blocks|key|2118588|text|您需要为要存档的内容声明一个二维数组。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2118589|Vertex+**faces+=+new+Vertex*[num_faces];
for(int+i=0;+i+<+num_faces;+%2B%2Bi)
++++faces[i]+=+new+Vertex[4];|code-block|syntax|javascript|2118590|当然，当你的num_faces是编译时间常数时，你可以像这样声明数组。|2118591|Vertex+faces[num_faces][4];|2118592|在c%2B%2B-faq+lite中可以找到多维动态数组的很好的资源|offset|length|2118593|entityMap|0|LINK|mutability|MUTABLE|url|http://www.parashift.com/c%2B%2B-faq-lite/freestore-mgmt.html#faq-16.16^0|0|0|0|0|1|C|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|10|8|@]|9|@$M|11|N|12|1|13]]|A|$]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

You need to declare a two dimensional array for what you want to archive.

<pre><code>Vertex **faces = new Vertex*[num_faces];
for(int i=0; i &lt; num_faces; ++i)
 faces[i] = new Vertex[4];
</code></pre>

Or of course when you num_faces is compile time constant you can just declare the array like this.

<pre><code>Vertex faces[num_faces][4];
</code></pre>

A good resource of dynamic arrays with multiple dimensions can be found in <a href="http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.16" rel="nofollow">c++-faq lite</a>

blocks|key|1534806|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1534807|无法从‘顶点(*)4’转换为‘顶点*’|blockquote|1534808|1534809|1534810|编译器已经告诉您需要知道的内容，只需更改faces的类型|offset|length|style|CODE|1534811|Vertex+(*faces)[4]+=+new+Vertex[num_faces][4];|code-block|syntax|javascript|1534812|entityMap^0|0|0|0|0|K|5|0|0^^$0|@$1|2|3|-4|4|5|6|S|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|T|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|U|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|V|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|W|7|@$H|X|I|Y|J|K]]|8|@]|9|$]]|$1|L|3|M|4|N|6|Z|7|@]|8|@]|9|$O|P]]|$1|Q|3|-4|4|5|6|10|7|@]|8|@]|9|$]]]|R|$]]

<blockquote>
<pre><code>cannot convert from 'Vertex (*)[4]' to 'Vertex *'
</code></pre>
</blockquote>

The compiler is already telling you what you need to know, just change the type of <code>faces</code>:

<pre><code>Vertex (*faces)[4] = new Vertex[num_faces][4];
</code></pre>

blocks|key|1534848|text|当向量做同样的事情时，永远不要使用数组。您需要的数据结构是std::vector<std::vector<Vertex>+>。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1534849|class+Vertex+{
++++...
++++Vertex(float+x,+float+y,+float+z)+:+x(x),+y(y),+z(z)+{}
};

int+main()+{
++++//+a+1D+array+of+vertices+is+a+polygon
++++std::vector<Vertex>+square;
++++square.push_back(Vertex(0,+0,+0));
++++square.push_back(Vertex(0,+1,+0));
++++square.push_back(Vertex(1,+1,+0));
++++square.push_back(Vertex(1,+0,+0));

++++//+Another+square,+in+a+different+plane
++++std::vector<Vertex>+square1;
++++square1.push_back(Vertex(0,+0,+1));
++++square1.push_back(Vertex(0,+1,+1));
++++square1.push_back(Vertex(1,+1,+1));
++++square1.push_back(Vertex(1,+0,+1));

++++//+A+2D+array+of+vertices+is+a+polyhedron.
++++std::vector<std::vector<Vertex>+>+cube;
++++cube.push_back(square);
++++cube.push_back(square1);
++++//+need+four+more+sides
}|code-block|syntax|javascript|1534850|注意完全没有指针、malloc或new。|1534851|为了提高可读性，我建议使用typedef+std::vector<Vertex>+polygon、typedef+std::vector<polygon>+polyhedron和自由使用boost::assign中的函数。|1534852|entityMap^0|T|X|0|0|9|6|G|3|0|D|Z|1D|13|2L|D|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|Z|8|@$9|10|A|11|B|C]|$9|12|A|13|B|C]|$9|14|A|15|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|P|$]]

Never use arrays when vectors will do the same thing. The data structure you need is <code>std::vector&lt;std::vector&lt;Vertex&gt; &gt;</code>.

