CODEWITHSUNDEEP
c++constructor
user1086635
user1086635

Reputation: 1544

Use of a trivial constructor

12.1/5 A constructor is trivial if it is an implicitly-declared default constructor and if:

— its class has no virtual functions (10.3) and no virtual base classes (10.1), and
— all the direct base classes of its class have trivial constructors, and
— for all the nonstatic data members of its class that are of class type (or array thereof), each such class has a trivial constructor.

First I thought a trivial constructor is just an implicit default constructor. But when reading the above text in standard, it seems trivial constructor is not only a implicit default constructor but it has other requirements as mentioned above. What does it mean? Whats the point of having a trivial constructor?

For example:

class X
{
  // ...
};

Does class X has a trivial or implicit default constructor?

Upvotes: 1

Views: 1196

Answers (2)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726539

Trivial constructor does nothing at all. The list from your post says that a constructor is trivial when:

  1. You did not make your constructor do work by providing a non-default constructor
  2. The bases of your class did not make your constructor do work associated with preparing to handle virtual functions, virtual base classes, or non-trivial constructors of their own
  3. The data members of your class did not make your constructor do work associated with calling their own non-trivial constructors

These rules taken together mean that the constructor has nothing to do, hence it is trivial.

In case of X, it all depends on its data members: if they all have trivial constructors, and if you did not provide a non-trivial constructor yourself, X will have a trivial constructor too.

Upvotes: 1

ruakh
ruakh

Reputation: 183290

It depends what's in the // ....

Every trivial constructor is an implicitly-declared default constructor, but not every implicitly-declared default constructor is a trivial constructor. Class X has a trivial destructor if it has an implicit default, and every one of its base-classes has an implicit default (as well as those base-classes' base-classes, and so on), and every one of its members is either a primitive like int or else is of a type with an implicit default (as well as its members' members, and so on, as well as its members' base-classes, and their base-classes, and so on, as well its base-classes' members, and their members, and so on, and so on, and so on).

It may be easier to look at it the opposite way: if a class doesn't have a trivial constructor, then no class that extends it ("is-a") or includes it ("has-a") has a trivial constructor, either.

Upvotes: 2

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