local variable as a non-typename argument
c++, c++11, templates
Solution
Because template arguments must be evaluated at compile time, and the compiler won't know the address of a local variable until run-time (in order to bind a reference to an object, the compiler needs to know the address of that object).
Notice, that the C++11 Standard tells exactly what non-type template arguments can be provided in paragraph 14.3.2/1:
A template-argument for a non-type, non-template template-parameter shall be one of:
— for a non-type template-parameter of integral or enumeration type, a converted constant expression (5.19) of the type of the template-parameter; or
— the name of a non-type template-parameter; or
— a constant expression (5.19) that designates the address of an object with static storage duration and external or internal linkage or a function with external or internal linkage, including function templates and function template-ids but excluding non-static class members, expressed (ignoring parentheses) as & id-expression, except that the & may be omitted if the name refers to a function or array and shall be omitted if the corresponding template-parameter is a reference; or
— a constant expression that evaluates to a null pointer value (4.10); or
— a constant expression that evaluates to a null member pointer value (4.11); or
— a pointer to member expressed as described in 5.3.1; or
— an address constant expression of type `std::nullptr_t`.
As you can see, local variables are not in this list.
Problem
Why is it illegal to use a local variable as a non-type argument? For example, in the next code `local_var` cannot be argument to `X`. ``` template<int& x> struct X {}; void f(int local_var) { X<local_var> x; } ```