I just can't find out how this simple example can be compiled in C++ :
class C
{
public:
static const void* noop = static_cast<const void*> (0x1);
};
Because of the static storage constraint I want, the only cast possible here would be static_cast but it is incompatible with this int-to-ptr cast.
error: invalid static_cast from type ‘int’ to type ‘const void*’
How can an integer value be statically casted to a pointer type?
The problem here is that although you are declaring a const void* the const qualifier doesn't apply to the pointer but to the address that this pointer points to.
This means that noop is not a static const member variable and as all non const static member variables needs to be defined and initialized in a single translation unit outside class's definition like the example below:
class C {
public:
static const void *noop;
};
const void* C::noop = (const void*) 0x1;
The following solution:
class C {
public:
static constexpr const void* noop = reinterpret_cast<const void*>(0x1);
};
Although, it compiles and works fine in GCC isn't valid C++ (e.g., it doesn't compile with either clang or VC++2013) because according to the standard § 5.19/2 Constant Expressions [expr.const] the result of a reinterpret_cast can't be a constant expression.
constexpr attempts.reinterpret_cast is what you are looking for (raw interpretation of the bytes of an int to a const void* pointer)
Also, you need constexpr for in-class initialization of static data member :
class C
{
public:
static constexpr const void* noop = reinterpret_cast<const void*>(0x1);
};
static_cast doesn't work since there is no combination of user-defined or implicit conversions that would transform an int to a const void*.
EDIT:
This solution is not standard C++ (reinterpret_cast is not allowed in constant expressions), 40two solution's is correct.
constexpr ! C++11 gives me headache... Thank you !
const void but the pointer is non-const.reinterpret_cast is never a constant expression.40two has fixed the compile error, but also made the member non-const, which is undesirable because it hinders optimization and opens to door to accidental mutation.
Best:
class C {
public:
static const void* const noop;
};
const void* const C::noop = reinterpret_cast<const void*>(0x1);
An alternate solution which also comes from the question 40two references: constexpr and initialization of a static const void pointer with reinterpret cast, which compiler is right?, is to use intptr_t which is:
integer type capable of holding a pointer
This also allows you to make the member variable a constexpr:
class C
{
public:
static constexpr intptr_t noop = 0x1 ;
};
then you can use reinterpret_cast at the point where it is used:
const void * vPtr = reinterpret_cast<void*>( C::noop ) ;
static_cast, I bet. Can you share it?error: non-const static data member must be initialized out of line(clang++)reinterpret_cast?intptr_tleads to a better solution.