>C++ Automatically cast void pointer into Type pointer in C++ in #define in case of type is not given (C
Hi!
I've used the following C macro, But in C++ it can't automatically cast void* to type* .
#define MALLOC_SAFE(var, size) {
var = malloc(size);
if (!var) goto error;
}
I know, I can do something like this:
#define MALLOC_SAFE_CPP(var, type, size) {
var = (type)malloc(size);
if (!var) goto error;
}
But I don't want to rewrite a big portion of code, where MALLOC_SAFE was used.
Is there any way to do this without giving the type to the macro? Maybe some MSVC 2005 #pragma / __declspec /other ?
ps: I can't use C compiler, because my code is part (one of hundreds modules) of the large project. And now it's on C++. I know, I can build my code separately. But it's old code and I just want to port it fast.
The question is about void* casting ;) If it's not possible, I'll just replace MACRO_SAFE with MACRO_SAFE_CPP
Thank You!
I do not recommend doing this; this is terrible code and if you are using C you should compile it with a C compiler (or, in Visual C++, as a C file)
If you are using Visual C++, you can use decltype :
#define MALLOC_SAFE(var, size)
{
var = static_cast<decltype(var)>(malloc(size));
if (!var) goto error;
}
To makes James' answer even dirtier, if you don't have decltype support you can also do this:
template <typename T>
class auto_cast_wrapper
{
public:
template <typename R>
friend auto_cast_wrapper<R> auto_cast(const R& x);
template <typename U>
operator U()
{
return static_cast<U>(mX);
}
private:
auto_cast_wrapper(const T& x) :
mX(x)
{}
auto_cast_wrapper(const auto_cast_wrapper& other) :
mX(other.mX)
{}
// non-assignable
auto_cast_wrapper& operator=(const auto_cast_wrapper&);
const T& mX;
};
template <typename R>
auto_cast_wrapper<R> auto_cast(const R& x)
{
return auto_cast_wrapper<R>(x);
}
Then:
#define MALLOC_SAFE(var, size)
{
var = auto_cast(malloc(size));
if (!var) goto error;
}
I expanded on this utility (in C++11) on my blog. Don't use it for anything but evil.
例如，像这样：
template <class T>
void malloc_safe_impl(T** p, size_t size)
{
*p = static_cast<T*>(malloc(size));
}
#define MALLOC_SAFE(var, size) {
malloc_safe_impl(&var, size);
if (!var) goto error;
}
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