Initialize parent class with empty constructor in initializer list?

Question

Are there any dangers in not initializing an empty constructor parent class in the child initialization list?

Example:

class Parent
{
  public:
    Parent(){}
    ~Parent(){}
};

class Child : public Parent
{
  public:
    Child(): Parent()
    {}
    ~Child(){}
};

Reason for the question: I often see code where a "Parent" class with an empty ctor is not initialized in the child ctor initialization list.

The default constructor is called implicitly if you don't explicitly call a constructor in the initialization list. — scohe001
– scohe001, Commented Sep 5, 2014 at 21:15

Kerrek SB · Accepted Answer · 2014-09-06 11:08:12Z

1

Suppose Parent doesn't have a user-provided constructor, e.g. if it is an aggregate:

struct Parent
{
    int x;
    int get_value() const { return x; }
};

Now there's a difference (cf. [dcl.init]/(8.1)), since value-initialization of Parent will zero-initialize the member x, whereas default-initialization will not:

struct GoodChild : Parent { GoodChild() : Parent() {} };

struct BadChild : Parent { BadChild() {} };

Therefore:

int n = GoodChild().get_value(); // OK, n == 0

int m = BadChild().get_value();  // Undefined behaviour

edited Sep 6, 2014 at 11:08

answered Sep 5, 2014 at 21:19

Kerrek SB

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3 Comments

Lightness Races in Orbit Over a year ago

Specifically, according to C++11 the base will be default-initialised when the initialiser is not given ([C++11: 12.6.2/8]), and default-initialising an aggregate does nothing useful. By contrast, including Parent() in the ctor-initialiser will end up value-initialising its members ([C++11: 12.6.2/7]).

Kerrek SB Over a year ago

Downvoter, care to explain your objection? I'd be happy to improve on unclear or missing points (or just edit it yourself).

Kerrek SB Over a year ago

@T.C.: That makes sense, thanks. I've edited the post to reflect that. (Cleaning up the comments.)

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