The rate at which the capacity of a vector grows is required by the standard to be exponential (which, IMHO, is over-specification). The standard specifies this in order to meet the amortized constant time requirement for the push_back operation. What amortized constant time means and how exponential growth achieves this is interesting. Every time a … Read more
You’ve been fooled. The iterator returned by std::back_inserter has it++ as a no-op. So those ‘gaps’ you are creating? Yeah that’s all doing nothing.
Use the .erase() method: // Remove the first N elements, and shift everything else down by N indices myvec.erase(myvec.begin(), myvec.begin() + N); This will require copying all of the elements from indices N+1 through the end. If you have a large vector and will be doing this frequently, then use a std::deque instead, which has … Read more
Because C++ is not C#/Java. std::vector<my_obj*> foo; This is a definition of an object, not a reference as in C#/Java. An object is a living instance of a type. new std::vector<my_obj*>() This expression returns a pointer. It returns a std::vector<my_obj*>*, which is not the same type as foo (the * at the end is what … Read more
See http://en.cppreference.com/w/cpp/language/default_initialization Default initialization is performed in three situations: when a variable with automatic storage duration is declared with no initializer when an object with dynamic storage duration is created by a new-expression without an initializer when a base class or a non-static data member is not mentioned in a constructor initializer list and that … Read more
You use a move_iterator with insert: v1.insert(v1.end(), make_move_iterator(v2.begin()), make_move_iterator(v2.end())); The example in 24.5.3 is almost exactly this. You’ll get the optimization you want if (a) vector::insert uses iterator-tag dispatch to detect the random-access iterator and precalculate the size (which you’ve assumed it does in your example that copies), and (b) move_iterator preserves the iterator category … Read more
There is no limit on how much automatic storage any std API uses. They could all require 12 terabytes of stack space. However, that API only requires Cpp17DefaultInsertable, and your implementation creates an extra instance over what is required by the constructor. Unless it is gated behind detecting the object is trivially ctorable and copyable, … Read more
Items in a vector must be assignable (or, in more recent versions of the standard, movable). const objects aren’t assignable, so attempting to store them in a vector will fail (or at least can fail — the code is invalid, but a compiler is free to accept it anyway, if it so chooses, though most … Read more
Both std::set and std::map are associative containers. The difference is that std::sets contain only the key, while in std::map there is an associated value. Choosing one over the other depends mainly on what the task at hand is. If you want to build a dictionary of all the words that appear in a text, you … Read more
In theory, it is called each time, since a for loop: for(initialization; condition; increment) body; is expanded to something like { initialization; while(condition) { body; increment; } } (notice the curly braces, because initialization is already in an inner scope) In practice, if the compiler understands that a piece of your condition is invariant through … Read more