Re: [eigen] Qt's container support

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independently of the QVector::fill() issue I'm in favor to allow
operator= to resize an uninitialized matrix. I'm pretty sure the
unique argument was this if()....

To make it clear to everyone, after this change you 'll be able to do:

MatrixXf a;
a = MatrixXf::Random(100,100); // will be ok

but not the following:

MatrixXf a;
a = MatrixXf::Random(100,100); // will still be OK
a = MatrixXf::Random(50,50);    // NOT OK => use .set()


On Tue, Jan 20, 2009 at 5:09 PM, Keir Mierle <mierle@xxxxxxxxx> wrote:
> On Tue, Jan 20, 2009 at 8:03 AM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
>> 2009/1/20 Keir Mierle <mierle@xxxxxxxxx>:
>>> What is wrong with (a)? I'd like to have this anyway.
>> In my understanding the main problem with (a) was that it would
>> require operator= to start with an if() to check if the matrix is
>> already initialized (so runtime overhead).
>> Perhaps this argument isn't convincing: dynamic-size matrices involve
>> runtime branching anyway eveytime you have to loop over their
>> coefficients, so this if() is going to be negligible.
> Exactly. Also, I believe that in some cases GCC can prove that a
> matrix is uninitialized even for dynamic sized matrices and skip the
> if, provided operator= is inlined. This will require some
> investigation, but my understanding is that the scalar promotion of
> aggregates optimization phase will expose this (scalar promotion of
> aggregates breaks structures up into individual basic types, exposing
> many optimization opportunities).
>> It's true that this aspect of our API is one of the things that's
>> causing the most trouble to users. I'm open to reconsidering it. But
>> first: is there another reason for the current behavior that i
>> forgot...?
>> Benoit

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