Why can an array of char be a template parameter but a const char* can't

Question

I'm trying to pass a literal string as a template parameter in a C++14 project. Google told me that I can do as below:

struct Test {
    static const char teststr[];

    template<const char* str>
    void p() {std::cout << str;}
};

const char Test::teststr[] = "Hello world!";

int main() {
    Test t;
    t.p<Test::teststr>();
}

It did work.

However, if I use const char*, instead of const char []. It won't work.

struct Test {
    static const char* teststr;

    template<const char* str>
    void p() {std::cout << str;}
};

const char* Test::teststr = "Hello world!";

int main() {
    Test t;
    t.p<Test::teststr>();
}

Now it doesn't work. The compiler told me that 'Test::teststr' is not a valid template argument because 'Test::teststr' is a variable, not the address of a variable.

Well, I don't know what it meant.

Answer 1

The error message from the compiler is clear enough:

error: 'Test::teststr' is not a valid template argument because 'Test::teststr' is a variable, not the address of a variable

So you need:

#include <iostream>

struct Test {
    static const char* teststr;

    template<const char **str>
    void p() {std::cout << *str;}
};

const char* Test::teststr = "Hello world!";

int main() {
    Test t;
    t.p <&Test::teststr>();
}

And then it works - the point being that [the contents of] a variable is not a compile-time constant, whereas the address of a variable (if it's a static or global variable) is.