What's the point in defaulting functions in C++11?

Question

C++11 adds the ability for telling the compiler to create a default implementation of any of the special member functions. While I can see the value of deleting a function, where's the value of explicitly defaulting a function? Just leave it blank and the compiler will do it anyway.

The only point I can see is that a default constructor is only created when no other constructor exists:

class eg {
public:
    eg(int i);
    eg() = default; 
};

But is that really better than how you do it now?

class eg {
public:
    eg(int i);
    eg() {}
};

Or am I missing a use-case?

Answer 1

A defaulted constructor will have a declaration, and that declaration will be subject to the normal access rules. E.g. you can make the default copy constructor protected. Without these new declarations, the default generated members are public.

Answer 2

Those examples from Stroustrup's website might help you understand the point:

defaulted and deleted functions -- control of defaults

The common idiom of "prohibiting copying" can now be expressed directly:

class X {
  // ...

  X& operator=(const X&) = delete;    // Disallow copying
  X(const X&) = delete;
};

Conversely, we can also say explicitly that we want to default copy behavior:

class Y {
  // ...
  Y& operator=(const Y&) = default;   // default copy semantics
  Y(const Y&) = default;

};

Being explicit about the default is obviously redundant, but comments to that effect and (worse) a user explicitly defining copy operations meant to give the default behavior are not uncommon. Leaving it to the compiler to implement the default behavior is simpler, less error-prone, and often leads to better object code. The "default" mechanism can be used for any function that has a default. The "delete" mechanism can be used for any function. For example, we can eliminate an undesired conversion like this:

struct Z {
  // ...

  Z(long long);     // can initialize with an long long
  Z(long) = delete; // but not anything less
};