我见过在为基于范围的for循环声明范围时使用的许多类型说明符，如:

#include <iostream>
#include <vector>
 
int main() {
    std::vector<int> v = {0, 1, 2, 3, 4, 5};
 
    for (const int& i : v) // access by const reference
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (auto i : v) // access by value, the type of i is int
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (auto&& i : v) // access by forwarding reference, the type of i is int&
        std::cout << i << ' ';
    std::cout << '\n';
 
    const auto& cv = v;
 
    for (auto&& i : cv) // access by f-d reference, the type of i is const int&
        std::cout << i << ' ';
    std::cout << '\n';
}

// some aren't mentioned here, I just copied this snippet from cppreference

预计哪一种性能更快（在-O2或-O3优化后）？ 选择是否取决于vector的数据类型，还是取决于我们在循环中执行的操作。

附注:循环内的cout仅用于演示。