Unable to print the value of nullptr on screen

Question

I was reading about nullptr and doing workout on g++ and also on VS2010.

When I did

#include 
using namespace std;

aut         


        

Answer 1

The pointer literal is the keyword nullptr. It is a prvalue of type std::nullptr_t.

Type nullptr_t should be convertible to T*, but compiler has no operator << for nullptr_t and don't know to which type you want to convert nullptr.

You can use this

cout << static_cast<void*>(nullptr) << endl;

Answer 2

This is because nullptr is of type std::nullptr_t, which does not define the appropriate operators for std::cout to be able print objects of that type. You can define the operator yourself like this:

//std::cout is of type std::ostream, and nullptr is of type std::nullptr_t
std::ostream& operator << (std::ostream& os, std::nullptr_t ptr)
{
    return os << "nullptr"; //whatever you want nullptr to show up as in the console
}

After this function is defined, it will be used to handle all attempts to print nullptr via an ostream. This way you don't need to cast nullptr every time you print it.

Answer 3

I ran into this problem while I was writing some type-parameterized test code (using templates). I needed to print a value of type T, where nullptr_t was a valid type for T. I came up with a solution where the value to be printed was wrapped inside a printable template function. This function then makes use of template specialization to provide the desired behavior when a nullptr_t is used.

#include <cstddef>
#include <iostream>

template <typename T> struct Printable
{
    Printable(const T& val) : val(val) {}
    void print(std::ostream& out) const {out << val;}
    const T& val;
};

template <> struct Printable<std::nullptr_t>
{
    Printable(nullptr_t) {}
    void print(std::ostream& out) const {out << "null";}
};

template <typename T>
Printable<T> printable(const T& value) {return Printable<T>(value);}

template <typename T>
std::ostream& operator<<(std::ostream& out, const Printable<T>& p)
{
    p.print(out);
    return out;
}

int main() {
    std::cout << printable(42) << " " << printable(nullptr) << "\n";
    return 0;
}

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