Hi there, consider the following line of code:

     std::vector<X> vec(1000);

It is my understanding that all 1000 elements of this vector are created
by the default constructor of X.  This understanding is based on "The
Standard C++ Library" by Josuttis page 232.

But my compiler (MS Visual Studio .NET 2003), appears to create the
first element with the default constructor, but the remaining 999
elements are created by copy-constructing the first element.

Have i completely missed something really obvious?  Is this correct
behaviour according to the standard?

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Mark van Dijk wrote:
>     std::vector<X> vec(1000);
>
> It is my understanding that all 1000 elements of this vector are created
> by the default constructor of X.

No. See 23.2.4.1. The vector constructor you are calling has a second
argument which is used as the initial value for the elements. If you
leave it out, it defaults to a default-constructed X, which is the single
call to the default constructor that you see. The vector elements are
copy constructed from this object.

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mark.vandijk@perform-sol.com (Mark van Dijk) wrote in message news:<3f94def6$1@news.iconz.co.nz>...
> Hi there, consider the following line of code:
>
>      std::vector<X> vec(1000);
>
> It is my understanding that all 1000 elements of this vector are created
> by the default constructor of X.  This understanding is based on "The
> Standard C++ Library" by Josuttis page 232.
>
> But my compiler (MS Visual Studio .NET 2003), appears to create the
> first element with the default constructor, but the remaining 999
> elements are created by copy-constructing the first element.
> Have i completely missed something really obvious?  Is this correct
> behaviour according to the standard?

The standard requires that X be a CopyConstructible type(23.1p3),
which means that an object created by copy construction is equivalent
to the original, and can therefore be substituted for it wherever
desired. Therefore, this is a legitimate implementation of that
function, and is an example of precisely why the CopyConstructible
requirement exists.

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On Tue, 21 Oct 2003 18:45:11 +0000, Mark van Dijk wrote:

> Hi there, consider the following line of code:
>
>      std::vector<X> vec(1000);
>
> It is my understanding that all 1000 elements of this vector are created
> by the default constructor of X.  This understanding is based on "The
> Standard C++ Library" by Josuttis page 232.
>
> But my compiler (MS Visual Studio .NET 2003), appears to create the
> first element with the default constructor, but the remaining 999
> elements are created by copy-constructing the first element.
>
> Have i completely missed something really obvious?  Is this correct
> behaviour according to the standard?

Yes, it is. But most probably, your implementation is default constructing
1 object, and copy constructing all the 1000 objects from that 1 object.
This version of the ctor takes 2 arguments instead of the 1 that you have
mentioned. The 2nd one is a default argument that has the default ctor
invoked T(), and the copies are created form that object.

Regards,
-Dhruv.



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