Tom Smith wrote:
Quote:
I hardly dare ask this given the furore in another thread over strings and
const... My problem is this. I am assured that casting away the constness of
the return value of std::string::c_str() is an Error according to the Standard.
No. It is perfectly legitimate to cast away the const using const_cast.
What is not legitimate (read: undefined behaviour) is to then try and
modify the now apparently non-const data.
Quote:
But I need to pass it to a function (in an old and unpleasant C library, ugh)
which takes an ordinary char*. What should I do? Is it really necessary to make
a fresh copy of the string?
Depends on what the library function does. If it modifies the data then
you do need to make a copy. The std::string class encapsulates its data
and makes available facilities to modify its data through its public
interface. std::string is not designed to be able to handle anything
outside directly modifying the data outside the scope of its public
interface.
However, many functions that take a C-style string passed as a char* do
not and are never intended to modify the data. Such functions (which
may be C or C++) are written with char* rather than const char* in the
signature for a number of reasons, e.g.
1 The function is a legacy function, written before const existed or
was widely supported
2 The programmer was careless in making the code const-correct
3 The programmer did not know of the existence of const
In a case where the parameter should logically be const, but happens
(e.g. for one of the reason above) not to be declared const, that is
exactly the situation const_cast is designed for. It allows you to keep
your code const-correct, by using const declarations and classes that
manage their own data, while still being able to interface with
const-incorrect legacy APIs.
Remember that a cast does not mean "I can't think of another way to
make this work. I hope it doesn't go wrong". A cast means "Dear
compiler, you won't like this code without a cast, but I know exactly
what I'm doing and why. I am casting off all the safety precautions you
usually give me and assuming full responsibility for the correct
behaviour of this code because in this case I know best".
There is no other way to tell the compiler that the API is incorrect
and should have been written to take a const char*, so if you really do
know best then tell the compiler that with a const_cast.
Gavin Deane
