C++ range for: To get item by value or by reference?

range for 就是 java 的 for each loop，C++ 里有三种形式：

std::vector<MyClass> vec;

for (auto x : vec) { ... }
for (auto &x : vec) { ... }
for (auto const &x : vec) { ... }

这三种形式是有区别的，尤其是 for (auto x : vec)，它其实会调用 copy constructor，将 vec 的元素 copy 一份给你，所以你操作 x 的时候就是在操作 copy，vec 的元素本身并没有改变。

C++11 range based loop: get item by value or reference to const 总结得非常好：

• Choose auto x when you want to work with copies.
• Choose auto &x when you want to work with original items and may modify them.
• Choose auto const &x when you want to work with original items and will not modify them.

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