blocks|key|1129477|text|您不能创建抽象类类型的向量，因为您不能创建抽象类的实例，并且像std::vector这样的C%2B%2B标准库容器存储值(即实例)。如果你想这样做，你必须创建一个指向抽象类类型的指针向量。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1129478|您的变通方法不会起作用，因为虚函数(这就是您首先想要抽象类的原因)只有在通过指针或引用调用时才起作用。你也不能创建引用的向量，所以这是你必须使用指针向量的第二个原因。|1129479|您应该意识到，C%2B%2B和C#几乎没有什么共同之处。如果你打算学习C%2B%2B，你应该认为它是从头开始的，并阅读一本很好的专门的C%2B%2B教程，比如Koenig和Moo的Accelerated+C%2B%2B。|offset|length|1129480|entityMap|0|LINK|mutability|MUTABLE|url|http://www.acceleratedcpp.com/^0|0|0|26|F|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@$F|S|G|T|1|U]]|A|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|O]]]]

You can't create a vector of an abstract class type because you cannot create instances of an abstract class, and C++ Standard Library containers like std::vector store values (i.e. instances). If you want to do this, you will have to create a vector of pointers to the abstract class type.

Your workround would not work because virtual functions (which is why you want the abstract class in the first place) only work when called through pointers or references. You cannot create vectors of references either, so this is a second reason why you must use a vector of pointers.

You should realise that C++ and C# have very little in common. If you are intending to learn C++, you should think of it as starting from scratch, and read a good dedicated C++ tutorial such as <a href="http://www.acceleratedcpp.com/" rel="noreferrer">Accelerated C++</a> by Koenig and Moo.

blocks|key|1344753|text|您不能实例化抽象类，因此抽象类的向量不能工作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1344754|但是，您可以使用指向抽象类的指针向量：|1344755|std::vector<IFunnyInterface*>+ifVec;|code-block|syntax|javascript|1344756|这也允许你实际使用多态行为--即使类不是抽象的，按值存储也会导致object+slicing的问题。|offset|length|1344757|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Object_slicing^0|0|0|0|W|E|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@$K|Y|L|Z|1|10]]|A|$]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

You can't instantiate abstract classes, thus a vector of abstract classes can't work.

You can however use a vector of pointers to abstract classes:

<pre><code>std::vector&lt;IFunnyInterface*&gt; ifVec;
</code></pre>

This also allows you to actually use polymorphic behaviour - even if the class wasn't abstract, storing by value would lead to the problem of <a href="http://en.wikipedia.org/wiki/Object_slicing" rel="noreferrer">object slicing</a>.

blocks|key|1344830|text|因为要调整向量的大小，需要使用默认的构造函数和类的大小，这反过来又要求它是具体的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1344831|您可以按照其他人的建议使用指针。|1344832|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Because to resize a vector you need to use the default constructor and the size of the class, which in turn requires it to be concrete.

You can use a pointer as other suggested.

blocks|key|1129607|text|我认为这种令人遗憾的限制的根本原因是构造函数不能虚化。因此，在不知道对象在编译时的时间的情况下，编译器不能生成复制对象的代码。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1129608|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I think that the root cause of this really sad limitation is the fact that constructors can not virtual. Thereof compiler can not generate code which copy the object without knowing its time in the compile time.

blocks|key|1129653|text|vector将尝试分配内存以包含您的类型。如果您的类是纯虚拟的，则向量无法知道它必须分配的类的大小。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1129654|我认为，使用您的变通方法，您将能够编译vector<IFunnyInterface>，但不能在其中操作FunnyImpl。例如，如果IFunnyInterface+(抽象类)的大小是20+(我真的不知道)，而FunnyImpl的大小是30，因为它有更多的成员和代码，那么您最终会尝试将30放入20的向量中|offset|length|style|CODE|1129655|解决方案是使用"new“在堆上分配内存，并将指针存储在vector<IFunnyInterface*>中|1129656|entityMap^0|0|J|N|0|R|O|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@$D|Q|E|R|F|G]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|K|$]]

std::vector will try to allocate memory to contain your type. If your class is purely virtual, the vector cannot know the size of the class it will have to allocate.

