C++11 基于范围的循环：按值获取项目或引用 const
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/15176104/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
C++11 range based loop: get item by value or reference to const
提问by masoud
Reading some examples of range based loops they suggest two main ways1, 2, 3, 4
阅读基于范围的循环的一些示例，他们提出了两种主要方式1, 2, 3, 4
std::vector<MyClass> vec;
for (auto &x : vec)
{
// x is a reference to an item of vec
// We can change vec's items by changing x
}
or
或者
for (auto x : vec)
{
// Value of x is copied from an item of vec
// We can not change vec's items by changing x
}
Well.
好。
When we don't need changing vecitems, IMO, Examples suggest to use second version (by value). Why they don't suggest something which constreferences (At least I have not found any direct suggestion):
当我们不需要更改vec项目时，IMO，示例建议使用第二个版本（按值）。为什么他们不建议const引用的东西（至少我没有找到任何直接的建议）：
for (auto const &x : vec) // <-- see const keyword
{
// x is a reference to an const item of vec
// We can not change vec's items by changing x
}
Isn't it better? Doesn't it avoid a redundant copy in each iteration while it's a const?
不是更好吗？当它是一个时，它不是在每次迭代中避免冗余副本const吗？
回答by Nawaz
If you don't want to change the items as well as want to avoidmaking copies, then auto const &is the correct choice:
如果您不想更改的项目，以及要避免复印，然后auto const &是正确的选择：
for (auto const &x : vec)
Whoever suggests you to use auto &is wrong. Ignore them.
谁建议你使用auto &是错误的。别理他们。
Here is recap:
这是回顾：
- Choose
auto xwhen you want to work with copies. - Choose
auto &xwhen you want to work with original items and may modify them. - Choose
auto const &xwhen you want to work with original items and will not modify them.
- 选择
auto x何时使用副本。 - 选择
auto &x何时使用原始项目并可以修改它们。 - 选择
auto const &x何时使用原始项目并且不修改它们。
回答by Mr.C64
If you have a std::vector<int>or std::vector<double>, then it's just fine to use auto(with value copy) instead of const auto&, since copying an intor a doubleis cheap:
如果您有std::vector<int>or std::vector<double>，那么使用auto（带值复制）代替 就可以了const auto&，因为复制 anint或 adouble很便宜：
for (auto x : vec)
....
But if you have a std::vector<MyClass>, where MyClasshas some non-trivial copy semantics (e.g. std::string, some complex custom class, etc.) then I'd suggest using const auto&to avoid deep-copies:
但是如果你有一个std::vector<MyClass>, whereMyClass有一些非平凡的复制语义（例如std::string，一些复杂的自定义类等），那么我建议使用const auto&以避免深层复制：
for (const auto & x : vec)
....
回答by Andy Prowl
When we don't need changing
vecitems, Examples suggest to use first version.
当我们不需要更改
vec项目时，示例建议使用第一个版本。
Then they give a wrong suggestion.
然后他们给出了错误的建议。
Why they don't suggest something which const references
为什么他们不建议 const 引用的东西
Because they give a wrong suggestion :-) What you mention is correct. If you only want to observean object, there is no need to create a copy, and there is no need to have a non-constreference to it.
因为他们给出了错误的建议 :-) 你提到的是正确的。如果只想观察一个对象，就没有必要创建副本，也没有必要对它进行非const引用。
EDIT:
编辑：
I see the references you link all provide examples of iterating over a range of intvalues or some other fundamental data type. In that case, since copying an intis not expensive, creating a copy is basically equivalent to (if not more efficient than) having an observing const &.
我看到您链接的所有参考资料都提供了在一系列int值或其他一些基本数据类型上进行迭代的示例。在这种情况下，由于复制 anint并不昂贵，因此创建副本基本上等同于（如果不是更有效）拥有一个观察const &。
This is, however, not the case in general for user-defined types. UDTs may be expensive to copy, and if you do not have a reason for creating a copy (such as modifying the retrieved object without altering the original one), then it is preferable to use a const &.
但是，对于用户定义的类型，通常情况并非如此。UDT 复制可能很昂贵，如果您没有创建副本的理由（例如修改检索到的对象而不更改原始对象），则最好使用const &.
回答by Benjamin Lindley
I'm going to be contrary here and say there is no need for auto const &in a range based for loop. Tell me if you think the following function is silly (not in its purpose, but in the way it is written):
我将在这里相反，说不需要auto const &基于范围的 for 循环。告诉我您是否认为以下函数很傻（不是它的目的，而是它的编写方式）：
long long SafePop(std::vector<uint32_t>& v)
{
auto const& cv = v;
long long n = -1;
if (!cv.empty())
{
n = cv.back();
v.pop_back();
}
return n;
}
Here, the author has created a const reference to vto use for all operations which do not modify v. This is silly, in my opinion, and the same argument can be made for using auto const &as the variable in a range based for loop instead of just auto &.
在这里，作者创建了一个 const 引用，v用于所有不修改 v 的操作。在我看来，这很愚蠢，并且可以auto const &在基于范围的 for 循环中用作变量而不是仅仅使用相同的参数auto &.
