std :: map中值的内存分配

Question

I've had some experience in C++ from school works. I've learned, among other things, that objects should be stored in a container (vector, map, etc) as pointers. The main reason being that we need the use of the new-operator, along with a copy constructor, in order to create a copy on the heap (otherwise called dynamic memory) of the object. This method also necessitates defining a destructor.

However, from what I've read since then, it seems that STL containers already store the values they contain on the heap. Thus, if I were to store my objects as values, a copy (using the copy constructor) would be made on the heap anyway, and there would be no need to define a destructor. All in all, a copy on the heap would be made anyway???

Also, if(true), then the only other reason I can think of for storing objects using pointers would be to alleviate resource needs for copying the container, as pointers are easier to copy than whole objects. However, this would require the use of std::shared_ptr instead of regular pointers, since you don't want elements in the copied container to be deleted when the original container is destroyed. This method would also alleviate the need for defining a destructor, wouldn't it?

Edit : The destructor to be defined would be for the class using the container, not for the class of the objects stored.

Edit 2 : I guess a more precise question would be : "Does it make a difference to store objects as pointers using the new-operator, as opposed to plain values, on a memory and resources used standpoint?"

Answer 1

The main reason to avoid storing full objects in containers (rather than pointers) is because copying or moving those objects is expensive. In that case, the recommended alternative is to store smart pointers in the container.

So...

vector<something_t> ................. Usually perfectly OK
vector<shared_ptr<something_t>> ..... Preferred if you want pointers
vector<something_t*> ................ Usually best avoided

The problem with raw pointers is that, when a raw pointer disappears, the object it points to hangs around causing memory and resource leaks - unless you've explicitly deleted it. C++ doesn't have garbage collection, and when a pointer is discarded, there's no way to know if other pointers may still be pointing to that object.

Raw pointers are a low-level tool - mostly used to write libraries such as vector and shared_ptr. Smart pointers are a high-level tool.

However, particularly with C++11 move semantics, the costs of moving items around in a vector is normally very small even for huge objects. For example, a vector<string> is fine even if all the strings are megabytes long. You mostly worry about the cost of moving objects if sizeof(classname) is big - if the object holds lots of data inside itself rather than in separate heap-allocated memory.

Even then, you don't always worry about the cost of moving objects. It doesn't matter that moving an object is expensive if you never move it. For example, a map doesn't need to move items around much. When you insert and delete items, the nodes (and contained items) stay where they are, it's just the pointers that link the nodes that change.