I think that with your workaround, you will be able to compile a <code>vector&lt;IFunnyInterface&gt;</code> but you won't be able to manipulate FunnyImpl inside of it. For example if IFunnyInterface (abstract class) is of size 20 (i dont really know) and FunnyImpl is of size 30 because it has more members and code, you will end up trying to fit 30 into your vector of 20

The solution would be to allocate memory on the heap with "new" and store pointers in <code>vector&lt;IFunnyInterface*&gt;</code>

blocks|key|1129695|text|在这种情况下，我们甚至不能使用以下代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1129696|std::vector+<IFunnyInterface*>+funnyItems;|code-block|syntax|javascript|1129697|或|1129698|std::vector+<std::tr1::shared_ptr<IFunnyInterface>+>+funnyItems;|1129699|因为FunnyImpl和IFunnyInterface之间不存在A关系，并且由于私有继承，FUnnyImpl和IFunnyInterface之间不存在隐式转换。|1129700|您应该按如下方式更新代码：|1129701|class+IFunnyInterface
{
public:
++++virtual+void+IamFunny()++=+0;
};

class+FunnyImpl:+public+IFunnyInterface
{
public:
++++virtual+void+IamFunny()
++++{
++++++++cout+<<+"<INSERT+JOKE+HERE>";
++++}
};|1129702|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

In this case we can't use even this code:

<pre><code>std::vector &lt;IFunnyInterface*&gt; funnyItems;
</code></pre>

or

<pre><code>std::vector &lt;std::tr1::shared_ptr&lt;IFunnyInterface&gt; &gt; funnyItems;
</code></pre>

Because there is no IS A relationship between FunnyImpl and IFunnyInterface and there is no implicit convertion between FUnnyImpl and IFunnyInterface because of private inheritance.

You should update your code as follows:

<pre><code>class IFunnyInterface
{
public:
 virtual void IamFunny() = 0;
};

class FunnyImpl: public IFunnyInterface
{
public:
 virtual void IamFunny()
 {
 cout &lt;&lt; "&lt;INSERT JOKE HERE&gt;";
 }
};
</code></pre>

Having spent quite some time developping in C#, I noticed that if you declare an abstract class for the purpose of using it as an interface you cannot instantiate a vector of this abstract class to store instances of the children classes.

<pre><code>#pragma once
#include &lt;iostream&gt;
#include &lt;vector&gt;

using namespace std;

class IFunnyInterface
{
public:
 virtual void IamFunny() = 0;
};

class FunnyImpl: IFunnyInterface
{
public:
 virtual void IamFunny()
 {
 cout &lt;&lt; "&lt;INSERT JOKE HERE&gt;";
 }
};

class FunnyContainer
{
private:
 std::vector &lt;IFunnyInterface&gt; funnyItems;
};
</code></pre>

The line declaring the vector of abstract class causes this error in MS VS2005:

<pre><code>error C2259: 'IFunnyInterface' : cannot instantiate abstract class
</code></pre>

I see an obvious workaround, which is to replace IFunnyInterface with the following:

<pre><code>class IFunnyInterface
{
public:
 virtual void IamFunny()
 {
 throw new std::exception("not implemented");
 }
};
</code></pre>

Is this an acceptable workaround C++ wise ?
If not, is there any third party library like boost which could help me to get around this ?

Thank you for reading this !

Anthony

Why can't we declare a std::vector<AbstractClass>?

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在C#中花了相当多的时间进行开发之后，我注意到，如果为了将其用作接口而声明一个抽象类，则无法实例化该抽象类的向量来存储子类的实例。#pragma once#include <iostream>#include <vector>using namespace std;class IFunnyInterface{publi...

问为什么我们不能声明一个std::vector<AbstractClass>呢？
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