Why can't templates be declared in a function?

Question

Reading C++ Templates: The Complete Guide and it says

Note that templates cannot be declared in a function

It does not give explanation and/or cross reference to any other chapter in the book or external resource.

Could someone help in explaining this. Probably it is explained later in the book but not there yet. If explained earlier, I must have missed it.

Example:

int main()
{
  class DummyClass  //  This compiles ok
  {
    int object;
  };

  template <typename T> //  compile error "expected primary-expression before "template""
  class DummyTemplate
  {
    T object;
  };

  return 0;
}

I do not understand the error message from gcc either. The error message says:

expected primary-expression before "template"

Answer 1

It means you cannot do something like the following

  void foo()
  {
       template <typename T> //Error
       T something;
  }

Template declarations are only permitted at global, namespace, or class scope. :)

What is the reasoning behind it?

It is not allowed because the Standard says so .

ISO C++-98 (Section 14.2)

A *template declaration* can appear only as a namespace or class scope declaration

Does that make sense?

Answer 2

What exactly would be the use? So you can declare template variables that you can only use within the function? Is that really useful?

Answer 3

The short answer to why this is, is because that how the guys who wrote the c/c++ compiler's and standards wanted it to be. Templates inside of functions must have been deemed to chaotic and or difficult to understand or parse, so they forbade it.

Answer 4

The only time this would be useful would be if you created multiple instances of the template with different types withing the one function. Move your private classes out of your functions anyway. If that starts to clutter up your classes then they are too big and need to be refactored.