C++ strings: [] vs. *

Question

Hi all..

Been thinking, what's the difference between declaring a variable with [] or * ? The way I see it:

char *str = new char[100];
char str2[] = "Hi world!";

.. should be the main difference, though Im unsure if you can do something like

char *str = "Hi all";

.. since the pointer should the reference to a static member, which I don't know if it can?

Anyways, what's really bugging me is knowing the difference between:

void upperCaseString(char *_str) {};
void upperCaseString(char _str[]) {};

So, would be much appreciated if anyone could tell me the difference? I have a hunch that both might be compiled down the same, except in some special cases?

Ty

Answer 1

The first option dynamically allocates 100 bytes.

The second option statically allocates 10 bytes (9 for the string + nul character).

Your third example shouldn't work - you're trying to statically-fill a dynamic item.

As to the upperCaseString() question, once the C-string has been allocated and defined, you can iterate through it either by array indexing or by pointer notation, because an array is really just a convenient way to wrap pointer arithmetic in C.

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(That's the simple answer - I expect someone else will have the authoritative, complicated answer out of the spec :))

Answer 2

The three different declarations let the pointer point to different memory segments:

char* str = new char[100];

lets str point to the heap.

char str2[] = "Hi world!";

puts the string on the stack.

char* str3 = "Hi world!";

points to the data segment.

The two declarations

void upperCaseString(char *_str) {};
void upperCaseString(char _str[]) {};

are equal, the compiler complains about the function already having a body when you try to declare them in the same scope.

Answer 3

As a supplement to the answers already given, you should read through the C FAQ regarding arrays vs. pointers. Yes it's a C FAQ and not a C++ FAQ, but there's little substantial difference between the two languages in this area.

Also, as a side note, avoid naming your variables with a leading underscore. That's reserved for symbols defined by the compiler and standard library.

Answer 4

Okay, I had left two negative comments. That's not really useful; I've removed them.

• The following code initializes a char pointer, pointing to the start of a dynamically allocated memory portion (in the heap.)

char *str = new char[100];

This block can be freed using delete [].

• The following code creates a char array in the stack, initialized to the value specified by a string literal.

char [] str2 = "Hi world!";

This array can be modified without problems, which is nice. So

str2[0] = 'N';
cout << str2;

should print Ni world! to the standard output, making certain knights feel very uncomfortable.

• The following code creates a char pointer in the stack, pointing to a string literal... The pointer can be reassigned without problems, but the pointed block cannot be modified (this is undefined behavior; it segfaults under Linux, for example.)

char *str = "Hi all";
str[0] = 'N'; // ERROR!

• The following two declarations

void upperCaseString(char *_str) {};
void upperCaseString(char [] _str) {};

look the same to me, and in your case (you want to uppercase a string in place) it really doesn't matters.

However, all this begs the question: why are you using char * to express strings in C++?

Answer 5

Please also take a look at the http://c-faq.com/aryptr/aryptr2.html The C-FAQ might prove to be an interesting read in itself.