std::nullptr_t - cppreference.com

Original:

std::nullptr_t is the type of the null pointer literal, nullptr.

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Se dois ou mais sobrecargas aceitar tipos diferentes de ponteiro, uma sobrecarga para std::nullptr_t é necessário aceitar um argumento de ponteiro nulo .

#include <cstddef>
#include <iostream>

void f(int* pi)
{
   std::cout << "Pointer to integer overload\n";
}

void f(double* pd)
{
   std::cout << "Pointer to double overload\n";
}

void f(std::nullptr_t nullp)
{
   std::cout << "null pointer overload\n";
}

int main()
{
    int* pi; double* pd;

    f(pi);
    f(pd);
    f(nullptr);  // would be ambiguous without void f(nullptr_t)
    // f(NULL);  // ambiguous overload: all three functions are candidates
}

Pointer to integer overload
Pointer to double overload
null pointer overload