c++ - 如何为 YAML::Emitter 重载 << 运算符以序列化包含另一个自定义类的 vector 的自定义类

Question

在我的应用程序中，我有一个类包含另一个类的 vector ，我无法为此编写重载的 YAML::Emitter& operator<<。

为了说明，我编写了重现错误的波纹管文件:

Department 类包含一个 Employee vector ，我需要以 Yaml 格式输出包含所有员工的 Department。

我收到以下似乎与我的方法签名不匹配的奇怪错误:

$ g++ ta.cpp -L ~/yaml-cpp/lib/ -I ../Thanga/yaml/yaml-cpp-0.5.1/include/ -lyamlcpp 2>&1|more ... ta.cpp:37:16: note: YAML::Emitter& operator<<(YAML::Emitter&, Department&) ta.cpp:37:16: note: no known conversion for argument 2 from ‘const Employee’ to ‘Department&’ ta.cpp:28:16: note: YAML::Emitter& operator<<(YAML::Emitter&, Employee&) ta.cpp:28:16: note: no known conversion for argument 2 from ‘const Employee’ to ‘Employee&’

完整的源文件如下:

#include <iostream>
#include <fstream>
#include "yaml-cpp/yaml.h"
#include <cassert>
#include <vector>
#include <string>

struct Employee
{
        std::string name;
        std::string surname;
        int age;
        std::string getName(){return name;}
        std::string getSurname(){return surname;}
        int getAge(){return age;}
};

struct Department
{
    std::string name;
    int headCount;
    std::vector<Employee>  staff;
    std::string getName(){return name;}
    int getHeadCount(){return headCount;}
    std::vector<Employee> & getStaff(){return staff;}
};

YAML::Emitter& operator << (YAML::Emitter& out, Employee& e) {
        out << YAML::BeginMap;
        out << YAML::Key <<"name"<<YAML::Value <<e.getName();
        out << YAML::Key <<"surname"<<YAML::Value <<e.getSurname();
        out << YAML::Key <<"age"<<YAML::Value <<e.getAge();
        out << YAML::EndMap;
        return out;
}

YAML::Emitter& operator << (YAML::Emitter& out, Department& d) {
        out << YAML::BeginMap;
        out << YAML::Key <<"name"<<YAML::Value <<d.getName();
        out << YAML::Key <<"headCount"<<YAML::Value <<d.getHeadCount();
        out << YAML::Key <<"Employees"<<YAML::Value <<d.getStaff();
        out << YAML::EndMap;
        return out;
}


int main()
{
    Employee k;
    Department d;

    d.name="Twidlers";
    d.headCount=5;

    k.name="harry";
    k.surname="person";
    k.age=70;
    d.staff.push_back(k);

    k.name="joe";
    k.surname="person";
    k.age=30;
    d.staff.push_back(k);

    k.name="john";
    k.surname="doe";
    k.age=50;
    d.staff.push_back(k);

    std::ofstream ofstr("output.yaml");
    YAML::Emitter out;
    out<<d;
    ofstr<<out.c_str();
}

Answer 1

为 vector 提供的重载拿个const std::vector<T>& , 所以你必须撒一些额外的 const贯穿始终:

YAML::Emitter& operator << (YAML::Emitter& out, const Employee& e)
...
YAML::Emitter& operator << (YAML::Emitter& out, const Department& d)

然后将它们放在您的成员函数中，例如:

const std::vector<Employee>& getStaff() const { return staff; }

(一般来说，你应该让你的 getter 默认为常量，如果你需要改变状态，添加 setter 而不是非常量的 getter。)