Question

Assume that we have that struct X; and we use C++11 compiler (e.g. gcc 4.7). I’d like to emit some code and attributes if and only if, say, opt = true.

template <bool opt>
struct X {
  void foo() {
    EMIT_CODE_IF(opt) {
      // optional code
    }

    // ...common code...
  }

  int optional_variable; // Emitted if and only if opt is true
};

1. As for the code, I assume that normal if suffices.
2. But as for the attributes, if one leaves them unused (when opt = false), will and COULD they be automatically omitted by the compiler? I definitely do not want them there when opt = false.

Answer 1

The approach to avoid attributes in a class template is to derive from a base class template which is specialized to be empty if the member shouldn’t be there. For example:

template <bool Present, typename T>
struct attribute {
    attribute(T const& init): attribute_(init) {}
    T attribute_;
};
template <typename T>
struct attribute<false, T> {
};

template <bool opt>
class X: attribute<opt, int> {
    ...
};

With respect to optional code you may get away with a conditional statement but often the code wouldn’t compile. In this case, you’d factor out the code into a suitable function object which be specialized to do nothing when not needed.