What restrictions does ISO C++03 place on structs defined at function scope?

Question

We\'re not allowed to define a functor struct inside a function because one is not allowed to use function declared structs in the instantiation of function templates.

Answer 1

1. C++ standard forbids using locally-defined classes with templates.

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.

A code example:

    template  class X { /* ... */ };
    void f()
    {
      struct S { /* ... */ };
      X x3;  // error: local type used as
                //  template-argument
      X x4; // error: pointer to local type
                //  used as template-argument
    }

Here is a little more reference from IBM documentation:

2. Declarations in a local class can only use type names, enumerations, static variables from the enclosing scope, as well as external variables and functions.

A Code Example:

int x;                         // global variable
void f()                       // function definition
{
      static int y;            // static variable y can be used by
                               // local class
      int x;                   // auto variable x cannot be used by
                               // local class
      extern int g();          // extern function g can be used by
                               // local class

      class local              // local class
      {
            int g() { return x; }      // error, local variable x
                                       // cannot be used by g
            int h() { return y; }      // valid,static variable y
            int k() { return ::x; }    // valid, global x
            int l() { return g(); }    // valid, extern function g
      };
}

int main()
{
      local* z;                // error: the class local is not visible
      return 0;
}

3. A local class cannot have static data members

A Code Example:

void f()
{
    class local
    {
       int f();              // error, local class has noninline
                             // member function
       int g() {return 0;}   // valid, inline member function
       static int a;         // error, static is not allowed for
                             // local class
       int b;                // valid, nonstatic variable
    };
}