I have a question about the Eigen VectorXf. Currently, I am doing the following in a loop as follows:

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for (int i = 0; i < num_iterations; ++i) {
    Eigen::VectorXf temp (some_size);
    temp << some_computed_value;
    temp << some_other_computed_value;
    temp << they_another_value;
}


Since the vector does not change size, it would be more desirable to do the following:

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Eigen::VectorXf temp (some_size);
for (int i = 0; i < num_iterations; ++i) {
   // Call some function here that will keep the size of the vector but
   // insert these values from the beginning of the array
    temp << some_computed_value_array;
    temp << some_other_computed_value_array;
    temp << yet_another_value_array;
}


So what I would like to do is to overwrite the array at each loop iteration. My understanding is that the << operator does a push_back which would resize the array if I declare it as I currently do.

"<<" does not change the size of the destination, however, it is not as efficient as operator = when assigning a matrix/vector to another matrix/vector. The recommended way in your case is thus:

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Eigen::VectorXf temp (some_size);
for (int i = 0; i < num_iterations; ++i) {
   // Call some function here that will keep the size of the vector but
   // insert these values from the beginning of the array
    temp = some_computed_value_array;
    temp = some_other_computed_value_array;
    temp = yet_another_value_array;
}


No resizing, no memory reallocation, unless the size of "some_whatever_array" is not some_size.

Sorry I made a small mistake in my original post. The use I was intending was something like:

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Eigen::VectorXf temp (some_size);
for (int i = 0; i < num_iterations; ++i) {
   // Call some function here that will keep the size of the vector but
   // insert these values from the beginning of the array
    temp  << array_block_1, array_block_2, array_block_3;

    // further computations...
}


Here the sum of the size of the three array blocks equal to some_size. Is it safe to do it like this in a loop. The array size should not change but it should get refreshed with the new values at every iteration.

Sorry for causing the confusion due to my stupidity.

Actually, to make << works the sum of sizes *must* equals to some_size. Otherwise, you will get an assertion error. So yes, your use case is perfectly fine, and no memory re-allocation occur.