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c++c++11nullc++-faqnullptr

What are the advantages of using nullptr?

This piece of code conceptually does the same thing for the three pointers (safe pointer initialization):

int* p1 = nullptr;
int* p2 = NULL;
int* p3 = 0;

And so, what are the advantages of assigning pointers nullptr over assigning them the values NULL or 0?

Solution

  • In that code, there doesn't seem to be an advantage. But consider the following overloaded functions:

    void f(char const *ptr);
void f(int v);

f(NULL);  //which function will be called?

    Which function will be called? Of course, the intention here is to call f(char const *), but in reality f(int) will be called! That is a big problem1, isn't it?

    So, the solution to such problems is to use nullptr:

    f(nullptr); //first function is called

    Of course, that is not the only advantage of nullptr. Here is another:

    template<typename T, T *ptr>
struct something{};                     //primary template

template<>
struct something<nullptr_t, nullptr>{};  //partial specialization for nullptr

    Since in template, the type of nullptr is deduced as nullptr_t, so you can write this:

    template<typename T>
void f(T *ptr);   //function to handle non-nullptr argument

void f(nullptr_t); //an overload to handle nullptr argument!!!

    1. In C++, NULL is defined as #define NULL 0, so it is basically int, that is why f(int) is called.