Template argument string vs. int

Question

Possible Duplicate:
Strings as template arguments?

Why the first declaration is OK but the second is not? Why std::string is not suitable?

template <typename T, T x> struct foo { };

using namespace std;

int main()
{
    foo<int, 0> f_int;              // ok
    foo<string, ""> f_string;      // not ok
}

I get:

error: a non-type template parameter cannot have type 'std::basic_string<char>'

using clang++.

Answer 1

You simply cannot have a template parameter of type std::string. The rules for non-type template parameters are defined by the standard as follows (§14.1/4):

A non-type template-parameter shall have one of the following (optionally cv-qualified) types:

• integral or enumeration type,
• pointer to object or pointer to function,
• lvalue reference to object or lvalue reference to function,
• pointer to member,
• std::nullptr_t.

In addition (§14.1/7):

A non-type template-parameter shall not be declared to have floating point, class, or void type.

As std::string is a class type, your instantiation of foo is not allowed.

Answer 2

Because a non-type template parameter (ugly name for "value") should be computable at compile time and std::string is not (it may require dynamic memory allocation thus its constructor is not constexpr).