c++c++11pointersshared-ptrnullptr

Why can I convert 0 to an std::shared_ptr<T> but not 1?

#include <memory>

void f1(std::shared_ptr<bool> ptr) {}

int main() {
    f1(0); // OK
    f1(1); // compilation error: could not convert ‘1’ from ‘int’ to ‘std::shared_ptr<bool>’
}

Both as int, why 0 but 1 can be converted to std::shared_ptr<T>?

How the disability of conversion from 1 to std::shared_ptr<T> be checked when compiling?

How the disability of conversion from 1 to std::nullptr_t be checked when compiling?

Solution

  • 0is a special value in C/C++. Many things work with 0 but not with 1. The reason(s) for that are the conversion rules of the language.

    f1(0); // OK

    That's ok because of the following conversions.

    0  -> nullptr
nullptr -> std::shared_ptr<bool> // Through a constructor

    However,

    f1(1);

    is not ok since there is no conversion available to convert 1 to a shared_ptr<bool>.