Element access time in a typical implementation of a std::vector is the same as element access time in an ordinary array available through a pointer object (i.e. a run-time pointer value) std::vector<int> v; int *pa; … v[i]; pa[i]; // Both have the same access time However, the access time to an element of an array … Read more
The problem you’re having is that the compiler doesn’t know which versions of your template to instantiate. When you move the implementation of your function to x.cpp it is in a different translation unit from main.cpp, and main.cpp can’t link to a particular instantiation because it doesn’t exist in that context. This is a well-known … Read more
That’s because push_back puts new elements onto the end of the vector. You can see the effect by running i to 9: the negative numbers will occupy v[5] to v[9]. Writing std::vector<int> v{-1, -2, -3, -4, -5}; instead is a particularly elegant fix.
Vector is another name for dynamic arrays. It is the name used for the dynamic array data structure in C++. If you have experience in Java you may know them with the name ArrayList. (Java also has an old collection class called Vector that is not used nowadays because of problems in how it was … Read more
You need to be careful because erase() will invalidate existing iterators. However, it will return a new valid iterator you can use: for ( it = Entities.begin(); it != Entities.end(); ) { if( (*it)->getXPos() > 1.5f ) { delete * it; it = Entities.erase(it); } else { ++it; } }
Although your question is not very clear, I’m going to assume you mean a 2D vector to mean a vector of vectors: vector< vector<int> > vvi; Then you need to use two iterators to traverse it, the first the iterator of the “rows”, the second the iterators of the “columns” in that “row”: //assuming you … Read more
subvec is probably the best way. The Clojure docs say subvec is “O(1) and very fast, as the resulting vector shares structure with the original and no trimming is done“. The alternative would be walking the vector and building a new one while skipping certain elements, which would be slower. Removing elements from the middle … Read more
It’s useful if you have an idea how many elements the vector will ultimately hold – it can help the vector avoid repeatedly allocating memory (and having to move the data to the new memory). In general it’s probably a potential optimization that you shouldn’t need to worry about, but it’s not harmful either (at … Read more
Synchronized collections are a waste of time and dangerous. A trivial example why they are bad is to consider two threads running a loop at the same time on the same collection: int i = 0; while (i < list.size()) { if (testSomeCondition(list.get())) { list.remove(i); else i++; } Our list could be synchronized (e.g. a … Read more
The reason is that an unchecked access can probably be done with a single processor instruction. A checked access will also have to load the size from memory, compare it with the index, and (assuming it’s in range) skip over a conditional branch to the error handler. There may be more faffing around to handle … Read more