Why can I not do this:
char* p = new char;
void SetString(char * const str)
p = str;
int _tmain(int argc, _TCHAR* argv)
MyString *strg = new MyString(10);
MyString *secondstr = new MyString(7);
void MyString::concat(MyString& a, MyString& b)
len = a.len + b.len;
s = new char[len + 1];
delete  s;
void MyString::SetString(char * const str)
s = str;
s = new char[n+1];
s[n+1] = '\0';
len = n;
MyString(int n = 80);
void SetString (char * const str);
void concat (MyString& a, MyString& b);
There is difference between constant pointer and pointer to constant. Constant pointer is a pointer (a number - memory address) that cannot be changed - it always point to the same object given via initialization:
int * const const_pointer = &some_int_var; // will be always pointing to this var const_pointer = &some_other_var; // illegal - cannot change the pointer *const_pointer = 2; // legal, the pointer is a pointer to non-const
Pointer to constant is a pointer whose pointed value cannot be changed:
const int * pointer_to_const = &some_int_var; // doesn't have to be always pointing to this var pointer = &some_other_var; // legal, it's not a constant pointer and we can change it *pointer = 2; // illegal, pointed value cannot be changed
You can always assign constant to variable i.e. const pointer to non-const pointer (a). You can cast pointer to non-const to a poionter to const (b). But you cannot cast pointer to const to a poionter to non-const (c):
int * pointer; int * const const_pointer = &var; const int * pointer_to_const; /* a */ pointer = const_pointer; // OK, no cast (same type) /* b */ pointer_to_const = pointer; // OK, casting 'int*' to 'const int*' /* c */ pointer = pointer_to_const; // Illegal, casting 'const int*' to 'int*'
[EDIT] Below, this is not standard c++. However, this is common.[/EDIT]
is converted to constant pointer to const (
const char * const):
char *pointer = "Hello"; // Illegal, cannot cast 'const char*' to 'char*' char * const const_pointer = "Hello"; // Illegal, cannot cast 'const char*' to 'char*' const char * pointer_to_const = "Hello"; // OK, we can assign a constant to a variable of the same type (and the type is 'const char*') "Hello" = pointer_to_const; // Illegal cannot re-assign a constant
In above examples the second is your case. You tried to initialize pointer-to-non-const with a pointer-to-const when passing string literal as argument of your function. No matter if these pointers are constants or not, it's matter what do they point to.
1) If you cast a pointer of some type to a pointer of another type, you cannot cast pointer-to-const to pointer-to-non-const.
2) If you have constant pointer, the same rules applies as to other constants - you can assign a constant to a variable but you cannot assign a variable to a constant (except initializing it).
As GMan pointed out, the C++98 standard (§4.2/2) allows to implicitly cast string literals (which are constant char arrays) to a non-const char pointer. This is because of backward compatibility (in C language there are no constants).
Of course such a conversion can lead to mistakes and compilers will violate the rule and show an error. However, GCC in compatibility mode shows only a warning.