Re: [eigen] Value-initializing a Matring

[Matan Nassau <matan.nassau@xxxxxxxxx>]

To be clear, my question stands still whether the code is

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

or

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

The question is, what's the value of v after construction? In the current state of affairs it is indeterminate. It can be anything. But one might expect it to be value-initialized just as std::vector or int would be (such that v.size()==0 or v==0 for the std::vector and int respectively) if that's what the type of v were. Yes, I can get this behaviour with the EIGEN_INITIALIZE_MATRICES_BY_ZERO define, but then I can't get the do-nothing performance anymore if that's what I want, so that's not a solution.

On Sat, Apr 18, 2015 at 18:26 Matan Nassau <matan.nassau@xxxxxxxxx> wrote:

But, there is no initializer list here. There is an =default, yes, but maybe we can do that conditionally?

#if __cplusplus >= 201103L
Matrix()=default;
#else
#ifdef EIGEN_INITIALIZE_MATRICES_BY_ZERO
...

If Eigen supports C++11 initialization semantics it opens the door for more performant code, and for more expressive flexibility in user code. I mean, I can clearly express what I want with

Matrix3d m;

and

Matrix3d m{};

controlling the performance or behaviour in case-by-case fashion, in my code.

Eigen can do this, while still providing the current behaviour for C++03.

On Sat, Apr 18, 2015 at 15:47 Gael Guennebaud <gael.guennebaud@xxxxxxxxx> wrote:

Hi,

Eigen aims to be compatible with C++03, and initializer-lists are not supported yet. So currently Vector2d v{}; is equivalent to Vector2d v; .

See this bug entry for further discussions on initializer-lists: http://eigen.tuxfamily.org/bz/show_bug.cgi?id=954. They will likely be supported in Eigen 3.3, and we already agreed on making the empty initializer-list initializes to 0.

In the meantime, you can compile with -DEIGEN_INITIALIZE_MATRICES_BY_ZERO, as documented there: http://eigen.tuxfamily.org/dox/TopicPreprocessorDirectives.html#title0.

cheers,

gael

On Sat, Apr 18, 2015 at 7:22 PM, Matan Nassau <matan.nassau@xxxxxxxxx> wrote:

I have a class,

 struct foo {

   foo() : v{} {}

   private:

   Eigen::Vector2d v;

 };

I just got bitten when I realized the value of v is undetermined.

From

http://eigen.tuxfamily.org/dox/classEigen_1_1Matrix.html#a3aede62f513da27e6f61ae7a972b4f96

I understand the default constructor of a fixed-size matrix does nothing. In particular,

 Eigen::Vector2d v{};

will create a vector with an undetermined value.

Why is that? Is this for speed?

If I value-initialize an object I expect it to initialize. To motivate, all standard templates and classes behave this way:

 std::string s{};  // assert(s=="");

 std::vector v{};  // assert(v.size() == 0);

Granted,

 std::string s;  // assert(s=="");

but if we want speed here then we can do

 template<...

 struct Matrix {

   Matrix() = default;
   // ....

This way, we'd get the best of both worlds:

 Eigen::Vector2d v;  // valid, undetermined value

 Eigen::Vector2d v{};  // assert(v==Eigen::Vector2d::Zero());

Matan