Why does a explicit constructor expecting std::shared_ptr accept a nullptr?

Question

The following code compiles fine with gcc 4.8.1

#include <memory>

class Foo {
public:
     explicit Foo(const std::shared_ptr<Foo>& foo) {

     }
};

int main() {
    Foo foo(nullptr);
}

Why is this possible? Shouldn't the explicit prevent the compiler from calling std::shared_ptr(nullptr) implicitly?

As a note, a universal or otherwise `template` constructor can use SFINAE to do what you want. — Yakk - Adam Nevraumont, Apr 02 '14 at 23:41

score 4 · Accepted Answer · answered Apr 02 '14 at 23:14

Shouldn't the explicit prevent the compiler from calling std::shared_ptr(nullptr) implicitly?

No, the explicit constructor would stop this from happening:

Foo foo = some_shared_ptr;

It has no effect on the constructors of shared_ptr, so implicit conversion from nullptr to shared_ptr is still allowed.

1 Answers1