CODEWITHSUNDEEP
c++stringpointersostream
Saeid
Saeid

Reputation: 568

Are there any exceptions to const char*?

If cstring is a pointer, then why can it get a value directly? Secondly, Why wasn't the ‍‍‍*cstring's result equal to whole of string? Third, cstring is a non-constant pointer to a constant character, so why change its value and not change its address?

#include <cstdio>
#include <conio.h>
#include <iostream>

using namespace std;

int main() 
{
    const char* cstring = "string";
    cout << cstring << endl << *cstring << endl << &cstring << endl;

    cstring = "foo";
    cout << cstring << endl << *cstring << endl << &cstring << endl;

    _getch();
    return 0;
}

Upvotes: 1

Views: 146

Answers (2)

Davide Spataro
Davide Spataro

Reputation: 7482

If cstring is a pointer, then why can it get a value directly?

The operator << of cout for const char* is specialized to behave that way. It will treat the pointer as a NULL terminated string and will print it instead of the pointer value. For different types you get different behaviour. For char* you have the whole string printed.

Why wasn't the ‍‍‍*cstring's result equal to whole of string?

That is because the type of *cstring is char and again, operator << behaves correctly by just printing a single char. a const char* is essentially an array of char.An array is essentially a pointer to the first element of the array. If you use the * operator on a pointer you are accessing whatever the pointer points to. If it points to the first element, well, you get the first element.

Third, cstring is a non-constant pointer to a constant character, so why change its value and not change its address?

As you said, cstring is a non-constant pointer to constant data. You cannot change the place it points to (it is a constant pointer), but you can substitute the content of the pointed data with other stuff. You point to the same location but the content of that cell changes.

Upvotes: 2

Blaze
Blaze

Reputation: 16876

If cstring is a pointer, then why can it get a value directly?

Anyone can get the value pointed at by a pointer by dereferencing the pointer. That's what happens when you do std::cout << cstring. The proper overload gets chosen that prints the string represented by cstring assuming that is correctly formed, null-terminated C-style string.

Secondly, Why wasn't the ‍‍‍*cstring's result equal to whole of string?

cstring is a const char*, so *cstring is a const char. Pass that to std::cout and it will call an overload that prints one char. The function that is called internally doesn't even know that this is just one char in a string.

Third, cstring is a non-constant pointer to a constant character, so why change its value and not change its address?

You can't change the address of a variable. cstring is in a fixed place on the stack. You change the value of cstring, which is the address of the string that it's pointing to (it is now pointing to a different string, which has a different address, "string" of course stil has the same address).

What you probably wanted to try is this:

const char* cstring = "string";
std::cout << (void*)cstring << std::endl;
cstring = "foo";
std::cout << (void*)cstring << std::endl;

Now you can see the different addresses. One is the address of "string" and one is the address of "foo".

Upvotes: 1

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