C / C++ / MFC

26-Feb-08 15:23

Hello everyone,

The space in string should use heap address memory space, not stack, right? But through debugging, for example,

string str = "hello";

why I can not see the invocation of new operator? Anyone could point out where STL string class allocates space on heap and using which function to allocate please (any other approach other than using new to allocate space on heap?)?

thanks in advance,
George

User 58385226-Feb-08 15:49

26-Feb-08 15:49

Most implementations of std::string have a smallish stack allocated buffer which is used for strings small enough to fit into it. Try assigning a much longer string and see what happens.

<blockquote class="FQ"><div class="FQA">George_George wrote:</div>Anyone could point out where STL string class allocates space on heap</blockquote>

Its done through the allocator. By default for a std::string this will be

allocator<char>

In my version of Visual Studio the actual call to new is around line 44 in the header file "xmemory"

George_George26-Feb-08 16:01

26-Feb-08 16:01

Hi Josh,

I doubt your below comments. If the space is allocated on stack,

1. How do you deal with global object instance? There is no global stack or something, right?
2. How do you pass one string object instance from one function to another -- safely to ensure address space is valid?

Josh Gray wrote:
Most implementations of std::string have a smallish stack

regards,
George

User 58385226-Feb-08 16:11

26-Feb-08 16:11

Hello George,

Im not sure I understand your questions

<blockquote class="FQ"><div class="FQA">George_George wrote:</div>1. How do you deal with global object instance? There is no global stack or something, right?</blockquote>

There is only one stack, you can consider it a global stack I guess

<blockquote class="FQ"><div class="FQA">George_George wrote:</div>2. How do you pass one string object instance from one function to another -- safely to ensure address space is valid?</blockquote>

Its either passed by value in which case the assignment operator of the destination string is invoked, allocating memory if nessasary or its passed by reference in which case the underlying char array is not copied

Think of it like this

class string
{
public:
void assign(char* str)
{
if(strlen(str) < 15)
{
//small string will fit in array on stack
strcpy(smallArray, str, strlen(str));
}
else
{
//to big for stack array so allocate space on heap
strPointer = new char[strlen(str)+1];
strcpy(strPointer, str, strlen(str));
}
}

private:
char smallArray[16];
char* strPointer;
};

George_George26-Feb-08 16:26

26-Feb-08 16:26

Hi Josh,

Thanks for so much information. Suppose in your code for class string, there are two functions, foo and goo, foo invokes goo. Then,

1. In goo, it allocates a string, small. In your point, it should be on the stack on function goo, right?

2. Goo returns the string by reference to foo;

3. Then foo will refer to some space in goo function stack, which is unwound when goo returns.

Is it safe?

BTW:

Josh Gray wrote:
There is only one stack, you can consider it a global stack I guess

I mean the function call stack. You mean there is another global stack? Intereted in that, do you have any recommended links for the memory space?

regards,
George

User 58385226-Feb-08 16:39

26-Feb-08 16:39

There is only one stack. It isued for storing objects as well as holding various memory addresses required for implementing function calls. The stack is a single FILO (first in, last out) data structure.

If you do this

void foo()
{
string& str = goo();
}

string& goo()
{
string myString("George is cool");
return myString;
}

it will fail (some compilers will give a warning)

The reason is that when goo() returns the stack pointer goes back to where it was before the call to goo and anything allocated on the stack by goo will be gone. This will also be true even if you assign a long string and the string class puts it on the heap, the reason is that myString is on the stack and its pointer to the heap will be lost.

George_George26-Feb-08 16:46

26-Feb-08 16:46

Thanks Josh,

I find the warning of your code in Visual Studio 2008,

warning C4172: returning address of local variable or temporary

But what makes me confused is, as you mentioned if the content is larger than 16 bytes, heap should be used to allocate, right?

But why if I increase the length of thecontent, the same warning exists? My code,

#include <string>

using namespace std;

string& goo()
{
	string myString("George is cool 111111111111111111111 11111111111111111111111111111111111");
	return myString;
}

void foo()
{
	string& str = goo();
}

regards,
George

User 58385226-Feb-08 16:56

26-Feb-08 16:56

You get the warning because myString is on the stack. If it is internally using the heao to store something it doesnt matter. When the method returns myString is lost and with it the pointer to the memory on the heap

You could try

void foo()
{
string myString;

goo(myString);
}

void goo(string& str)
{
str = "George is cool";
}

George_George26-Feb-08 17:31

26-Feb-08 17:31

Thanks Josh,

Got your idea. My bad before.

regards,
George

David Crow26-Feb-08 17:16

David Crow

26-Feb-08 17:16

George_George wrote:
1. In goo, it allocates a string, small. In your point, it should be on the stack on function goo, right?

The allocated memory is either going to be from the heap or from the stack.

George_George wrote:
Is it safe?

char *goo1( void )
{
    char *s = new char[5];
    return s;
}
 
char *goo2( void )
{
    char s[5];
    return s;
}
 
string s1 = goo1(); // this is fine since goo1() allocated from heap
string s2 = goo2(); // the local variable in goo2() will be 
                    // invalid once the function goes out of scope

George_George26-Feb-08 17:32

silently installing .reg file

26-Feb-08 17:32

Thanks DavidCrow,

My question is answered.

regards,
George

act_x26-Feb-08 10:58

act_x

26-Feb-08 10:58

From my application , I need to load the contents of a reg file programmatically .

I have tried to use Shellexecute supplying the filepath to the reg file however it still prompts me by asking if i need to insert the file into the registry.
I would want to do this silently as the regedit /s option does it silently.

thanks

Engineering is the effort !

Re: silently installing .reg file

Dave Calkins26-Feb-08 16:16

Dave Calkins

26-Feb-08 16:16

Why not just shellexecute the command you mentioned, "regedit /s file.reg"?

Re: silently installing .reg file

ThatsAlok27-Feb-08 16:10

ThatsAlok

27-Feb-08 16:10

thats the nice tip i must say!

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow
"If it were machines, the pair_programming seem to work, but for humans it is pair_crackdown that seems to work! " - Nisamudheen
Never mind - my own stupidity is the source of every "problem" - Mixture

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
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Is this book on the standard library worth it?

markt26-Feb-08 5:53

markt

26-Feb-08 5:53

I ran across this book and it looks like it could be really good. does anyone have it and is it worth it.

The C++ Standard Library: A Tutorial and Reference (Hardcover)
by Nicolai M. Josuttis

http://www.amazon.com/C-Standard-Library-Tutorial-Reference/dp/0201379260/ref=pd_sim_b_title_2[^]

Dave Calkins26-Feb-08 6:17

Dave Calkins

26-Feb-08 6:17

That's the book I learned STL on. I thought it was a good read. Over the years, while using it, I think it seemed to be a little lacking as a reference for some reason. But overall a good book.

One of the quirks of STL is the way strings evolved over the years, so you end up having stuff talking about char * strings and the std::string class and the other classes have evolved to use std::string. Anyway, it can be a little confusing. I seem to recall this book covering both.

Nemanja Trifunovic26-Feb-08 7:21

Nemanja Trifunovic

26-Feb-08 7:21

That's "the" book on STL, if you ask me.

Programming Blog

utf8-cpp

markt26-Feb-08 11:29

markt

26-Feb-08 11:29

the reviews for it were all good too but i just wanted to make sure. thanks guys. I feel a little stupid now i didnt realize it was so popular. i wouldnt have asked if i knew it was like asking if scott meyers was worth buying.

User 58385226-Feb-08 15:50