Re: [eigen] Value-initializing a Matring

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Am 19.04.2015 um 00:44 schrieb Matan Nassau:
> To be clear, my question stands still whether the code is
>
> struct foo {
> foo() : v{} {} // braces
> private:
> Eigen::Vector2d v();

That would declare (but not implement) a member function named v which 
takes no parameters and returns an Eigen::Vector2d. Of course, you can't 
initialize a funtion in the constructor.

Assuming this was just a typo, to initialize v by zeros, you currently 
need to write:

struct foo {
  foo() : v(Eigen::Vector2d::Zero()) {}
private:
  Eigen::Vector2d v;
};

or (shorter, but probably slightly harder to optimize for a compiler):

struct foo {
  foo() : v(0,0) {}
private:
  Eigen::Vector2d v;
};

The latter only works for sizes up to 4. (And for 1d vectors only with 
the devel-branch, IIRC).

Both v() and v{} will lead to v not getting initalized.

About your comparisons with std::string and std::vector:
For dynamic vectors (VectorXd), you will already get a vector of 
dimension 0 by default (nothing else would make sense here), however
  VectorXd a{2};
will call VectorXd a(2) and allocate two (uninitialized) doubles.
  VectorXd b{3,1}; // calls VectorXd(Index, Index)
will accidentially work and allocate a 3x1-matrix, i.e. a 3d vector.
  VectorXd c{1,2,3};
and more parameters won't compile at the moment.

A better comparision would have been with std::array<double, 2>, but 
please check out the bugzilla entry Gael already referred to:
http://eigen.tuxfamily.org/bz/show_bug.cgi?id=954

As Gael already said, we plan to support initializor-list constructors 
and -- to put Billy's mind at ease -- there are no plans to give up 
C++03-compatibility, so all C++11 features will be activated only if the 
compiler supports them.



Christoph






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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
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