<pre><code>class Vertex {
 ...
 Vertex(float x, float y, float z) : x(x), y(y), z(z) {}
};

int main() {
 // a 1D array of vertices is a polygon
 std::vector&lt;Vertex&gt; square;
 square.push_back(Vertex(0, 0, 0));
 square.push_back(Vertex(0, 1, 0));
 square.push_back(Vertex(1, 1, 0));
 square.push_back(Vertex(1, 0, 0));

 // Another square, in a different plane
 std::vector&lt;Vertex&gt; square1;
 square1.push_back(Vertex(0, 0, 1));
 square1.push_back(Vertex(0, 1, 1));
 square1.push_back(Vertex(1, 1, 1));
 square1.push_back(Vertex(1, 0, 1));

 // A 2D array of vertices is a polyhedron.
 std::vector&lt;std::vector&lt;Vertex&gt; &gt; cube;
 cube.push_back(square);
 cube.push_back(square1);
 // need four more sides
}
</code></pre>

Notice the complete lack of pointers, <code>malloc</code> or <code>new</code>.

For increased readability, I'd recommend <code>typedef std::vector&lt;Vertex&gt; polygon</code>, <code>typedef std::vector&lt;polygon&gt; polyhedron</code>, and liberal use of the functions from <code>boost::assign</code>.

blocks|key|2118643|text|“二维数组”是邪恶的.它们通常不能完全实现您希望实现的目标。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2118644|问题是，要编译你的代码，它要么需要改变索引，在这种情况下，你会得到：|2118645|Vertex+*faces[4]+=+{
++++new+Vertex[num_faces],
++++new+Vertex[num_faces],
++++new+Vertex[num_faces],
++++new+Vertex[num_faces]
};|code-block|syntax|javascript|2118646|否则你将不得不使用mkaes发布的代码。|2118647|这样做的问题是，它至少需要四次(如果不是五次)对new()的调用，理论上任何一次调用都可能失败。你也不能在一次操作中delete这个“东西”。|offset|length|style|CODE|2118648|对于像这样完全“类结构”的对象，可以使用以下技巧：|2118649|char+*memblk+=
++++malloc(num_faces+*+4+*+sizeof(Vertex)+%2B+num_faces+*+sizeof(Vertex+*));

Vertex+**faces+=+static_cast<Vertex+**>memblk;
Vertex+*first+=+static_cast<Vertex+*>(memblk+%2B+num_faces+*+sizeof(Vertex+*));

for(i+=+0;+i+<+num_faces;+first+%2B=+4,+i%2B%2B)
++++faces[i]+=+first;|2118650|其中，您分配的单个内存块大小足以容纳：|2118651|2118652|大小为Vertex+*的数组，大小为num_faces的and|ordered-list-item|2118653|num_faces数组，大小为4.|2118654|的数组|2118655|然后初始化前者以包含各自的起始地址。|2118656|要“删除”它，你需要再次使用C函数free(faces)+--但只需调用一次就可以了。|2118657|上面的方法实际上非常有效，并且与OpenGL顶点缓冲区对象用于数据的低级表示非常接近(实际上，可以使其与这些数据相匹配)。|2118658|entityMap^0|0|0|0|0|O|5|1M|6|0|0|0|0|0|3|8|I|9|0|0|9|F|1|0|0|0|H|B|0|0^^$0|@$1|2|3|4|5|6|7|1C|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1D|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|1E|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|1F|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|1G|8|@$M|1H|N|1I|O|P]|$M|1J|N|1K|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1L|8|@]|9|@]|A|$]]|$1|S|3|T|5|F|7|1M|8|@]|9|@]|A|$G|H]]|$1|U|3|V|5|6|7|1N|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1O|8|@]|9|@]|A|$]]|$1|X|3|Y|5|Z|7|1P|8|@$M|1Q|N|1R|O|P]|$M|1S|N|1T|O|P]]|9|@]|A|$]]|$1|10|3|11|5|Z|7|1U|8|@$M|1V|N|1W|O|P]|$M|1X|N|1Y|O|P]]|9|@]|A|$]]|$1|12|3|13|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|14|3|15|5|6|7|20|8|@]|9|@]|A|$]]|$1|16|3|17|5|6|7|21|8|@$M|22|N|23|O|P]]|9|@]|A|$]]|$1|18|3|19|5|6|7|24|8|@]|9|@]|A|$]]|$1|1A|3|-4|5|6|7|25|8|@]|9|@]|A|$]]]|1B|$]]

