers do not point to valid data; dereferencing such a pointer is a runtime error.
Any pointerset to 0 is called a null pointer, and since there is no memory location 0, it is an invalid pointer. One should generally check whether a pointer is null before dereferencing it. Pointers are often set to 0 to signal that they are not currently valid. Dereferencing pointers(unlocated pointers) to data that has been erased from memory also usually causes runtime errors. Example:
int * myFunc () {
int phantom = 4;
return & phantom ;
}
phantom is deallocated when myFunc exits, so the pointer the function returns is invalid. As with any other variable, the value of a pointer is undefined until it is initialized, so it may be invalid.