Using `volatile` has unexpected results

Question

I am puzzled by following difference discovered by trying to replicate different bug. This program works as expected:

#include <memory>
#include <string>
#include <iostream>


int main() {
    std::string s = "Hi\n";
    std::cout << s;

    auto p = std::make_unique<const char *>(s.c_str());
    std::cout << *(p.get());

    return 0;
}

It prints

Hi
Hi

But adding single volatile keyword:

#include <memory>
#include <string>
#include <iostream>


int main() {
    std::string s = "Hi\n";
    std::cout << s;

    auto p = std::make_unique<volatile const char *>(s.c_str());
    std::cout << *(p.get());

    return 0;
}

makes it print:

Hi
1

This did not help either:

#include <memory>
#include <string>
#include <iostream>


int main() {
    std::string s = "Hi\n";
    std::cout << s;

    auto p = std::make_unique<volatile const char *>(reinterpret_cast<volatile const char *>(s.c_str()));
    std::cout << *(p.get());

    return 0;
}

Why?

Answer 1

The answer is in the comment by Jarod42:

std::ostream has overload for operator << for const char* and bool (pointer has conversion to bool), not for volatile const char *

The difference then can be replicated simply by:

#include <iostream>

int main() {
    /*volatile*/ const char * s = "abc\n";
    std::cout << s;

    return 0;
}

Gives abc with volatile commented out and 1 (without newline) with uncommenting it.