I'm confused!

Hey guys!
I hope you can help me sort one thing out. Why i cannot do this?

Code:

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char* ptr = new char[40];
ptr = "First string";
strcpy(ptr, "New string");

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However, if i would write this

Code:

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char* ptr = new char[40];
strcpy(ptr, "First string");
strcpy(ptr, "New string");

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then everything would work out and contents of ptr would change to the new string. In other words, how does function strcpy change from normal assignment. Thank you.

In the first case, you are assigning the address of "First string" into ptr and then you are trying to overwrite it with strcpy() which is a no-no. In the other case, strcpy() uses the pointer as an array and does not change the address contained in the pointer. You could prevent this by having a const pointer to a char like this:

Code:

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char const* ptr = new char[40];
ptr = "First string"; // compile-error : cannot change memory address, declared char const*
strcpy(ptr, "New string");

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Besides, using new[] with a constant size isn't much useful at all. Just to make sure you know.

Quote:

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Code:

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char const* ptr = new char[40];
ptr = "First string"; // compile-error : cannot change memory address, declared char const*

---

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Actually, no. char const* is the same as const char* so you will get a compile error on the strcpy line. I think you meant:

Code:

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char *const ptr = new char[40];
ptr = "First string";//error
strcpy(ptr,"New string"); //ok

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So, to make sure i got it - using strcpy is like overwriting existing address with new value. And using operator = is just changing the address to a new value.

Character arrays tend to be rather confusing unless you do simple stuff with them or have a good understanding of pointers, arrays and constants. If you just want to work with text, you may be better off using C++ style "string" objects, which are safe and easy to handle (even if you do have a good understanding of character arrays): http://www.cprogramming.com/tutorial/string.html

This is a good read on the subject too: http://pw1.netcom.com/~tjensen/ptr/pointers.htm

It says C but applies to C++ if you're using char arrays, in case you were wondering.