Question

To use a struct in a function outside of main(), can you use forward declaration and define it in main(), or does it have to be defined outside of a block?

Answer 1

If you define a structure inside of main(), it will hide the global name of the structure. So the function outside of main() will only be able to reference the global name. This example is taken from the C++ 2011 draft, Section 9.1 p2:

struct s { int a; };

void g() {
    struct s;              // hide global struct s
                           // with a block-scope declaration
    s* p;                  // refer to local struct s
    struct s { char* p; }; // define local struct s
    struct s;              // redeclaration, has no effect
}

There is no syntax to refer to the locally defined type from outside the function scope. Because of that, even using a template will fail, because there is no way to express the instantiation of the template:

template <typename F> void bar (F *f) { f->a = 0; }

int main () {
    struct Foo { int a; } f = { 3 };
    bar(&f);                         // fail in C++98/C++03 but ok in C++11
}

Actually, this is now allowed in C++11, as explained in this answer.