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Re: Understanding default member initializers of constructorless structs

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On 8 Feb 2022 11:31, Juha Nieminen wrote:
> I have always understood that the default member initializers (introduced
> in C++11) merely added that particular initialization value to the
> initialization list of a constructor, if it's not explicitly specified
> there. In other words, if you do this:
> 
>      struct A
>      {
>          int a = 5, b = 10;
> 
>          A() {}
>          A(int v): a(v) {}
>      };
> 
> it results in pretty much the exact equivalent of:
> 
>      struct A
>      {
>          int a, b;
> 
>          A(): a(5), b(10) {}
>          A(int v): a(v), b(10) {}
>      };
> 
> The description at https://en.cppreference.com/w/cpp/language/data_members
> seems to corroborate this notion. It says there:
> 
>      Non-static data members may be initialized in one of two ways:
>      ...
>      2) Through a default member initializer, which is a brace or equals
>      initializer included in the member declaration and is used if the
>      member is omitted from the member initializer list of a constructor.
> 
> I can't find any other explanation of the meaning and behavior of default
> member initializers. (If another behavior is explained somewhere, I can't
> find it.)
> 
> All the above is find and dandy, except that the meaning of default member
> initializers seems to be rather different with constructorless structs
> (and classes, I suppose). Consider this:
> 
> //---------------------------------------------------------------------------
> #include <iostream>
> 
> struct S1 { int a = 5, b = 15, c = 25; };
> struct S2 { int a, b, c = 35; };
> struct S3 { int a = 1, b, c = 3; };
> 
> int main()
> {
>      S1 s1 = { 100 };
>      S2 s2 = { 200, 300 };
>      S3 s3 = { 400 };
> 
>      std::cout << "s1 = { " << s1.a << ", " << s1.b << ", " << s1.c << " }\n"
>                << "s2 = { " << s2.a << ", " << s2.b << ", " << s2.c << " }\n"
>                << "s3 = { " << s3.a << ", " << s3.b << ", " << s3.c << " }\n";
> }
> //---------------------------------------------------------------------------
> 
> It behaves as you would expect it to behave. The members not specified in
> the initialization of the variables get the specified default values. So
> this is printed:
> 
> s1 = { 100, 15, 25 }
> s2 = { 200, 300, 35 }
> s3 = { 400, 0, 3 }
> 
> But how does that work, exactly? The structs have no user-declared
> constructors, so where exactly are those default values being "put",
> so to speak?

I believe but you'll have to check this, that there is a compiler 
generated constructor.

Note: there was some change regarding what's regarded as an aggregate 
class, an extension I think in C++17, sort of just to baffle people and 
screw up old code.


> How are the objects constructed from the initializater
> lists? Does the compiler, like, create an implicitly constructor
> for those structs, with three int parameters with the specified
> default values? In other words, is for example S1 above equivalent to:
> 
>      struct S1
>      {
>          int a, b, c;
> 
>          S1(int ia = 5, int ib = 15, int ic = 25):
>              a(ia), b(ib), c(ic) {}
>      };
> 
> I think it can't be *completely* equivalent to that because gcc,
> if you specify -Wextra, will issue a warning from the line:
> 
>      S3 s3 = { 400 };
> 
> warning: missing initializer for member 'S3::b'
> 
> s3.b will be initialized to 0 (IIRC this is guaranteed), but gcc
> will still give a warning (if enough warning flags are turned on).

It looks like a warning without name, so can't be turned off?


- Alf

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Thread

Understanding default member initializers of constructorless structs Juha Nieminen <nospam@thanks.invalid> - 2022-02-08 10:31 +0000
  Re: Understanding default member initializers of constructorless structs "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-08 11:55 +0100
    Re: Understanding default member initializers of constructorless structs Juha Nieminen <nospam@thanks.invalid> - 2022-02-08 12:37 +0000
      Re: Understanding default member initializers of constructorless structs red floyd <no.spam.here@its.invalid> - 2022-02-08 10:56 -0800
  Re: Understanding default member initializers of constructorless structs Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-02-08 07:29 -0800
    Re: Understanding default member initializers of constructorless structs Juha Nieminen <nospam@thanks.invalid> - 2022-02-09 07:26 +0000

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