CODEWITHSUNDEEP
c++constructordefault
Veritas
Veritas

Reputation: 2210

Are constructors specified with the default keyword trivial?

I know that among other things, a trivial constructor has to be implicitly defined.

Does this also apply when we use the default keyword?

Say we specify a T()=default constructor , is it considered user-provided or is it treated like an implicit constructor?

Upvotes: 0

Views: 510

Answers (2)

Kerrek SB
Kerrek SB

Reputation: 477710

Yes, a user-declared constructor that is defaulted on its first declaration may be trivial:

struct Foo
{ 
    Foo() = default;
    Foo(int, int);

    char x;
};

#include <type_traits>
static_assert(std::is_trivially_constructible<Foo>::value, "Works");

The example demonstrates how to define a POD class even in the presence of user-defined (non-default) constructors.

From the standard (12.1), "a default constructor is trivial if it is not user-provided" (plus conditions), and (8.4.2):

A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration.

However, note that triviality of a default constructor depends on more than just its declaration and definition. To expand the quote from 12.1:

A default constructor is trivial if it is not user-provided and if:

— its class has no virtual functions (10.3) and no virtual base classes (10.1), and

— no non-static data member of its class has a brace-or-equal-initializer, and

— all the direct base classes of its class have trivial default constructors, and

— for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.

Upvotes: 4

Dr. Debasish Jana
Dr. Debasish Jana

Reputation: 7128

Implicit constructor is one provided by the compiler if you don't define one. That's a default constructor having no argument, unless you would like to have your own constructor with or without arguments to precisely control initialization of your object instance data members.

Upvotes: 0

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