String template parameters in c++

Question

When using templates in c++ I sometime need to pass strings as value template parameters.

I found quite difficult to understand why certain parameters are allowed and other are not.

For example a const char * can be given as template argument if static member of a class, can not if defined outside.

I did a small program to test all this, commenting lines that do not compile. I also did a couple of supposition based on compiler output but they might be wrong.

What are the rules of template param values. I saw that the object needed external linkage but a bool is authorized although it obviously doesn't have any kind of linkage.

#include <iostream>
using namespace std;

struct tag {
    static char array[];
    static const char carray[];
    static char *ptr;
    static const char *cptr;
    static const char *const cptrc;
    static string str;
    static const string cstr;
};
char tag::array[] = "array";
const char tag::carray[] = "carray";
char *tag::ptr = (char*)"ptr"; // cast because deprecated conversion
const char *tag::cptr = "cptr";
const char *const tag::cptrc = "cptrc";
string tag::str = "str";
const string tag::cstr = "cstr";


namespace ntag {
    char array[] = "array";
    const char carray[] = "carray";
    char *ptr = (char *)"ptr"; // cast because deprecated conversion
    const char *cptr = "cptr";
    const char *const cptrc = "cptrc";
    string str = "str";
    const string cstr = "cstr";
};

template <class T, T t>
void print() { cout << t << endl; };

int main()
{
    cout << "-- class --" << endl;
    // Works
    print<char *, tag::array>();
    print<const char *, tag::carray>();

    // Does not work because it is a lvalue ?
    // print<char *, tag::ptr>();
    // print<const char *, tag::cptr>();
    // print<const char *const, tag::cptrc>();

    // Template type param must be a basic type ?
    // print<string, tag::str>();
    // print<const string*, tag::cstr>();

    cout << "-- namespace --" << endl;
    // Works
    print<char *, ntag::array>();

    // No external linkage ?
    // print<const char *, ntag::carray>();

    // Does not work because it is an lvalue ?
    // print<char *, ntag::ptr>();
    // print<const char *, ntag::cptr>();
    // print<const char *const, ntag::cptrc>();

    // The type of a template value param must a basic type
    // print<string, ntag::str>();
    // print<const string*, ntag::cstr>();
}

Answer 1

When using non-type template parameters you need to specify a constant. When the non-type template parameter is a pointer or a reference it is sufficient to specify a constant which can be determined at link-time. In any case, the compiler won't accept anything which can be possibly mutated after link-time. Even variable initialized during link-time are initialized too late:

print<char *, tag::array>();               // OK: the address of the array won't change
print<const char *, tag::carray>();        // OK: the address of the array won't change
print<char *, tag::ptr>();                 // not OK: tag::ptr can change
print<const char *, tag::cptr>();          // not OK: tag::ptr can change
print<const char *const, tag::cptrc>();    // not OK: a [run-time initialized] variable
print<string, tag::str>();                 // not OK: few types are supported (*)
print<const string*, tag::cstr>();         // not OK: tag::cstr has a different type
print<const string*, &tag::cstr>();        // (added) OK: address won't change

print<char *, ntag::array>();              // OK: address of array won't change
print<const char *, ntag::carray>();       // OK: address of array won't change (**)
print<char *, ntag::ptr>();                // not OK: ntag::ptr can change
print<const char *, ntag::cptr>();         // not OK: ntag::cptr can change
print<const char *const, ntag::cptrc>();   // not OK: a [run-time initialized] variable

print<string, ntag::str>();                // not OK: few types are supported (*)
print<const string*, ntag::cstr>();        // not OK: ntag::cstr has a different type
print<const string*, &ntag::cstr>();       // (added) OK: address won't change

Notes:

• (*) Only integral types, pointers, and references can be used a non-type template parameters. There is no concept of user-define constants which can be used as template parameters.

• (**) gcc doesn't like this use while clang likes it. gcc not accepting this code seems to be an error! I can't see any restriction which would prohibit the use a const char[] as a template argument. Instead, there is an example in 14.3.2 [temp.arg.nontype] paragraph 2 which is exactly equivalent:

  template<class T, const char* p> class X {
      / ... /
  };
  X<int, "Studebaker"> x1; // error: string literal as template-argument
  const char p[] = "Vivisectionist";
  X<int,p> x2; // OK
  

• Casting string literals to non-const pointer to char is OK, however, trying to change one of these values is undefined behavior. I strongly recommend not to use this cast!
• Don't overuse std::endl: in your code is no use for std::endl at all.