Pointers / Destructor etc

Hi Guys,

Just wandered if there was anyway through Visual Studio or a third part app that will let me see what variables are currently still in memort when i am writing an apllication??

I'm trying to get my head around memory allocation and pointers - in particular char* and const char* pointers!!

basically what i'd like to see is;

char* var[] = "Hello";
var = "Hello again";

Is "Hello" still in memory and now var points to "Hello Again" or have i modified the original "Hello" string and only one portion of memorty allocated??

any help much appreciated.

Dale

In the snippet you have given, both "Hello" and "Hello again" are called string literals, they are both const strings and cannot be modified (attempting to modify them will yeild undefined behaviour).

Also, this line:

Code:

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char* var[] = "Hello";

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is not valid C++, I expect you meant:

Code:

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char* var = "Hello";

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That said, although the above is valid, it is better to write:

Code:

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const char* var = "Hello";

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since the const better describes the state of the data to which the pointer is being assigned.

By the way, string literals have static storage duration (they will exist for the lifetime of the application or loaded dll in which they are contained).

Quote:

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Originally Posted by PredicateNormative

Also, this line:

Code:

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char* var[] = "Hello";

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is not valid C++, I expect you meant:

Code:

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char* var = "Hello";

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Another alternative (with a different semantic meaning) would be

Code:

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char var[] = "Hello";

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This will create a local, modifiable array just big enough to hold the contents of the string literal, which will be copied in. Note that although this array is modifiable, you can never put any string into it longer than 5 letters.

Thanks for the replies.

It seems as though I've been getting muddled up with char arrays and pointers :)

The book I'm learning from starts off using char arrays as appose to strings to try and show how to use pointers etc.

Come to think of it i don't suppose any of you know of a good article about passing pointers and references to functions and class member functions / constructors??

The ones i have found seem very over-complicated examples.

Thanks

There's a book list on the forum somewhere, perhaps someone else will have more to suggest, my days of reaching introductory material were decades ago, so I have nothing to offer on your recent question.

I wanted to go back an an unanswered portion of your first post.

You want a debugger....

You'll be able to see the locations and nature of all the objects running.

I assume you're in Linux, so the command line oriented GDB is the "base line" option. An IDE would have a GUI front end to GDB, to make it easier to use.

It's hard to beat the Visual Studio debugger, even the free "Express" edition (obviously that's going to mean Windows)

In addition to what has been pointed out above, you don't do this to change the values of a character array:

Code:

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var = "Hello again";

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You will have to use strcpy or strncpy. According to the C++ standards [2.13.4/2]:

Code:

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A string literal that does not begin with u, U, or L is an ordinary string literal,
also referred to as a narrow string literal. An ordinary string literal has type
“array of n const char”, where n is the size of the string as defined below; it has
static storage duration (3.7) and is initialized with the given characters.

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So, the following would have been invalid in C++:

Code:

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char* var = "Hello";

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"Hello" is an array of 6 constant characters. var is the pointer to the first element of the array and since that element is const, the pointer cannot be declared as non-const char*. The pointer has to be of the type 'pointer to a const char'. But to have backward compatibility with C where the above works an implicit conversion happens for array to pointer conversion where a string literal would be converted to an r-value of type "pointer to a char". This is however deprecated as per Annexure D. Section [4.2/2] says:

Code:

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A string literal (2.13.4) with no prefix, with a u prefix, with a U prefix, or
with an L prefix can be converted to an rvalue of type “pointer to char”,
“pointer to char16_t”, “pointer to char32_t”, or “pointer to wchar_t”, respectively.
In any case, the result is a pointer to the first element of the array. This conversion
 is considered only when there is an explicit appropriate pointer target type,
and not when there is a general need to convert from an lvalue to an rvalue.
[ Note: this conversion is deprecated. See Annex D. —end note ]

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And annexure D.4/1 mentions: (also mentioned in note section above)

Code:

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The implicit conversion from const to non-const qualification for string
literals (4.2) is deprecated.

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On a side note that might be related, when you declare a pointer, it is just a pointer to "something". That "something" might be already allocated or already "there" and declaring a pointer to point to it does not cause any allocation of memory for that "something". Pointer just points to the already existing memory/object. It is when you try to "copy" that object, new memory would be allocated for the copy apart from the existing one.