Slightly extend discussions on auto and move the content of the Pit falls wiki page here.

http://eigen.tuxfamily.org/index.php?title=Pit_Falls

doc/Pitfalls.dox

@@ -7,14 +7,30 @@ namespace Eigen {
 
 See this \link TopicTemplateKeyword page \endlink.
 
+
 \section TopicPitfalls_aliasing Aliasing
 
 Don't miss this \link TopicAliasing page \endlink on aliasing,
 especially if you got wrong results in statements where the destination appears on the right hand side of the expression.
 
+
+\section TopicPitfalls_alignment_issue Alignment Issues (runtime assertion)
+
+%Eigen does explicit vectorization, and while that is appreciated by many users, that also leads to some issues in special situations where data alignment is compromised.
+Indeed, since C++17,  C++ does not have quite good enough support for explicit data alignment.
+In that case your program hits an assertion failure (that is, a "controlled crash") with a message that tells you to consult this page:
+\code
+http://eigen.tuxfamily.org/dox/group__TopicUnalignedArrayAssert.html
+\endcode
+Have a look at \link TopicUnalignedArrayAssert it \endlink and see for yourself if that's something that you can cope with.
+It contains detailed information about how to deal with each known cause for that issue.
+
+Now what if you don't care about vectorization and so don't want to be annoyed with these alignment issues? Then read \link getrid how to get rid of them \endlink.
+
+
 \section TopicPitfalls_auto_keyword C++11 and the auto keyword
 
-In short: do not use the auto keywords with Eigen's expressions, unless you are 100% sure about what you are doing. In particular, do not use the auto keyword as a replacement for a Matrix<> type. Here is an example:
+In short: do not use the auto keywords with %Eigen's expressions, unless you are 100% sure about what you are doing. In particular, do not use the auto keyword as a replacement for a \c Matrix<> type. Here is an example:
 
 \code
 MatrixXd A, B;
@@ -22,23 +38,81 @@ auto C = A*B;
 for(...) { ... w = C * v;  ...}
 \endcode
 
-In this example, the type of C is not a MatrixXd but an abstract expression representing a matrix product and storing references to A and B. Therefore, the product of A*B will be carried out multiple times, once per iteration of the for loop. Moreover, if the coefficients of A or B change during the iteration, then C will evaluate to different values.
+In this example, the type of C is not a \c MatrixXd but an abstract expression representing a matrix product and storing references to \c A and \c B.
+Therefore, the product of \c A*B will be carried out multiple times, once per iteration of the for loop.
+Moreover, if the coefficients of A or B change during the iteration, then C will evaluate to different values.
 
 Here is another example leading to a segfault:
 \code
 auto C = ((A+B).eval()).transpose();
 // do something with C
 \endcode
-The problem is that eval() returns a temporary object (in this case a MatrixXd) which is then referenced by the Transpose<> expression. However, this temporary is deleted right after the first line, and there the C expression reference a dead object. The same issue might occur when sub expressions are automatically evaluated by Eigen as in the following example:
+The problem is that \c eval() returns a temporary object (in this case a \c MatrixXd) which is then referenced by the \c Transpose<> expression.
+However, this temporary is deleted right after the first line, and then the \c C expression references a dead object.
+One possible fix consists in applying \c eval() on the whole expression:
+\code
+auto C = (A+B).transpose().eval();
+\endcode
+
+The same issue might occur when sub expressions are automatically evaluated by %Eigen as in the following example:
 \code
 VectorXd u, v;
 auto C = u + (A*v).normalized();
 // do something with C
 \endcode
-where the normalized() method has to evaluate the expensive product A*v to avoid evaluating it twice. On the other hand, the following example is perfectly fine:
+Here the \c normalized() method has to evaluate the expensive product \c A*v to avoid evaluating it twice.
+Again, one possible fix is to call \c .eval() on the whole expression:
 \code
 auto C = (u + (A*v).normalized()).eval();
 \endcode
-In this case, C will be a regular VectorXd object.
+In this case, \c C will be a regular \c VectorXd object.
+Note that DenseBase::eval() is smart enough to avoid copies when the underlying expression is already a plain \c Matrix<>.
+
+
+\section TopicPitfalls_header_issues Header Issues (failure to compile)
+
+With all libraries, one must check the documentation for which header to include.
+The same is true with %Eigen, but slightly worse: with %Eigen, a method in a class may require an additional <code>#include</code> over what the class itself requires!
+For example, if you want to use the \c cross() method on a vector (it computes a cross-product) then you need to:
+\code
+#include<Eigen/Geometry>
+\endcode
+We try to always document this, but do tell us if we forgot an occurrence.
+
+
+\section TopicPitfalls_ternary_operator Ternary operator
+
+In short: avoid the use of the ternary operator <code>(COND ? THEN : ELSE)</code> with %Eigen's expressions for the \c THEN and \c ELSE statements.
+To see why, let's consider the following example:
+\code
+Vector3f A;
+A << 1, 2, 3;
+Vector3f B = ((1 < 0) ? (A.reverse()) : A);
+\endcode
+This example will return <code>B = 3, 2, 1</code>. Do you see why?
+The reason is that in c++ the type of the \c ELSE statement is inferred from the type of the \c THEN expression such that both match.
+Since \c THEN is a <code>Reverse<Vector3f></code>, the \c ELSE statement A is converted to a <code>Reverse<Vector3f></code>, and the compiler thus generates:
+\code
+Vector3f B = ((1 < 0) ? (A.reverse()) : Reverse<Vector3f>(A));
+\endcode
+In this very particular case, a workaround would be to call A.reverse().eval() for the \c THEN statement, but the safest and fastest is really to avoid this ternary operator with %Eigen's expressions and use a if/else construct.
+
+
+\section TopicPitfalls_pass_by_value Pass-by-value
+
+If you don't know why passing-by-value is wrong with %Eigen, read this \link TopicPassingByValue page \endlink first.
+
+While you may be extremely careful and use care to make sure that all of your code that explicitly uses %Eigen types is pass-by-reference you have to watch out for templates which define the argument types at compile time.
+
+If a template has a function that takes arguments pass-by-value, and the relevant template parameter ends up being an %Eigen type, then you will of course have the same alignment problems that you would in an explicitly defined function passing %Eigen types by reference.
+
+Using %Eigen types with other third party libraries or even the STL can present the same problem.
+<code>boost::bind</code> for example uses pass-by-value to store arguments in the returned functor.
+This will of course be a problem.
+
+There are at least two ways around this:
+  - If the value you are passing is guaranteed to be around for the life of the functor, you can use boost::ref() to wrap the value as you pass it to boost::bind. Generally this is not a solution for values on the stack as if the functor ever gets passed to a lower or independent scope, the object may be gone by the time it's attempted to be used.
+  - The other option is to make your functions take a reference counted pointer like boost::shared_ptr as the argument. This avoids needing to worry about managing the lifetime of the object being passed.
+
 */
 }