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C++11中引入了对基于范围的for循环(range-based for loop)的支持:该循环对一系列值(例如容器中的所有元素)进行操作。代码段如下:
- const std::vector<int> vec{ 1,2,3,4,5 };
- for (const auto& i : vec)
- std::cout << i << ", ";
- std::cout << std::endl;
C++17中增加对结构化绑定(structured bingding)声明的支持。代码段如下:
- std::map<int, int> m{ {1,-1}, {2,-2}, {3,-3} };
- for (const auto& [key, value] : m)
- std::cout << key << "," << value << "; ";
- std::cout << std::endl;
C++20中增加支持声明任意多个变量,必须以分号结尾。代码段如下:
- const std::vector<int> vec{ 1,2,3,4,5 };
- for (auto n = vec.size(), m = static_cast<size_t>(0); const auto & i : vec)
- std::cout << --n + i << "," << ++m << "; ";
- std::cout << std::endl;
-
- for (const auto vec = std::vector{ -1,-2,-3,-4,-5 }; const auto& i : vec)
- std::cout << i << ", ";
- std::cout << std::endl;
以下为完整测试代码:
- int test_for_20()
- {
- const std::vector<int> vec{ 1,2,3,4,5 };
- // C++11
- for (const auto& i : vec)
- std::cout << i << ", ";
- std::cout << std::endl;
-
- // C++17
- std::map<int, int> m{ {1,-1}, {2,-2}, {3,-3} };
- for (const auto& [key, value] : m)
- std::cout << key << "," << value << "; ";
- std::cout << std::endl;
-
- // C++20
- for (auto n = vec.size(), m = static_cast<size_t>(0); const auto & i : vec)
- std::cout << --n + i << "," << ++m << "; ";
- std::cout << std::endl;
-
- for (const auto vec = std::vector{ -1,-2,-3,-4,-5 }; const auto& i : vec)
- std::cout << i << ", ";
- std::cout << std::endl;
-
-
- return 0;
- }
执行结果如下图所示:
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