string question

[Kristofer Pettijohn]

I'm a bit unsure about this and the results... but given the following:

....
std::string mystring;
mystring.reserv e(256);

strcpy(mystring .c_str(), "Some string C-style"); // C-style copy
....

Is something like this even legal? I've tested it, and it works,
but I just don't know if its "proper".

of course, I wouldn't use strcpy; I'm inquiring because I'd like to use
BSD read()'s without using char[]'s.

Thanks!

--
Kristofer Pettijohn
kristofer@cyber netik.net

[White Wolf]

Re: string question

Kristofer Pettijohn wrote:[color=blue]
> I'm a bit unsure about this and the results... but given the
> following:
>
> ...
> std::string mystring;
> mystring.reserv e(256);
>
> strcpy(mystring .c_str(), "Some string C-style"); // C-style copy
> ...
>
> Is something like this even legal?[/color]

No.
[color=blue]
> I've tested it, and it works,
> but I just don't know if its "proper".[/color]

It is not. In standard C++ std::string::c_ str() returns a const pointer
_and_ special care has been taken in the standard to ensure that library
implementations can give you a char buffer, which is a *copy* of the string
internals.
[color=blue]
> of course, I wouldn't use strcpy; I'm inquiring because I'd like to
> use BSD read()'s without using char[]'s.[/color]

Then read into an std:: vector<char>

--
WW aka Attila

[Andrey Tarasevich]

Re: string question

Kristofer Pettijohn wrote:[color=blue]
> I'm a bit unsure about this and the results... but given the following:
> ...
> std::string mystring;
> mystring.reserv e(256);
>
> strcpy(mystring .c_str(), "Some string C-style"); // C-style copy
> ...
>
> Is something like this even legal? I've tested it, and it works,
> but I just don't know if its "proper".
> ...[/color]

No, it is not legal. It won't work for several reasons.

Firstly, in general case the pointer returned by 'c_str()' doesn't
really give you access to the innards of a 'std::string' object. In
other words, there is no guarantee that returned pointer points to the
actual character sequence controlled by this 'std::string' object. It is
possible that 'c_str()' returns a pointer to a temporary buffer
allocated specifically for this purpose. The modifications may affect
this temporary buffer, but have no effect on the actual character
sequence controlled by this 'std::string' object.

Secondly, the character sequence stored in a 'std::string' object is not
guaranteed to reside in a continuous block of memory. It can be split
across several blocks.

Thirdly, null-character has no special meaning within character sequence
stored in a 'std::string' object. 'std::string's (unlike C-strings) are
not "terminated " by any special character, which means that in one way
or another 'std::string' objects have to keep the current length of the
stored sequence as a separate piece(s) of data. For this reason, all
operations that may modify the length of the sequence must go through
'std::string's public interface, thus giving 'std::string' objects the
ability keep the length information up-to-date. Any attempts to "hack
around" that interface using, for example, direct pointers to internal
string data, will almost inevitably destroy the integrity of
'std::string' object.

--
Best regards,
Andrey Tarasevich
Brainbench C and C++ Programming MVP

[Kevin Goodsell]

Re: string question

Kristofer Pettijohn wrote:
[color=blue]
> I'm a bit unsure about this and the results... but given the following:
>
> ...
> std::string mystring;
> mystring.reserv e(256);[/color]

There is no reserve() member for std::string. That's a vector-only function.
[color=blue]
>
> strcpy(mystring .c_str(), "Some string C-style"); // C-style copy[/color]

Clearly wrong, for reasons that others have explained.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Default User]

Re: string question

Kristofer Pettijohn wrote:
[color=blue]
> strcpy(mystring .c_str(), "Some string C-style"); // C-style copy[/color]


What compiler are you using that allows this?




Brian Rodenborn

[llewelly]

Re: string question

Default User <first.last@com pany.com> writes:
[color=blue]
> Kristofer Pettijohn wrote:
>[color=green]
>> strcpy(mystring .c_str(), "Some string C-style"); // C-style copy[/color]
>
>
> What compiler are you using that allows this?[/color]

IIRC MSVC6 and 7 will compile this. Howver, they don't 'allow' it
per se; it has weird behavior.

[Kevin Goodsell]

Re: string question

llewelly wrote:
[color=blue]
> Default User <first.last@com pany.com> writes:
>
>[color=green]
>>Kristofer Pettijohn wrote:
>>
>>[color=darkred]
>>>strcpy(mystr ing.c_str(), "Some string C-style"); // C-style copy[/color]
>>
>>
>>What compiler are you using that allows this?[/color]
>
>
> IIRC MSVC6 and 7 will compile this. Howver, they don't 'allow' it
> per se; it has weird behavior.[/color]

I tried the following code in MSVC 6:

#include <string>
#include <string.h>


int main()
{
std::string mystring;
mystring.reserv e(256);

strcpy(mystring .c_str(), "Some string C-style"); // C-style copy

return 0;
}

It gave the following error:

C:\Documents and Settings\Kevin\ My Documents\temp\ fun\fun.cpp(10) :
error C2664: 'strcpy' : cannot convert parameter 1 from 'const char *'
to 'char *'
Conversion loses qualifiers


-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.