Is C NULL equal to C++11 nullptr

问题

I like to use nullptr instead of NULL. Now I call a C function (from libjansson in this case).

NULL in C is implementation defined.

For nullptr I found that "A null pointer constant is an integral constant expression (5.19) rvalue of integer type that evaluates to zero".

So the safest thing to do:

auto string_obj=json_object_get(m_handle,name);
if(string_obj!=NULL)
    {
    auto string=json_string_value(string_obj);
    if(string!=NULL)
        {return string;}
    }
return nullptr;

Do I really need that or can I do it simpler:

auto string_obj=json_object_get(m_handle,name);
if(string_obj!=nullptr)
    {
    return json_string_value(string_obj); //Assume there is no difference between C NULL and C++11 nullptr
    }
return nullptr;

回答1:

In C++11 and beyond, a pointer that is ==NULL will also ==nullptr and vice versa.

Uses of NULL other than comparing with a pointer (like using it to represent the nul byte at the end of a string) won't work with nullptr.

In some cases, NULL is #define NULL 0, as the integer constant 0 is special-cased in C and C++ when you compare it with pointers. This non-type type information causes some problems in both C and C++, so in C++ they decided to create a special type and value that does the same thing in the "proper" use cases, and reliably fails to compile in most of the "improper" use cases.

Insofar as your C++ implementation is compatible with the C implementation you are interoping with (very rare for this not to be true), everything should work.

To be very clear, if ptr is any kind of pointer, then the following expressions are equivalent in C++:

ptr == nullptr
ptr == NULL
ptr == 0
!ptr

As are the following:

ptr = nullptr
ptr = NULL
ptr = 0

and if X is some type, so are the following statements:

X* ptr = nullptr;
X* ptr = NULL;
X* ptr = 0;

nullptr differs when you pass it to a template function that deduces type (NULL or 0 become an int unless passed to an argument expecting a pointer, while nullptr remains a nullptr_t), and when used in some contexts where nullptr won't compile (like char c = NULL;) (note, not char* c=NULL;)

Finally, literally:

NULL == nullptr

is true.

The NULL constant gets promoted to a pointer type, and as a pointer it is a null pointer, which then compares equal to nullptr.

来源：https://stackoverflow.com/questions/36484473/is-c-null-equal-to-c11-nullptr

标签

c++11

null

interop