blocks|key|240238|text|是的，它是安全的，标准库实现也是如此。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|240239|我相信实现者将这个需求追溯到23.2/11，但我不知道是怎么回事，我也找不到更具体的东西。我能找到的最好的是这篇文章：|240240|http://www.drdobbs.com/cpp/copying-container-elements-from-the-c-li/240155771|offset|length|240241|对libc%2B%2B和libstdc%2B%2B实现的检查表明，它们也是安全的。|240242|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|25|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|S|8|@]|9|@$F|T|G|U|1|V]]|A|$]]|$1|H|3|I|5|6|7|W|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|E]]]]

Yes, it's safe, and standard library implementations jump through hoops to make it so.

I believe implementers trace this requirement back to 23.2/11 somehow, but I can't figure out how, and I can't find something more concrete either. The best I can find is this article:

<a href="http://www.drdobbs.com/cpp/copying-container-elements-from-the-c-li/240155771">http://www.drdobbs.com/cpp/copying-container-elements-from-the-c-li/240155771</a>

Inspection of libc++'s and libstdc++'s implementations shows that they are also safe.

blocks|key|24016|text|第一个示例是否安全并不明显，因为push_back最简单的实现是，如果需要，首先重新分配向量，然后复制引用。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|24017|但至少在Visual+Studio2010中看起来是安全的。它的push_back实现在您推回向量中的元素时会对这种情况进行特殊处理。代码的结构如下：|24018|void+push_back(const+_Ty&+_Val)
++++{+++//+insert+element+at+end
++++if+(_Inside(_STD+addressof(_Val)))
++++++++{+++//+push+back+an+element
++++++++++++++++++++...
++++++++}
++++else
++++++++{+++//+push+back+a+non-element
++++++++++++++++++++...
++++++++}
++++}|code-block|syntax|javascript|24019|entityMap^0|G|9|0|W|9|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|R|8|@$9|S|A|T|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|U|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

It is not obvious that the first example is safe, because the simplest implementation of <code>push_back</code> would be to first reallocate the vector, if needed, and then copy the reference.

But at least it seems to be safe with Visual Studio 2010. Its implementation of <code>push_back</code> does special handling of the case when you push back an element in the vector.
The code is structured as follows:

<pre><code>void push_back(const _Ty&amp; _Val)
 { // insert element at end
 if (_Inside(_STD addressof(_Val)))
 { // push back an element
 ...
 }
 else
 { // push back a non-element
 ...
 }
 }
</code></pre>

blocks|key|24077|text|这不是标准的保证，但是作为另一个数据点，v.push_back(v[0])对于LLVM's+libc%2B%2B是安全的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|24078|当libc%2B%2B's+std::vector::push_back需要重新分配内存时，它调用__push_back_slow_path：|24079|void+__push_back_slow_path(_Up&+__x)+{
++allocator_type&+__a+=+this->__alloc();
++__split_buffer<value_type,+allocator_type&>+__v(__recommend(size()+%2B+1),+
++++++++++++++++++++++++++++++++++++++++++++++++++size(),+
++++++++++++++++++++++++++++++++++++++++++++++++++__a);
++//+Note+that+we+construct+a+copy+of+__x+before+deallocating
++//+the+existing+storage+or+moving+existing+elements.
++__alloc_traits::construct(__a,+
++++++++++++++++++++++++++++_VSTD::__to_raw_pointer(__v.__end_),+
++++++++++++++++++++++++++++_VSTD::forward<_Up>(__x));
++__v.__end_%2B%2B;
++//+Moving+existing+elements+happens+here:
++__swap_out_circular_buffer(__v);
++//+When+__v+goes+out+of+scope,+__x+will+be+invalid.
}|code-block|syntax|javascript|24080|entityMap|0|LINK|mutability|MUTABLE|url|http://libcxx.llvm.org/|1|http://llvm.org/svn/llvm-project/libcxx/trunk/include/vector^0|K|H|13|D|0|0|A|M|19|L|1|V|1|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@$9|17|A|18|1|19]]|E|$]]|$1|H|3|I|5|J|7|1A|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]|U|$5|P|Q|R|E|$S|V]]]]

This isn't a guarantee from the standard, but as another data point, <code>v.push_back(v[0])</code> is safe for <a href="http://libcxx.llvm.org/" rel="nofollow">LLVM's libc++</a>.

<a href="http://llvm.org/svn/llvm-project/libcxx/trunk/include/vector" rel="nofollow">libc++'s <code>std::vector::push_back</code></a> calls <code>__push_back_slow_path</code> when it needs to reallocate memory:

<pre><code>void __push_back_slow_path(_Up&amp; __x) {
 allocator_type&amp; __a = this-&gt;__alloc();
 __split_buffer&lt;value_type, allocator_type&amp;&gt; __v(__recommend(size() + 1), 
 size(), 
 __a);
 // Note that we construct a copy of __x before deallocating
 // the existing storage or moving existing elements.
 __alloc_traits::construct(__a, 
 _VSTD::__to_raw_pointer(__v.__end_), 
 _VSTD::forward&lt;_Up&gt;(__x));
 __v.__end_++;
 // Moving existing elements happens here:
 __swap_out_circular_buffer(__v);
 // When __v goes out of scope, __x will be invalid.
}
</code></pre>

<pre><code>vector&lt;int&gt; v;
v.push_back(1);
v.push_back(v[0]);
</code></pre>

If the second push_back causes a reallocation, the reference to the first integer in the vector will no longer be valid. So this isn't safe?

<pre><code>vector&lt;int&gt; v;
v.push_back(1);
v.reserve(v.size() + 1);
v.push_back(v[0]);
</code></pre>

This makes it safe?

Is it safe to push_back an element from the same vector?

Visual Studio

vector<int> v;v.push_back(1);v.push_back(v[0]);如果第二个push_back导致重新分配，则对向量中第一个整数的引用将不再有效。所以这不安全？vector<int> v;v.push_back(1);v.reserve(v.size() + 1);v.push_back(v...

问从相同的向量push_back一个元素是安全的吗？
EN

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问从相同的向量push_back一个元素是安全的吗？
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