"Twodimensional arrays" are of evil ... they often don't quite do what you're hoping to achieve.

Thing is, to compile your code, it either needs to turn the indices around, in which case you get:

<pre><code>Vertex *faces[4] = {
 new Vertex[num_faces],
 new Vertex[num_faces],
 new Vertex[num_faces],
 new Vertex[num_faces]
};
</code></pre>

Or you'd have to use the code that mkaes posted.

The problem with this is that it requires at least four (if not five) calls to <code>new()</code>, any of which theoretically could fail. You can also not <code>delete</code> this "thing" in a single operation.

For fully "struct-like" objects as this one, one can do the following trick:

<pre><code>char *memblk =
 malloc(num_faces * 4 * sizeof(Vertex) + num_faces * sizeof(Vertex *));

Vertex **faces = static_cast&lt;Vertex **&gt;memblk;
Vertex *first = static_cast&lt;Vertex *&gt;(memblk + num_faces * sizeof(Vertex *));

for(i = 0; i &lt; num_faces; first += 4, i++)
 faces[i] = first;
</code></pre>

where you allocate a single block of memory large enough to hold:

<ol>
<li>an array of <code>Vertex *</code> of size <code>num_faces</code>, and</li>
<li><code>num_faces</code> arrays of <code>Vertex</code> of size <code>4</code>.</li>
</ol>

and then initialize the former to contain the respective starting addresses.

To "delete" this, you'd need to use again the C function <code>free(faces)</code> - but it's ok with a single call.

The above is actually quite efficient and corresponds closely to the sort of low-level representation that e.g. OpenGL Vertex buffer objects use for the data (in fact, it can be made to match those).

So I made my own class in order to hold a 3D vertex. That basically looks like:

<pre><code>class Vertex // Vertex Class
{
public:
 float x; // X Component
 float y; // Y Component
 float z; // Z Component

 float getX() {
 return x;
 }
 float getY() {
 return y;
 }
 float getZ() {
 return z;
 }
};
</code></pre>

Now I need to make a 2D array of these but when I initialize it it won't work. Basically each row will be a face of a polygon. And each column will contain a vertex of that row. So if one row is

(0,0,0) (1, 1, 1) (3, 3, 3) (4,4,4);

Then that row will represent a face with vertices (0,0,0) (1, 1, 1) (3, 3, 3) (4,4,4);

Now when I try to initialize it using 

<pre><code>Vertex faces = new Vertex[num_faces][4];
</code></pre>

It work work. This seems pretty simple so what am I doing wrong?

EDIT: I changed it to

<pre><code>Vertex *faces = new Vertex[num_faces][4];
</code></pre>

and I get this error:

cannot convert from 'Vertex (*)[4]' to 'Vertex *'

2D array help in C++

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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所以我做了我自己的类来保存3D顶点。这基本上看起来像是：class Vertex                                    // Vertex Class{public:    float x;                                // X Component   ...

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