C ++向量对象的指针 [英] C++ Vector of Pointers to Objects
问题描述
我使用指向对象的向量。
I'm using a vector of pointers to objects. These objects are derived from a base class, and are being dynamically allocated and stored.
例如，我有类似的东西：
For example, I have something like:
vector<Enemy*> Enemies;
，我将从Enemy类派生，然后为派生类动态分配内存，例如这：
and I'll be deriving from the Enemy class and then dynamically allocating memory for the derived class, like this:
enemies.push_back(new Monster());
我需要注意什么才能避免内存泄漏和其他问题？
What are things I need to be aware of to avoid memory leaks and other problems?
推荐答案
std :: vector 会像往常一样管理你的内存，但这个内存将是指针，而不是对象。
std::vector will manage the memory for you, like always, but this memory will be of pointers, not objects.
这意味着你的类将在内存中丢失，一旦你的向量超出范围。例如：
What this means is that your classes will be lost in memory once your vector goes out of scope. For example:
#include <vector>
struct base
{
virtual ~base() {}
};
struct derived : base {};
typedef std::vector<base*> container;
void foo()
{
container c;
for (unsigned i = 0; i < 100; ++i)
c.push_back(new derived());
} // leaks here! frees the pointers, doesn't delete them (nor should it)
int main()
{
foo();
}
您需要做的是确保删除所有对象该向量超出范围：
What you'd need to do is make sure you delete all the objects before the vector goes out of scope:
#include <algorithm>
#include <vector>
struct base
{
virtual ~base() {}
};
struct derived : base {};
typedef std::vector<base*> container;
template <typename T>
void delete_pointed_to(T* const ptr)
{
delete ptr;
}
void foo()
{
container c;
for (unsigned i = 0; i < 100; ++i)
c.push_back(new derived());
// free memory
std::for_each(c.begin(), c.end(), delete_pointed_to<base>);
}
int main()
{
foo();
}
这很难维护，因为我们必须记住执行一些行动。更重要的是，如果在元素的分配和重新分配循环之间发生异常，重新分配循环将永远不会运行，并且您仍然遇到内存泄漏！这称为异常安全，这是解除分配需要自动完成的一个关键原因。
This is difficult to maintain, though, because we have to remember to perform some action. More importantly, if an exception were to occur in-between the allocation of elements and the deallocation loop, the deallocation loop would never run and you're stuck with the memory leak anyway! This is called exception safety and it's a critical reason why deallocation needs to be done automatically.
更好的是如果指针删除自己。这些称为智能指针，标准库提供 std :: unique_ptr 和 std :: shared_ptr 。
Better would be if the pointers deleted themselves. Theses are called smart pointers, and the standard library provides std::unique_ptr and std::shared_ptr.
std :: unique_ptr 表示指向某些资源的唯一（非共享，单一所有者）指针。这应该是您的默认智能指针，并完全替换任何原始指针使用。
std::unique_ptr represents a unique (unshared, single-owner) pointer to some resource. This should be your default smart pointer, and overall complete replacement of any raw pointer use.
auto myresource = /*std::*/make_unique<derived>(); // won't leak, frees itself
std :: make_unique 从C ++ 11标准中缺少监督，但你可以自己做一个。要直接创建 unique_ptr （不建议在 make_unique 如果可以），请执行以下操作：
std::make_unique is missing from the C++11 standard by oversight, but you can make one yourself. To directly create a unique_ptr (not recommended over make_unique if you can), do this:
std::unique_ptr<derived> myresource(new derived());
唯一指针只有移动语义;它们不能复制：
Unique pointers have move semantics only; they cannot be copied:
auto x = myresource; // error, cannot copy
auto y = std::move(myresource); // okay, now myresource is empty
这是我们需要在容器中使用它：
And this is all we need to use it in a container:
#include <memory>
#include <vector>
struct base
{
virtual ~base() {}
};
struct derived : base {};
typedef std::vector<std::unique_ptr<base>> container;
void foo()
{
container c;
for (unsigned i = 0; i < 100; ++i)
c.push_back(make_unique<derived>());
} // all automatically freed here
int main()
{
foo();
}
shared_ptr 计数复制语义;它允许多个所有者共享对象。它跟踪一个对象存在多少 shared_ptr ，并且当最后一个不存在（该计数变为零）时，它释放指针。复制只是增加了引用计数（并且以更低的，几乎免费的成本转移所有权）。你使用 std :: make_shared （或直接如上所示，但因为 shared_ptr 必须在内部进行分配，使用 make_shared ）通常更高效，技术上更安全。
shared_ptr has reference-counting copy semantics; it allows multiple owners sharing the object. It tracks how many shared_ptrs exist for an object, and when the last one ceases to exist (that count goes to zero), it frees the pointer. Copying simply increases the reference count (and moving transfers ownership at a lower, almost free cost). You make them with std::make_shared (or directly as shown above, but because shared_ptr has to internally make allocations, it's generally more efficient and technically more exception-safe to use make_shared).
#include <memory>
#include <vector>
struct base
{
virtual ~base() {}
};
struct derived : base {};
typedef std::vector<std::shared_ptr<base>> container;
void foo()
{
container c;
for (unsigned i = 0; i < 100; ++i)
c.push_back(std::make_shared<derived>());
} // all automatically freed here
int main()
{
foo();
}
记住，你通常想使用 std :: unique_ptr 作为默认值，因为它更轻量级。另外， std :: shared_ptr 可以由 std :: unique_ptr 构成（反之亦然），所以
Remember, you generally want to use std::unique_ptr as a default because it's more lightweight. Additionally, std::shared_ptr can be constructed out of a std::unique_ptr (but not vice versa), so it's okay to start small.
或者，您可以使用创建的容器来存储指向对象的指针，例如 boost :: ptr_container ：
Alternatively, you could use a container created to store pointers to objects, such as a boost::ptr_container:
#include <boost/ptr_container/ptr_vector.hpp>
struct base
{
virtual ~base() {}
};
struct derived : base {};
// hold pointers, specially
typedef boost::ptr_vector<base> container;
void foo()
{
container c;
for (int i = 0; i < 100; ++i)
c.push_back(new Derived());
} // all automatically freed here
int main()
{
foo();
}
而 boost :: ptr_vector< T& code>在C ++ 03中有明显的用途，我现在不能说相关性，因为我们可以使用 std :: vector< std :: unique_ptr< T>>
While boost::ptr_vector<T> had obvious use in C++03, I can't speak of the relevance now because we can use std::vector<std::unique_ptr<T>> with probably little to no comparable overhead, but this claim should be tested.
无论如何，永远不要在代码中显式地释放任何东西 。包装，以确保资源管理自动处理。
Regardless, never explicitly free things in your code. Wrap things up to make sure resource management is dealt with automatically. You should have no raw owning pointers in your code.
作为游戏中的默认值，我可能会使用 std :: vector< std :: shared_ptr< T>> 。我们期望共享，无论如何，它足够快，直到剖析说，否则，它的安全，它很容易使用。
As a default in a game, I would probably go with std::vector<std::shared_ptr<T>>. We expect sharing anyway, it's fast enough until profiling says otherwise, it's safe, and it's easy to use.
这篇关于C ++向量对象的指针的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
