Re: [eigen] Value-initializing a Matring

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