RE: Error with basic string

RE: Error with basic string

[Rupert Wood]

Ajay Bansal wrote:

> char *func(string a, int n)
> {
> 	a.resize(n);
> 	return a.begin();
> }

a.begin() is an iterator. You'll have to dereference it to get the
character and then re-reference that for the pointer, i.e. (bracketed
for clarity)

    char *func(string a, int n)
    {
        a.resize(n);
        return &(*(a.begin()));
    }

*However*

    1) you're not passing your string by reference; what you're
       telling it to do is to take a copy of string a, resize the
       copy to n characters and then return a pointer to the first
       character in the copy - i.e. a pointer to memory owned by a
       dead object

    2) (I'm not an STL guru so take this with a pinch of salt)
       I don't think there's a requirement that the string be
       stored continuously in memory, except for a read-only copy
       between c_str() the next non-const operation. It looks like
       you want func to allocate you an n-byte continuous character
       buffer in a basic_string and, whilst some implementations will
       give you this, I don't think you can assume all will. (But you
       probably know this - the comment about changing the return
       type?) I can't tell you if G++'s STL plays along.

Rup.

Re: Error with basic string

[Sebastian Huber]

Hello!

On Thursday 23 January 2003 19:31, Rupert Wood wrote:
> Ajay Bansal wrote:
> > char *func(string a, int n)
> > {
> > 	a.resize(n);
> > 	return a.begin();
> > }
>
> a.begin() is an iterator. You'll have to dereference it to get the
> character and then re-reference that for the pointer, i.e. (bracketed
> for clarity)
>
>     char *func(string a, int n)
>     {
>         a.resize(n);
>         return &(*(a.begin()));
>     }

You should use 'return const_cast<char*>( a.data())' or 'a.c_str()'. But you 
have to keep in mind, that every non-constant method of std::string may make 
the data invalid.

> *However*
>
>     1) you're not passing your string by reference; what you're
>        telling it to do is to take a copy of string a, resize the
>        copy to n characters and then return a pointer to the first
>        character in the copy - i.e. a pointer to memory owned by a
>        dead object
>
>     2) (I'm not an STL guru so take this with a pinch of salt)
>        I don't think there's a requirement that the string be
>        stored continuously in memory, except for a read-only copy
>        between c_str() the next non-const operation. It looks like
>        you want func to allocate you an n-byte continuous character
>        buffer in a basic_string and, whilst some implementations will
>        give you this, I don't think you can assume all will. (But you
>        probably know this - the comment about changing the return
>        type?) I can't tell you if G++'s STL plays along.
>
> Rup.

RE: Error with basic string

[Ajay Bansal]

This is actually not dead object. Func is a member of a class & "string
a" is also a member of the same class.. :)

This was just a sample program.. 

But anyway. Thanks a lot.. :).. It has worked.. 

-----Original Message-----
From: Rupert Wood [mailto:me@rupey.net] 
Sent: Friday, January 24, 2003 12:02 AM
To: Ajay Bansal
Cc: gcc-help@gcc.gnu.org
Subject: RE: Error with basic string 


Ajay Bansal wrote:

> char *func(string a, int n)
> {
> 	a.resize(n);
> 	return a.begin();
> }

a.begin() is an iterator. You'll have to dereference it to get the
character and then re-reference that for the pointer, i.e. (bracketed
for clarity)

    char *func(string a, int n)
    {
        a.resize(n);
        return &(*(a.begin()));
    }

*However*

    1) you're not passing your string by reference; what you're
       telling it to do is to take a copy of string a, resize the
       copy to n characters and then return a pointer to the first
       character in the copy - i.e. a pointer to memory owned by a
       dead object

    2) (I'm not an STL guru so take this with a pinch of salt)
       I don't think there's a requirement that the string be
       stored continuously in memory, except for a read-only copy
       between c_str() the next non-const operation. It looks like
       you want func to allocate you an n-byte continuous character
       buffer in a basic_string and, whilst some implementations will
       give you this, I don't think you can assume all will. (But you
       probably know this - the comment about changing the return
       type?) I can't tell you if G++'s STL plays along.

Rup.

RE: Error with basic string

[Ajay Bansal]

Ooops.. Copy-pasted wrong func defination

Correct function is..........

char *func(string a, int n)
{
	a.resize(n);
	return a.begin();
}



And error is
-----------------------

string_test.cpp: In function `char* func(std::basic_string<char, 
   std::char_traits<char>, std::allocator<char> >, int)':
string_test.cpp:7: cannot convert `__gnu_cxx::__normal_iterator<char*, 
   std::basic_string<char, std::char_traits<char>, std::allocator<char>
> >' to 
   `char*' in return



-----Original Message-----
From: Ajay Bansal 
Sent: Thursday, January 23, 2003 11:45 PM
To: gcc-help@gcc.gnu.org
Subject: Error with basic string 


Hi All

I am trying to compile the following simple function on RH73 with gcc
3.2.1

*********string_test.cpp**************
#include <iostream>
#include <string>
using namespace std;
char * func(string a, int n)
{
	a.resize(n);
	return (char *)a.begin();
}

int main() 
{
	string a;
	char *b;
	b=func(a,5);

	return 0;
}

***************

I get the folowing error

[ajay@linux1 stl]$ g++ string_test.cpp 
string_test.cpp: In function `char* func(std::basic_string<char, 
   std::char_traits<char>, std::allocator<char> >, int)':
string_test.cpp:7: cannot convert `std::basic_string<_CharT, _Traits, 
   _Alloc>::begin() [with _CharT = char, _Traits =
std::char_traits<char>, 
   _Alloc = std::allocator<char>]()' from type `
   __gnu_cxx::__normal_iterator<char*, std::basic_string<char, 
   std::char_traits<char>, std::allocator<char> > >' to type `char*'

How to resolve this error. This program works fine on my solaris and HP
machine with their compilers (workshop on sun and aCC on HP)

I can not change the return type of func.

Regards
Ajay

Error with basic string

[Ajay Bansal]

Hi All

I am trying to compile the following simple function on RH73 with gcc
3.2.1

*********string_test.cpp**************
#include <iostream>
#include <string>
using namespace std;
char * func(string a, int n)
{
	a.resize(n);
	return (char *)a.begin();
}

int main() 
{
	string a;
	char *b;
	b=func(a,5);

	return 0;
}

***************

I get the folowing error

[ajay@linux1 stl]$ g++ string_test.cpp 
string_test.cpp: In function `char* func(std::basic_string<char, 
   std::char_traits<char>, std::allocator<char> >, int)':
string_test.cpp:7: cannot convert `std::basic_string<_CharT, _Traits, 
   _Alloc>::begin() [with _CharT = char, _Traits =
std::char_traits<char>, 
   _Alloc = std::allocator<char>]()' from type `
   __gnu_cxx::__normal_iterator<char*, std::basic_string<char, 
   std::char_traits<char>, std::allocator<char> > >' to type `char*'

How to resolve this error. This program works fine on my solaris and HP
machine with their compilers (workshop on sun and aCC on HP)

I can not change the return type of func.

Regards
Ajay