【C++17】结构化绑定

C++17带来了一个新特性，它结合了语法糖和自动类型推断：结构化绑定。这有助于将对、元组和结构体中的值赋给单独的变量。在其他编程语言中，这也称为解包?

#include <iostream>
#include <chrono>

std::pair<int, std::string> func(int a, int b)
{
    if (b <= 0){
        return { -1, "error" };
    }else{
        return { a / b, "ok" };
    }
}

std::tuple<int, std::string, std::chrono::system_clock::time_point> func1()
{
    return std::make_tuple(0, "ok", std::chrono::system_clock::now());
}

int main()
{
    //C++17 之前
    const auto result = func(1, 0);
    std::cout << "result: " << result.first << "," << result.second << std::endl;

    //c++17 之后
    auto [code, err] = func(1, 0);
    std::cout << "result: " << code << "," << err << std::endl;

    auto [code1, err1, nowclock] = func1();

    std::cout << "Hello World!\n";
}

我们现在可以将单个值分配给具有表达性名称的单个变量，这样读起来更好。

结构化绑定应用到自定义结构体：

#include <iostream>
#include <chrono>
#include <vector>

struct employee 
{
    unsigned int id;
    std::string name;
    std::string role;
    unsigned salary;
};

int main()
{
    std::vector<employee> vecEly;
    employee ely;
    ely.id = 0;
    ely.name = "张三";
    ely.role = "员工";
    ely.salary = 5000;
    vecEly.emplace_back(ely);
    ely.id = 1;
    ely.name = "李四";
    ely.role = "员工";
    ely.salary = 6000;
    vecEly.emplace_back(ely);
    ely.id = 2;
    ely.name = "王五";
    ely.role = "员工";
    ely.salary = 8000;
    vecEly.emplace_back(ely);

    for (const auto &[id, name, role, salary] : vecEly)
    {
        std::cout << "id: " << id << " name: " << name << " role: " 
            << role << " salary: " << salary << std::endl;
    }

    std::cout << "Hello World!\n";
}

STL中的许多基本数据结构都可以使用结构化绑定立即访问，而无需更改任何内容。例如，考虑一个循环，它输出std::map的所有项：

#include <iostream>
#include <chrono>
#include <vector>
#include <map>

struct employee
{
    unsigned int id;
    std::string name;
    std::string role;
    unsigned salary;
};

int main()
{
    std::map<int, employee> mapEly;
    std::vector<employee> vecEly;
    employee ely;
    ely.id = 0;
    ely.name = "张三";
    ely.role = "员工";
    ely.salary = 5000;
    vecEly.emplace_back(ely);
    mapEly[0] = ely;
    ely.id = 1;
    ely.name = "李四";
    ely.role = "员工";
    ely.salary = 6000;
    vecEly.emplace_back(ely);
    mapEly[1] = ely;
    ely.id = 2;
    ely.name = "王五";
    ely.role = "员工";
    ely.salary = 8000;
    vecEly.emplace_back(ely);
    mapEly[2] = ely;
    for (const auto& [id, name, role, salary] : vecEly)
    {
        std::cout << "id: " << id << " name: " << name << " role: "
            << role << " salary: " << salary << std::endl;
    }

    for (const auto& [index, elyData] : mapEly)
    {
        std::cout << "index: " << index
            << " id: " << elyData.id << " name: " << elyData.name << " role: "
            << elyData.role << " salary: " << elyData.salary << std::endl;
    }

    std::cout << "Hello World!\n";
}

性能担忧：

除了C语言中缺少的语言特性外，通过返回值返回复杂结构在很长一段时间内被认为是缓慢的，因为对象必须在返回函数中初始化，然后复制到应该包含调用方返回值的变量中。现代编译器支持返回值优化(RVO)，它允许省略中间副本。