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.

Are there any other significant pitfalls to be aware of? E.g. would this be bad:

int foo()
{
    struct Scratch
    {
        int a, b, c;
    };
    std::vector<Scratch> workingBuffer;
    //Blah Blah
}

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