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What is the null pointer in C++?

Danil

Grokking Modern System Design Interview for Engineers & Managers

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nullptr is a keyword that denotes the pointer literal with the unique std::nullptr_t type.

It’s convertible to a null pointer value of any pointer type or any pointer to member type:

#include <iostream>
using namespace std;
struct MyStruct { };
void pointer_to_fundamental_type(int* ptr) {
std::cout << "Hello from void pointer_to_fundamental_type(int*)\n";
}
void pointer_to_user_defined_type(MyStruct* ptr) {
std::cout << "Hello from void pointer_to_user_defined_type(MyStruct*)\n";
}
// Accepts pointer to member of MyStruct that has
// int type
void pointer_to_member_object(int MyStruct::*) {
std::cout << "Hello from void pointer_to_member_object(int MyStruct::*)\n";
}
// Accepts pointer to member function of MyStruct
// that has int return type and no arguments
void pointer_to_member_function(int (MyStruct::*)(void)) {
std::cout << "Hello from pointer_to_member_function(int (MyStruct::*)(void))\n";
}
int main() {
std::nullptr_t null = nullptr;
// nullptr is convertible to null pointer of
// any pointer type:
pointer_to_fundamental_type(nullptr);
pointer_to_user_defined_type(null);
// and any pointer to member type:
pointer_to_member_object(nullptr);
pointer_to_member_function(null);
}

The difference between the nullptr keyword and NULL macro (along with a zero literal) is that NULL, along with zero literal (0), don’t preserve the meaning of the null pointer constant unless they’re no longer a literal.

#include <cstddef> // contains NULL pointer constant definition
#include <iostream>
template<class T>
constexpr T clone(const T& t)
{
return t; // Return copy of t
}
void g(int*)
{
std::cout << "Function g called\n";
}
int main()
{
g(nullptr); // Fine
g(NULL); // Fine
g(0); // Fine
g(clone(nullptr)); // Fine
// g(clone(NULL)); // ERROR: invalid conversion from ‘long int’ to ‘int*’
// g(clone(0)); // ERROR: invalid conversion from ‘int’ to ‘int*’
int* ptr = nullptr; // Fine
int* null_ptr = NULL; // Fine
int* zero_ptr = 0; // Fine
int* clone_ptr = clone(nullptr); // Fine
// int* clone_null_ptr = clone(NULL); // error: error: invalid conversion from ‘long int’ to ‘int*’
// int* clone_zero_ptr = clone(0); // error: invalid conversion from `int` to `int*`
}

RELATED TAGS

nullptr
c++
keyword
communitycreator

Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

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