Wrapper around C lib, problem with c_str and const

[Fred Paris]

Hi

I'm writing a class to act as a wrapper around a C library.

This C library exposes functions like:

SetSomeInfo( char *pTheInfo );

In my wrapper class, the info in question is in a STL string.

std::string m_TheInfo;

So inside the wrapper class I would like to call:

SetSomeInfo( m_TheInfo.c_str () );

But, problem:

c_str returns a const char *, and SetSomeInfo accepts a char * (not
const), so it doesn't compile. Maybe SetSomeInfo is poorly written,
because it should have const in its signature, but it's not mine and
I have no control over it.

So my (rather ugly) workaround right now is:

char *p = (char *)(DWORD) m_TheInfo.c_str () ; //get rid of const

but surely there has to be a more standard way ?

Thanks

[Shezan Baig]

Fred Paris wrote:

[snip]

char *p = (char *)(DWORD) m_TheInfo.c_str () ; //get rid of const

but surely there has to be a more standard way ?

Sure:

char *p = const_cast<char *>(m_TheInfo.c_ str());

But make sure that the function *really* doesn't modify 'p', otherwise
you might run into undefined behaviour.

If you want to be absolutely safe and more elegant (avoiding the
const_cast and possible undefined behaviour), you can do something
like:

enum { BUF_SIZE = 1024 };

char p[BUF_SIZE];
strncpy(p, m_TheInfo.c_str (), BUF_SIZE);
p[BUF_SIZE-1] = 0;

Hope this helps,
-shez-

[benben]

"Fred Paris" <no**@nono.inva lid> wrote in message
news:pl******** *************** *********@4ax.c om...

Hi

I'm writing a class to act as a wrapper around a C library.

This C library exposes functions like:

SetSomeInfo( char *pTheInfo );

In my wrapper class, the info in question is in a STL string.

std::string m_TheInfo;

So inside the wrapper class I would like to call:

SetSomeInfo( m_TheInfo.c_str () );
The following should get you through compilation...b ut really ask yourself
if that is really what you want (notice the low level operations and
explicit exception handling which are rather ugly):

char* buff = new char[m_TheInfo.lengt h() + 1];
std::copy(buff, m_TheInfo.begin (), m_TheInfo.end() );
buff[m_TheInfo.lengt h()] = '\0';
try
{
SetSomeInfo(buf f);
}
catch (...)
{
delete[] buff;
throw;
}
delete[] buff;


But, problem:

c_str returns a const char *, and SetSomeInfo accepts a char * (not
const), so it doesn't compile. Maybe SetSomeInfo is poorly written,
because it should have const in its signature, but it's not mine and
I have no control over it.

So my (rather ugly) workaround right now is:

char *p = (char *)(DWORD) m_TheInfo.c_str () ; //get rid of const
Don't do this. You may corrupt the string object and your program may not
recover from such brutal operation. NOT COOL!! std::string::c_ str() returns
a const char* for a reason, most likely to set up protection against misuse.

but surely there has to be a more standard way ?

Thanks

[John Carson]

"Fred Paris" <no**@nono.inva lid> wrote in message
news:pl******** *************** *********@4ax.c om

Hi

I'm writing a class to act as a wrapper around a C library.

This C library exposes functions like:

SetSomeInfo( char *pTheInfo );

In my wrapper class, the info in question is in a STL string.

std::string m_TheInfo;

So inside the wrapper class I would like to call:

SetSomeInfo( m_TheInfo.c_str () );

But, problem:

c_str returns a const char *, and SetSomeInfo accepts a char * (not
const), so it doesn't compile. Maybe SetSomeInfo is poorly written,
because it should have const in its signature, but it's not mine and
I have no control over it.

So my (rather ugly) workaround right now is:

char *p = (char *)(DWORD) m_TheInfo.c_str () ; //get rid of const

but surely there has to be a more standard way ?

Thanks

SetSomeInfo(con st_cast<char*>( m_TheInfo.c_str () ));
--
John Carson

[Shezan Baig]

benben wrote:

The following should get you through compilation...b ut really ask yourself
if that is really what you want (notice the low level operations and
explicit exception handling which are rather ugly):

Or, you could just use a vector<char>:

vector<char> buff(m_TheInfo. length() + 1);
....

This gets rid of all the exception handling ugliness. Note however
that vector<char> will likely allocate, and if you know that the string
is never going to be more than a certain length, its probably better if
you just use a local buffer. You might want to throw an exception if
it exceeds this "maximum" length.

Many people often underestimate the cost of allocation/deallocation,
especially in multi-threaded environments where memory contention
becomes a much bigger issue than in single-threaded environments.

Hope this helps,
-shez-

[benben]

"Shezan Baig" <sh************ @gmail.com> wrote in message
news:11******** *************@z 14g2000cwz.goog legroups.com...

benben wrote:

The following should get you through compilation...b ut really ask
yourself
if that is really what you want (notice the low level operations and
explicit exception handling which are rather ugly):
Or, you could just use a vector<char>:

vector<char> buff(m_TheInfo. length() + 1);

Eureka! Sure, I just forgot that a vector guarantees continuous memory block
haha.
...

This gets rid of all the exception handling ugliness. Note however
that vector<char> will likely allocate, and if you know that the string
is never going to be more than a certain length, its probably better if
you just use a local buffer. You might want to throw an exception if
it exceeds this "maximum" length.
That is to say we need a better allocator.

Many people often underestimate the cost of allocation/deallocation,
especially in multi-threaded environments where memory contention
becomes a much bigger issue than in single-threaded environments.

Hope this helps,
-shez-