blocks|key|407094|text|矢量传统上使用三个指针(存储的开始、结束和结束)来实现。缩小它们的唯一方法是真正地改变内部表示。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|407095|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|$]]

Vectors are traditionally implemented using three pointers (begin, end, and end of storage).
The only way to shrink them is to change the internal representation indeed.

blocks|key|1450552|text|不，你不能。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1450553|分配器最初的想法是，它们可以为数据的指针和引用类型定义对象。然而，这不再是真正允许的；allocator<T>::pointer必须是T*，并且容器可以自由地假定它是。|offset|length|style|CODE|1450554|因此，如果你真的需要一个向量来使用某种紧凑的指针表示或其他东西，你必须自己实现一个。|1450555|对于大多数应用来说，标准库容器都是很好的默认值。但对于那些有特殊需求的人，你可能不得不用其他东西来代替他们。这似乎就是这里的情况。|1450556|entityMap^0|0|17|L|1V|2|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@$D|P|E|Q|F|G]|$D|R|E|S|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|T|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|U|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|M|$]]

No, you can't.

The original idea with allocators was that they could define objects for pointers and reference types to the data. However, that's not really allowed anymore; <code>allocator&lt;T&gt;::pointer</code> must be <code>T*</code>, and containers are freely allowed to assume that it is.

So if you truly need a vector to use some kind of compact pointer representation or something, you're going to have to implement one yourself.

The standard library containers are good defaults for most uses. But for those with specialized needs, you may have to replace them with something else. That would appear to be the case here.

blocks|key|408115|text|这个大小内置于vector的实现中，您不能方便地减小它(构建在32位模式中当然会因为指针较小而减小大小)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|408116|也许，如果您在代码中使用了数万亿个vectors，那么您可能想要尝试减少这个数字(将它们组合成一个更大的vector?)而不是试图将每个字节都刮掉几个字节。|408117|entityMap^0|7|6|0|H|7|1G|6|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|R|8|@]|D|@]|E|$]]]|I|$]]

This size is built in to the <code>vector</code>'s implementation, and you can't reduce that conveniently (building in 32-bit mode would of course reduce the size due to smaller pointer sizes).

Perhaps if you're using trillions of <code>vectors</code> in your code you might want to try to reduce that number (combine them into a larger <code>vector</code>?) rather than trying to shave a couple bytes off each one.

blocks|key|1450633|text|有一些事情需要弄清楚:当向量大小增加时，向量大小和容量数据变得相对非常非常小。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1450634|假设你有一个大小为100万的双倍向量。大小和容量数据总共仍将占用8个字节，与向量持有的主要数据相比，这是相当小的。|1450635|如果您经常使用非常小的向量，比如从1到50，那么您可以向数组编写一个包装器并使用它，而std向量对于小型数据组可能不是一个好的选择。我不知道你的内存是100TB，还是只是你的硬盘容量，但保留许多小尺寸的数据结构可能不是一个好主意，因为这可能会导致严重的memory+fragmentation。|offset|length|style|CODE|1450636|entityMap^0|0|0|3I|K|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|N|8|@$F|O|G|P|H|I]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|K|$]]

There is something to clear up: vector size and capacity data become relatively very very small when vector size grows. 

Lets say you have a double vector of size 1 million. Size and capacity data will still occupy 8 bytes in total and it is quite minute compared to the main data the vector holds. 

If you frequently use very small sized vectors like from 1 to 50, then you can write a wrapper to arrays and use it and std vectors may not be a good choice for small sized data groups . I do not know whether your RAM is 100TB or it is just your hard drive capacity but keeping many small sized data structures may not be a good idea while it can result in severe <code>memory fragmentation</code>s.

I'm using Linux on a 64-bit machine and I use g++4.8.
The <code>sizeof</code> operator applied on a <code>std::vector</code> returns :

<pre><code>sizeof(std::vector&lt;float&gt;) = 24
sizeof(std::vector&lt;double&gt;) = 24
sizeof(std::vector&lt;long double&gt;) = 24
</code></pre>

My question is : is there any way to create a vector with a smaller size based on the fact that my application will never need vectors of more than <code>1 GB</code> of elements ? Because optimally, I could have a vector of 16 bytes : the pointer to the beginning (8 bytes), the current size (4 bytes) and the current capacity (4 bytes). Do I have to recode my own vector from scratch or can I reduce the size with a custom allocator ?

This size is very critical for me as I work on supercomputers with more than 100 TB of memory, and every byte I can gain on my basic classes could finally save several TB of memory.

Reducing the sizeof a vector using custom allocator?

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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我在一台64位机器上使用Linux，我使用的是g++4.8。应用于std::vector的sizeof运算符返回：sizeof(std::vector<float>) = 24sizeof(std::vector<double>) = 24sizeof(std::vector<long double>) = 24我的问...

问使用自定义分配器减少向量的大小？
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问使用自定义分配器减少向量的大小？EN