The Mysterious char*

I have always been confused about the working of char* pointer.
It seems to behave quite strangely.

Unlike the int or float pointer, the char* pointer seems to be able to store an address and also a string?
1.can some one explain this dual behavior of char*?

Code:

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Program

int main( )
{
char* p;

//Works
p="Hello";

//Prints correctly
cout<<p;

//Does not work
cin>>p;

return 0;

}

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2.why p="Hello" works? its a pointer shouldn't it give an error because "hello" is not an address, its a string?

3. And why cin>>p doesn't work?

Because "strings" are arrays, and as you know, an array reference decays into a pointer to the first element of that array.

The compiler takes each "string", and effectively turns it into

Code:

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static const char anonymous_string_1[] = { 'H', 'e', 'l', 'l', 'o', '\0' };

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So when you write
p="Hello";
the compiler has done something like
p = anonymous_string_1;


> 3. And why cin>>p doesn't work?
Because the "string" is a const, and you can't modify things which are constant.
On most systems, a "string" is also stored in read-only memory. Attempting to write to it causes the OS to just kill the program with a segmentation fault (or similar).

Quote:

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1.can some one explain this dual behavior of char*?

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O_o

There isn't any; none so different from other pointers anyway.

An array may always decay into a pointer (to the first element of the array). That's true for every kind of type.

The string literal syntax is just convenience to define a character array instead of doing it one character at a time.

The type of "Hello" is `const char[6]'; it naturally decays into `const char *' exactly as it should.

Code:

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p="Hello";

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This does not work; that's a deprecated conversion from a constant string literal to a mutable character pointer. If your compiler doesn't complain, consider supplementing your existing compiler with another one. You should make `p' a pointer t a constant value.

Code:

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cin>>p;

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This does work. It is an illustration of a bug because, as above, `p' points to memory that isn't supposed to be changed. The expression is fine and will behave correctly if `p' pointed to a writable address. Define a character array (`char lBuffer[16];') and point `p' at that location and enter a string less than 16 characters long.

Soma

hmm a little bit clear. Just asking, is the C++ string data type an alternative to char*? Any potential side effects of not using char*?

Quote:

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Originally Posted by phantomotap

There isn't any; none so different from other pointers anyway.
.....
The string literal syntax is just convenience to define a character array instead of doing it one character at a time.

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Doesn't the second sentence contradict the first ?
I mean, you can't give cout an address to a null terminated integer array and expect the array to be printed.

>_<

Ninja'd!

Quote:

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Just asking, is the C++ string data type an alternative to char*?

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If you use `std::string' correctly you'll find that it is not an "alternative" to `char *'; it is a facility that is better to represent the default character set and more limitedly wide characters string resources in every conceivable way.

Soma

Quote:

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Originally Posted by Waleed Mujeeb

hmm a little bit clear. Just asking, is the C++ string data type an alternative to char*? Any potential side effects of not using char*?

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I can't think of 'side-effects', but there sure are many benefits.

Quote:

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Doesn't the second sentence contradict the first ?

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How so?

The first part describes the behavior of all pointers with respect `char *'.

The second part describes the syntactical sugar the language provides to define a "C string" which is an array.

The array isn't a pointer; it decays into a pointer. The type `const char[6]' is not the same as the type `const char *'. However, the type `const int[6]' behaves with respect to `const int *' in the same ways.

Quote:

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I mean, you can't give cout an address to a null terminated integer array and expect the array to be printed.

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Yes, you can.

A "C string" is a convention that says "This is an array of unknown length terminated by a null character.". The "null" character is just a value. The operation you reference conceptually reads values one character at a time from the known location and prints them until the terminating character is found.

If you choose to define an "integer series" as a sequence of values terminated by `~static_cast<uint_t>(0)' a function for this sequence that follows the same conceptual logic can be made.

[Edit]
For context, by using Salem's post, here is bit more information.

Code:

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static const char anonymous_string_2[] = {'H', 'e', 'l', 'l', 'o'};

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The array `anonymous_string_2' is not a "C string"; the array `anonymous_string_2' is of type `const char[5]' and decays into the pointer `const char *' exactly as `anonymous_string_1'.
[/Edit]

Soma