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39 lines
1.7 KiB
Plaintext
39 lines
1.7 KiB
Plaintext
namespace Eigen {
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/** \page TopicPitfalls Common pitfalls
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\section TopicPitfalls_template_keyword Compilation error with template methods
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See this \link TopicTemplateKeyword page \endlink.
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\section TopicPitfalls_auto_keyword C++11 and the auto keyword
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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:
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\code
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MatrixXd A, B;
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auto C = A*B;
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for(...) { ... w = C * v; ...}
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\endcode
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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.
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Here is another example leading to a segfault:
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\code
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auto C = ((A+B).eval()).transpose();
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// do something with C
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\endcode
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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:
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\code
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VectorXd u, v;
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auto C = u + (A*v).normalized();
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// do something with C
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\endcode
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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:
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\code
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auto C = (u + (A*v).normalized()).eval();
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\endcode
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In this case, C will be a regular VectorXd object.
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*/
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}
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