Why can I define structures and classes within a function in C++?

Question

I just mistakenly did something like this in C++, and it works. Why can I do this?

int main(int argc, char** argv) {
    struct MyStruct
    {
      int somevalue;
    };

    MyStruct s;
    s.somevalue = 5;
}

Now after doing this, I kind of remembered reading about this trick someplace, a long time ago, as a kind of poor-man's functional programming tool for C++, but I can't remember why this is valid, or where I read it.

Answers to either question are welcome!

Note: Although when writing the question I didn't get any references to this question, the current side-bar points it out so I'll put it here for reference, either way the question is different but might be useful.

Answer 1

[EDIT 18/4/2013]: Happily, the restriction mentioned below has been lifted in C++11, so locally defined classes are useful after all! Thanks to commenter bamboon.

The ability to define classes locally would make creating custom functors (classes with an operator()(), e.g. comparison functions for passing to std::sort() or "loop bodies" to be used with std::for_each()) much more convenient.

Unfortunately, C++ forbids using locally-defined classes with templates, as they have no linkage. Since most applications of functors involve template types that are templated on the functor type, locally defined classes can't be used for this -- you must define them outside the function. :(

[EDIT 1/11/2009]

The relevant quote from the standard is:

14.3.1/2: .A local type, a type with no linkage, an unnamed type or a type compounded from any of these types shall not be used as a template-argument for a template type-parameter.