A defaulted default constructor, why is it not a user-provided default constructor?

Question

For example, clang does not compile this code, because, the defaulted default constructor for struct A below, A() = default; is not considered to be user-provided.

struct A{ A() = default; }; const A a;

But if you look at [dcl.fct.def.general]/1 you'll see:

function-body:
     ctor-initializer_opt compound-statement
     function-try-block
    = default ;
    = delete ;

That is, = default; is the function body for the default constructor A::A(), which is the same as saying that the definition A() = default; above is equivalent to A(){} as {}is the body for a default constructor.

By the way, g++ compiles the snippet above, but I know g++ has other issues in this regard, according to this comment by Jonathan Wakely.

Strange, as you state, clang fails, gcc compiles. So does VS2015. — wally
– wally, Commented Oct 22, 2016 at 18:23
User-provided explicitly-defaulted would look like struct A{A();}; A::A()=default; — David G
– David G, Commented Oct 22, 2016 at 18:34

T.C. · Accepted Answer · 2016-10-22 18:27:46Z

9

Because the standard says so ([dcl.fct.def.default]/5):

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

Doing it this way allows you to maintain the triviality property with = default;. Otherwise, there's no way to give a class with another constructor a trivial default constructor.

answered Oct 22, 2016 at 18:27

T.C.

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4 Comments

Alexander Over a year ago

I think the term "deleted" on the quote you provided is not necessary, as [[dcl.fct.def.delete]/4](eel.is/c++draft/dcl.fct.def.delete#4) already prohibits a deleted definition from occurring after the function's first declaration. Am I missing something here?

T.C. Over a year ago

@Alexander that makes A() = delete; not user-provided.

Alexander Over a year ago

But at least for the defaulted case we know that the function is either user-provided or implicitly provided by the compiler. But for a function that's explicitly deleted, I'm confused, because, as far as I know, this function is never defined. What would be the purpose of defining such a function, that will never be invoked?

T.C. Over a year ago

@Alexander I don't get your question. = delete; is a definition. Classifying A() = delete; as not-user-provided doesn't mean you can define it yourself.

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