Why can't I use std::string as a non-type template parameter?

Question

I understand that non-type template parameters should be a constant integral expression. Can someone shed light why this is so?

template <std::string temp>
void foo()
{
     // ...
}

error C2993: 'std::string' : illegal type for non-type template parameter 'temp'.

I understand what a constant integral expression is. What are the reasons for not allowing non-constant types like std::string as in the above snippet ?

Answer 1

The reason you can't do this is because non-constant expressions can't be parsed and substituted during compile-time. They could change during runtime, which would require the generation of a new template during runtime, which isn't possible because templates are a compile-time concept.

Here's what the standard allows for non-type template parameters (14.1 [temp.param] p4):

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.