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nullptr - Pointer Literal

nullptr is a pointer literal introduced in C++11, used to represent null pointers. It addresses the shortcomings of traditional null pointer representations (such as NULL and 0) in terms of type safety and overload resolution.

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Why was it introduced?

  • Resolve ambiguity issues with NULL macro and integer 0 in overload resolution
  • Provide type-safe null pointer representation
  • Clearly distinguish between pointer and integer types
  • Support type deduction in template programming

What's the difference between nullptr and NULL?

  • nullptr is a keyword introduced in C++11, with type std::nullptr_t
  • NULL is a preprocessor macro, typically defined as integer 0 or (void*)0
  • nullptr is more precise in overload resolution and won't be confused with integer types

I. Basic Usage and Scenarios

Replacing NULL and 0

Used for pointer variable initialization and assignment, replacing traditional NULL and 0

int* ptr1 = nullptr;        // Recommended usage
int* ptr2 = NULL;           // Traditional usage
int* ptr3 = 0;              // Not recommended

// Check if pointer is null
if (ptr1 == nullptr) {
    // Handle null pointer case
}

Resolving Overload Ambiguity

Explicitly passing null pointers in function calls, nullptr can avoid overload ambiguity issues and prevent confusion with integer types

void func(int* ptr) {
    if (ptr != nullptr) {
        *ptr = 42;
    }
}

void func(int value) {
    // Handle integer parameter
}

int main() {
    func(nullptr);  // Explicitly call pointer version
    func(0);        // May call integer version, causing ambiguity
    func(NULL);     // May call integer version, causing ambiguity
}

For example, in the code above, calling func(NULL) will report an overload ambiguity error

main.cpp: In function 'int main()':
main.cpp:16:9: error: call of overloaded 'func(NULL)' is ambiguous
   16 |     func(NULL);     // May call integer version, causing ambiguity
      |     ~~~~^~~~~~

Ensuring Type Safety in Template Programming

In template functions and classes, nullptr provides better type deduction and safety

// https://en.cppreference.com/w/cpp/language/nullptr.html

template<class T>
constexpr T clone(const T& t) {
    return t;
}

void g(int*) {
    std::cout << "Function g called\n";
}

int main() {
    g(nullptr);        // ok
    g(NULL);           // ok
    g(0);              // ok

    g(clone(nullptr)); // ok
    g(clone(NULL));    // ERROR: NULL might be deduced to non-"pointer" type
    g(clone(0));       // ERROR: 0 will be deduced to non-"pointer" type
}

When using function templates, NULL and 0 are usually deduced to non-"pointer" types, while nullptr can avoid this problem

main.cpp:19:12: error: invalid conversion from 'int' to 'int*' [-fpermissive]
   19 |     g(clone(0));       // ERROR: 0 will be deduced to non-"pointer" type
      |       ~~~~~^~~
      |            |
      |            int

Smart Pointers and Containers

Used with modern C++ features (such as smart pointers, STL containers)

#include <memory>
#include <vector>

int main() {
    std::shared_ptr<int> sp1 = nullptr;
    std::unique_ptr<int> up1 = nullptr;

    std::vector<int*> vec;
    vec.push_back(nullptr);

    // Check if smart pointer is null
    if (sp1 == nullptr) {
        sp1 = std::make_shared<int>(42);
    }
}

II. Important Notes

Type Deduction and std::nullptr_t

The type of nullptr is std::nullptr_t, which is a special type that can be implicitly converted to any pointer type:

#include <cstddef>  // Contains definition of std::nullptr_t

void func(int*) {}
void func(double*) {}
void func(std::nullptr_t) {}

int main() {
    auto ptr = nullptr;  // ptr's type is std::nullptr_t

    func(nullptr);       // Call std::nullptr_t version
    func(ptr);           // Call std::nullptr_t version

    int* intPtr = nullptr;
    func(intPtr);        // Call int* version
}

Implicit Conversion to Boolean Type

nullptr can be implicitly converted to bool type, which is very convenient in conditional checks:

int* ptr = nullptr;

if (ptr) { // Equivalent to if (ptr != nullptr)
    // Pointer is not null
} else {
    // Pointer is null
}

bool isEmpty = (ptr == nullptr);  // true

III. Practice Code

Practice Code Topics

Auto-Checker Command

d2x checker nullptr

IV. Additional Resources