implementation safe nullptr

问题

I'd like to keep my code compilable both on legacy C++ (C++ code using "NULL") and new C++11 standard (C++ code using "nullptr")

I'm using GCC, but planning to recompile the whole codebase also for VS when I'll finish most important things.

Should I expect both GCC and VS will do something like

#define NULL nullptr

Or Is better I'll do that myself (using of course a different name, where MY_LIB will be replaced by my library suffix)?

#ifndef nullptr
    #define MY_LIB_NULL NULL
#else
    #define MY_LIB_NULL nullptr
#endif

What I want to achieve is code that compiles regardless of wich C++11 features have been implemented or not (and since i'm not using templates, there are very few of them).

For example the keywords "override" and "final" are already done.

MY_LIB_OVERRIDE //macro, defines to "override" if c++11 is present.
MY_LIB_FINAL    //macro, defines to "final" if c++11 is present.

I'm asking the question because I know the "nullptr" question is a bit strange, so maybe just doing the same I already did for override and final, is wrong. Needs opinions about that. Any help is wellcome.

回答1:

You could probably create a "false" my_nullptr of type my_nullptr_t the following way:

const class my_nullptr_t
{
    public:

        /* Return 0 for any class pointer */
        template<typename T>
        operator T*() const
        {
            return 0;
        }

        /* Return 0 for any member pointer */
        template<typename T, typename U>
        operator T U::*() const
        {
            return 0;
        }

        /* Safe boolean conversion */
        operator void*() const
        {
            return 0;
        }

    private:

        /* Not allowed to get the address */
        void operator&() const;

} my_nullptr = {};

This works with C++03 and C++11 and should always be safe, whichever C++11 features are implemented. That solution was actually already discussed in this topic that proposed a version of nullptr_t based on the Official proposal.

回答2:

NULL is a macro that expands to a null pointer constant. It still works just like it used to. Code that has to work with non-C++11 compilers should use NULL.

回答3:

I think following will works:

#include <cstddef>

#ifndef MY_LIB_NULL
    #ifndef NULL //check for NULL
        #define MY_LIB_NULL nullptr
    #else
        #define MY_LIB_NULL NULL ///use NULL if present
    #endif
#endif

basically I check for "NULL". wich is a macro and can be checked, until the compiler is shipped with that macro (likely to be), than it's valid using the macro, when compiler will only provides "nullptr" and no longer have NULL then nullptr is used (maybe in a far future, but seems we can happily continue to use NULL!)

I think that's safer than redefining "nullptr" (like most people trying to do)

来源：https://stackoverflow.com/questions/13675203/implementation-safe-nullptr

标签

c++

c++11

overriding

final

nullptr