Is there any disadvantages if I'm using Array instead of Vector?

Question

I wrote a MPC controller in C++, which included a Matrix class, where I stored the data in an array and I used C memory functions(memcpy, memset etc). Today I replaced the array with c++ vector and I used copy to move the memory etc... I faced one problem, by replacing the array with vector the calculation time of the control signal almost doubled by using vector.

Is there any disadvantages if I'm using alloc,memcpy,memset, free ins c++ code? If there is any what are those?

Answer 1

Vector adds value in 2 aspects:

1. Provides additional functionality which is not present in C array, such as resizing, checking for current size etc. You may find better alternatives from its interface to your custom solution.
2. Catches some bugs, such as addressing an address beyond the scope of the vector.

If you are satisfied with your proprietary data management and confident in quality of your code, you don't need vector. Note, though, this might pose some software issues, such as decreased maintainability of C array compared to vector (e.g., future code may access an out of bounds value, even if your current code doesn't).

Edit: See @Als answer for a possible alternative in your case (std::array).

Answer 2

Using manual memory management means you have more work to do. Therefore you have more opportunity to make errors.

Manual memory management is almost always wrong in C++. If you really need a naked array, you should at least use a wrapper like boost::scoped_array or boost::shared_array. Also std::unique_ptr can be used to manage arrays.

Answer 3

Vector has 1 major(relatively speaking) problem if you are inserting or deleting many objects, besides the case of inserting in begining/middle of the vector which is irrelevant here (array has same issue).

If you have large number of inserts to be done, you would typically add a call to reserve before the inserts. Call it as vector.reserve(<best guess number of objects to be inserted>)

This is because, as you would know, vector allocates memory on an as needed basis. If you are inserting a large number of items at a time, that might involve a lot of reallocations/copies which might consume a lot of time.

There are other solutions to this problem. One is to use std::deque which is a little more efficient in such cases as it does not copy over the entire vector every time. Or to use std::array which is a vector style wrapper around a C array.

Edit: The title has changed, but this information might still be useful..