nontypes template parameter

Question

I have learned that:

• Nontype template parameters carry some restrictions. In general, they may be constant integral values (including enumerations) or pointers to objects with external linkage.

So i made following code

1.

 template <char const* name> 

 class MyClass { 
    … 
 }; 

 char const* s = "hello";  

 MyClass<s> x;         // ERROR:

This code didn't work and produce error 's' is not a valid template argument

My second code also didn't work

2.

template <char const* name> 
class MyClass { 
  … 
}; 

extern char const *s = "hello";

MyClass<s> x;     //error 's' is not a valid template argument` 

But strangely this code is fine

3.

template <char const* name> 
class MyClass { 
  … 
};

extern char const s[] = "hello";

MyClass<s> x;        // OK  

please tell what is happening in all of these three codes??

also tell how to correct errors to make other two codes working also.

Answer 1

I think the problem is here: even

const char * const p="hello";

only define a pointer variable which stores the address of a memory, the value of the memory cannot be determined when compilation. but

const char pp[]="hello";

the compiler will know when compile the memory is "hello", not a pointer to somewhere else. that's why

printf(" p=%p, &p=%p\n", p, &p);

will get the same value. but

printf("pp=%p, &pp=%p\n", pp, &pp);

will not show the same value.

Answer 2

From here: "Non-type template argument provided within a template argument list is an expression whose value can be determined at compile time".

You get a problem because your char pointer is not really constant in the first two examples. Have a look at this short example:

int main() {
        char const *c = "foor";
        std::cout << "c: " << c << std::endl;
        c = "bar";
        std::cout << "c: " << c << std::endl;
}

Which will give you

c: foo
c: bar