类私有成员C ++的动态内存分配与std :: vector

Question

While learning c++ I usually (very often) encounter the following advice: "Avoid dynamic memory allocation as much as you can ; use std::vectors instead as they handle this for you".

So my question is: When do I have to use dynamic memory allocation? All the exercises I have done (I am just a beginner) are much easier using std::vector ; nevertheless my lecturer forces us to use dynamic memory for simple classes (like matrices, geometric vectors, etc.) and delete[] in the destructor.

The only advantage I have found so far for new;delete[]; (or at least what I tell my self in order to feel that it is worth using dynamic memory) is using move copy and move assignment.

Answer 1

std::vector also does dynamic memory allocation behind the scenes (by using the new operator). Copy and move assignment are also defined for std::vector , as you can see here , so there is no speed gained if you do it by hand.

Probably your question refers to when should you manually allocate memory (by explicitly using new and delete ), as opposed to relying on another class (such as vector) to do it for you.

The "Modern C++" answer to this question is to never do memory management by hand. If an std::vector does the job, then use that instead. If you need to allocate a single item, then use std::unique_ptr .

Answer 2

There are situations where you may need to implement a custom container (because neither std::vector nor any of the other standard library containers fit your purpose), and in those cases it may make sense to do the memory management manually.

However, unless you are providing the low-level core components in some industrial grade code base and have very specific goals, doing manual memory management is probably still not necessary here either.

I agree with the strict rules of your lecturer insofar that it pays to understand what happens behind the scenes. You rarely, if ever, should have to use manual memory management, but it helps to understand how std::unique_ptr / std::vector ( have to ) do it for you in order to understand why C++ was built the way it is. Teaching C++ in a way that is useful in the real world would eventually allow/force you to use standard library containers though, because there is (as you correctly noticed) essentially no reason to ever write a delete .

Answer 3

The lecturer most likely forces you so you get an idea of how things work under the hood. The std::vector class as you mentioned handles all the dynamic memory management for you, such that you do not need to write all the code to handle this yourself. It is easy, especially as a beginner to mess up while handling dynamic memory and create a leak/forget to deallocate something.

A vector is however not the solution for every problem, and you may find cases where it does not fit all your critera - and in those cases it could be better to implement your own data structure.