-Wreorder and constructor initializer list

Question

I declared a class template as follows:

template<typename T>

    class C{
             public:
                      C(T&,        
                        shared_ptr<C<T>>&
                       );
                // rest of the public interface

             private:

                     T& rData;
                     shared_ptr<C<T>>& rP;            
              };

Subsequently, I defined the template constructor as:

template<typename T> C<T>::C(T& rDataArg,
                             shared_ptr<C<T>>& rPArg
                             ):rData(rDataArg),
                               rP(rPArg)
                            {}

For the above definition I got the following -Wreorder warning from the g++ compiler:

warning: field 'rData' will be initialized after field 'rP' [- Wreorder]

I reversed the order of initialization in my constructor definition and the warning disappeared.

Since both the members of the template class are references, I am curious about why the initialization in the constructor should adhere to the order specified by the compiler.

Please share your thoughts.

Maybe [this](https://stackoverflow.com/questions/1242830/constructor-initialization-list-evaluation-order) helps you. — FERNman, Jan 11 '18 at 08:57

score 2 · Accepted Answer · answered Jan 11 '18 at 08:58

Since both the members of the template class are references, I am curious about why the initialization in the constructor should adhere to the order specified by the compiler.

It's not specified by the compiler, it's specified by you. You specify it right here:

template<typename T>
class C{
  private:

    T& rData;               // first
    shared_ptr<C<T>>& rP;   // second 
};

Members will always be initialized by declaration order. It's a common source of bugs, when one ends up depending on another one that is yet of an indeterminate value. The warning is trying to help you prevent that. Though it's not a problem in your particular case, because the members don't depend on each other's initialization order.

-Wreorder and constructor initializer list

1 Answers1