Why must a pointer to a char array need strcpy to assign characters to its array and double quotes assignment will not work?

Question

The first example does not work when you go to delete the pointer. The program either hangs when I add the null terminator or without it I get:

Debug Assertion Failed Expression: _BLOCK_TYPE_IS_VALID(pHead->nBlockUse) from Visual Studio 2008

//Won't work when deleting pointer:
    char *at = new char [3];
    at = "tw"; //   <-- not sure what's going on here that strcpy does differently
    at[2] = '\0'; // <-- causes program to hang
    delete at;

//Works fine when deleting pointer:
    char *at = new char [3];
    strcpy(at,"t");
    at[1] = 'w';
    at[2] = '\0';
    delete at;

So what's going on when I use double quotes instead of strcpy? Both of them will cout the string perfectly and debugger does not show anything different.

Answer 1

When you do

char *at = ...;

at = "hello";

You're basically overwriting the pointer value (i.e., the address of the memory allocated for you by new[]) with the address of a static constant string. This means that when you later delete that memory, you're passing delete a pointer not previously returned by new.

That is a bad thing to be doing.

In C and C++, assignments to pointers typically don't do anything to the memory being pointed at, they change the pointer itself. This might be confusing if you're used to a language where strings are more of "first class citizens".

Also, you should use delete[] if you used new[].