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Add an explicit example for auto and re-evaluation

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Gael Guennebaud 2019-11-20 17:31:23 +01:00
parent e78ed6e7f3
commit c79b6ffe1f
1 changed files with 11 additions and 1 deletions
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12
doc/Pitfalls.dox
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@ -40,7 +40,17 @@ for(...) { ... w = C * v; ...}
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.
Moreover, if the coefficients of `A` or `B` change during the iteration, then `C` will evaluate to different values as in the following example:
\code
MatrixXd A = ..., B = ...;
auto C = A*B;
MatrixXd R1 = C;
A = ...;
MatrixXd R2 = C;
\endcode
for which we end up with `R1` ≠ `R2`.
Here is another example leading to a segfault:
\code