Why can't templates be declared in a function?

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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"
templates c++
7 Answers

The answer "because standard says so", is of course correct, but let's consider generic lambdas.

In C++14 and C++17 generic lambdas are the only way of writing template-like code that I know of:

    auto lambda = [](auto x) { };
    lambda.operator()<int>(0);

Technically, you can write any kind of template code just with that. Though you'll have to work hard to work around various limitations of this approach.

That will be simpler in C++20 though. With template parameter list in generic lambdas you will be able to write code like this:

    auto size = []<class T>() { return sizeof(T); };
    static_assert(4 == size.operator()<int>());

GCC already supports this syntax.

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