Question

是否实际上保证了以下降低能力陷阱“工作”?

int main() {
   std::string s = "lololololol";
   s = "";                        // capacity still non-zero

   string(s).swap(s);             // ?
}

对我来说,“工作”似乎只是“工作”(因为能力仍然是非零),我可以发现标准中的任何内容,即“参与者”必须在两个[相同]物体之间交换。

同样,关于序列集装箱:

int main() {
   vector<int> v { 1,2,3,4,5 };
   v.clear();                   // capacity still non-zero

   vector<int>(v).swap(v);      // ?
}

As far as I m aware, this "trick" is semi-widely used; perhaps this widespread adoption is misguided?

<>up>(课程,C++11,我们有shrink_to_fit [尽管不具约束力],但还是使这种模糊不清。

Answer 1

I ve always been taught that there is no guaranteed standard way to lower the capacity. All methods have been (and still are) implementation defined.

第23.2.18条指出:

The expression a.swap(b), for containers a and b of a standard container type other than array, shall exchange the values of a and b without invoking any move, copy, or swap operations on the individual container elements...

This guarantees that the internal pointers of vectors must be swapped.
However, I cannot find anything that guarantee on the capacity of a newly created vector.

§ 21.4.21 says that one of the basic_string default constructor s post conditions is that capacity() returns an unspecified value.
§ 21.4.23 says that one of the basic_string copy constructor s post conditions is that capacity() returns a value at least as big as size().
§ 21.4.6.82 says that string::swap runs in constant time, which (effectively) requires that the internal pointers are swapped.

就我所知,符合规定的执行可以有<编码>,其中:最大程度(){返回4;,并将所有内部从一个缓冲地带向另一个缓冲地带进行互换。 (主人可以这样做)

显然,这都是用盐类提取的。取自C++草案的Im,从Feb28, 11, ;我可以找到病媒复印机的规格。另外,没有找到for的证据与查询证据并不相同。

Answer 2

rel=“nofollow”>errata page for “Effective STL,” Scott Meyers notes:

When string implementations use reference counting, the swap trick using the copy constructor doesn t decrease the capacity, because the copy constructor isn t allocating any memory; it s just adjusting a reference count. A more reliable way to perform shrink-to-fit is to create the temporary string via the range constructor, e.g., string(s.begin(), s.end()).swap(s); This version of the swap trick is safer for vectors, too, because it eliminates any chance that the copy constructor will copy the other vector s excess capacity (which implementations are permitted to do).

关于担保,Meyers指出:

The language police require that I inform you that there s no guarantee that this technique will truly eliminate excess capacity. Implementers are free to give vectors and strings excess capacity if they want to, and sometimes they want to. [Effective STL, Item 17]

Answer 3

http://www.gotw.ca/gotw/054.htm“rel=“nofollow”http://www.gotw.ca/gotw/054.htm:

Some implementations may choose to round up the capacity slightly to their next larger internal "chunk size," with the result that the capacity actually ends up being slightly larger than the size.

Answer 4

如何界定这项工作,可能完全是执行性的。与病媒等集装箱不同的是,铺设装置可产生非常不同的执行。

如果扼杀性执行工作采用小规模优化,那么你的能力就会低于某一门槛值。如果扼杀装置使用印本,则不写字,不作真实复制。

http://www.gotw.ca/gotw/054.htm rel=“nofollow”http://www.gotw.ca/gotw/054.htm, shrink-to-fit and clear-completely are different tricks. 如果意图是完全清楚的,那么,如果用不设建筑的扼杀手段进行抽取,就会取得更好的结果。

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