C++17 新特性简解

C++17 新特性简解

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一、核心语言特性

1. 结构化绑定（Structured Bindings）

用途：解构复合类型（如元组、结构体）为独立变量
示例：

#include <iostream>
#include <tuple>int main() {// 解构 std::pairstd::pair<int, double> p{42, 3.14};auto [x, y] = p;std::cout << "Pair: " << x << ", " << y << "\n";// 解构结构体struct Point { int a; int b; };Point pt{10, 20};auto& [a, b] = pt; // 引用绑定a = 100;std::cout << "Point: " << pt.a << ", " << pt.b << "\n";return 0;
}

2. if/switch 初始化语句

用途：在条件语句中声明临时变量，限制作用域
示例：

#include <iostream>
#include <vector>int getValue() { return 42; }int main() {std::vector<int> vec{1, 2, 3};// if 初始化if (auto it = std::find(vec.begin(), vec.end(), 2); it != vec.end()) {std::cout << "Found: " << *it << "\n";}// switch 初始化switch (int key = getValue(); key) {case 42: std::cout << "Answer\n"; break;default: break;}return 0;
}

3. 内联变量（Inline Variables）

用途：允许头文件中直接定义全局变量
示例：

// header.h
inline int globalCount = 0; // 多文件包含安全struct MyClass {inline static int instanceCount = 0; // 类内初始化静态成员
};// main.cpp
#include <iostream>
#include "header.h"int main() {MyClass::instanceCount++;std::cout << "Global: " << globalCount << ", Instance: " << MyClass::instanceCount << "\n";return 0;
}

4. 折叠表达式（Fold Expressions）

用途：简化可变参数模板展开
示例：

#include <iostream>template<typename... Args>
auto sum(Args... args) {return (args + ...); // 折叠求和
}int main() {std::cout << "Sum: " << sum(1, 2, 3, 4) << "\n"; // 输出 10return 0;
}

5. 类模板参数推导（CTAD）

用途：自动推导模板参数类型
示例：

#include <vector>
#include <tuple>int main() {std::pair p{1, "hello"};    // 推导为 pair<int, const char*>std::vector v{1, 2, 3};     // 推导为 vector<int>std::tuple t{4, 3.14, "π"}; // 推导为 tuple<int, double, const char*>return 0;
}

6. constexpr 扩展

用途：支持更多编译期计算
示例：

#include <iostream>constexpr int factorial(int n) {if (n <= 1) return 1;return n * factorial(n - 1);
}int main() {constexpr int val = factorial(5);static_assert(val == 120); // 编译期验证std::cout << "5! = " << val << "\n";return 0;
}

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二、标准库增强

1. 文件系统库（Filesystem）

头文件：<filesystem>
示例：

#include <iostream>
#include <filesystem>namespace fs = std::filesystem;int main() {fs::path p = fs::current_path() / "test.txt";if (fs::exists(p)) {std::cout << "文件大小: " << fs::file_size(p) << "字节\n";}return 0;
}

2. 新容器类型

类型	用途	示例代码
std::optional<T>	表示可能不存在的值	[见下方]
std::variant<T...>	类型安全的联合体	[见下方]
std::any	存储任意类型	[见下方]
std::string_view	非拥有字符串视图	[见下方]

完整示例：

#include <iostream>
#include <optional>
#include <variant>
#include <any>
#include <string_view>int main() {// optionalstd::optional<int> opt = 42;if (opt) std::cout << "Optional: " << *opt << "\n";// variantstd::variant<int, std::string> v = "hello";std::cout << "Variant: " << std::get<std::string>(v) << "\n";// anystd::any a = 3.14;if (a.type() == typeid(double)) {std::cout << "Any: " << std::any_cast<double>(a) << "\n";}// string_viewstd::string_view sv = "Hello World";std::cout << "View: " << sv.substr(0, 5) << "\n";return 0;
}

3. 并行算法

头文件：<execution>
示例：

#include <iostream>
#include <vector>
#include <algorithm>
#include <execution>int main() {std::vector<int> data{5, 3, 1, 4, 2};// 并行排序std::sort(std::execution::par, data.begin(), data.end());// 并行遍历std::for_each(std::execution::par, data.begin(), data.end(), [](int x) {std::cout << x << " ";});return 0;
}

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三、其他改进

1. 嵌套命名空间简化

namespace A::B::C { // 等价于 namespace A { namespace B { namespace C {class MyClass {};
}}int main() {A::B::C::MyClass obj;return 0;
}

2. 强制复制省略

struct NonCopyable {NonCopyable() = default;NonCopyable(const NonCopyable&) = delete;
};NonCopyable create() {return NonCopyable{}; // C++17 必须省略拷贝
}int main() {NonCopyable obj = create();return 0;
}

3. 新增属性

[[nodiscard]] int critical() { return 42; }int main() {critical(); // 警告：返回值未使用return 0;
}

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四、完整可编译代码示例

#include <iostream>
#include <tuple>
#include <vector>
#include <filesystem>
#include <optional>
#include <algorithm>
#include <execution>// 结构化绑定
void demo_structured_binding() {auto [x, y] = std::make_tuple(42, 3.14);std::cout << "Tuple: " << x << ", " << y << "\n";
}// 文件系统
void demo_filesystem() {namespace fs = std::filesystem;fs::path p = fs::current_path();std::cout << "当前路径: " << p << "\n";
}// 并行算法
void demo_parallel() {std::vector<int> data{5, 3, 1, 4, 2};std::sort(std::execution::par, data.begin(), data.end());for (int n : data) std::cout << n << " ";
}int main() {demo_structured_binding();demo_filesystem();demo_parallel();return 0;
}

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五、编译与运行

1. 编译命令：

   g++ -std=c++17 -o cpp17_demo cpp17_demo.cpp -lstdc++fs