c++ - 使用 Maps 和 make_pair 编译错误

Question

我正在尝试复习一些关键的 STL 概念。我编写了以下程序来理解 std::pair 和 maps 的工作原理，但我遇到了一个严重的编译错误。也尝试添加所有 header ，但到目前为止无法找出问题所在。请帮忙。

void testMaps()
{
    DB_Type phoneDb;
    PhoneInfoList_Type v1(8);
    PhoneInfo_Type(*phoneInfoGenerator)() = phoneInfoGen;
    DBIT_Type it;
    PhoneInfoList_Type::iterator phit;
    int i = 0, j = 0;

    for (phit = v1.begin(); phit != v1.end(); ++phit)
    {
        v1.push_back(phoneInfoGen());
    }

    for (phit = v1.begin(); phit != v1.end(); ++phit)
    {
        phoneDb.insert(*phit);
    } 
    int choice;
    std::cout << "Enter choice 0. Search by name, 1. Search by no., 2. Exit\n";
    while (1)
    {
        switch (choice)
        {
            case 0:
            {
                std::cout << "Enter name:";
                std::string name;
                std::cin >> name;
                DBIT_Type it = std::find(phoneDb.begin(), phoneDb.end(), name);
                if (it != phoneDb.end())
                {
                    std::cout <<"\n Phone no of " << name << "is " << it->second << "\n";
                }
                else
                {
                    std::cout << "Name not found\n";
                }
            }break;
            case 1:
            {
                std::cout << "Enter phone no:";
                int ph;
                std::cin >> ph;
                DBIT_Type it = std::find(phoneDb.begin(), phoneDb.end(), ph);
                if (it != phoneDb.end())
                {
                    std::cout << "\n Name of person with phone no " << ph << "is " << it->first << "\n";
                }
                else
                {
                    std::cout << "Name not found\n";
                }
            }
            break;
            default:
                break;
        }
    }
}

我遇到的编译错误:

warning C4244: 'argument' : conversion from 'time_t' to 'unsigned int', possible loss of data
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\xutility(3026): error C2678: binary '==' : no operator found which takes a left-hand operand of type 'std::pair<const _Kty,_Ty>' (or there is no acceptable conversion)
1>          with
1>          [
1>              _Kty=std::string
1>  ,            _Ty=int
1>          ]
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\system_error(410): could be 'bool std::operator ==(const std::error_condition &,const std::error_code &) throw()'
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\system_error(402): or       'bool std::operator ==(const std::error_code &,const std::error_condition &) throw()'
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\exception(507): or       'bool std::operator ==(const std::exception_ptr &,std::nullptr_t)'
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\exception(502): or       'bool std::operator ==(std::nullptr_t,const std::exception_ptr &)'
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\exception(497): or       'bool std::operator ==(const std::exception_ptr &,const std::exception_ptr &)'
1>          while trying to match the argument list '(std::pair<const _Kty,_Ty>, const std::string)'
1>          with
1>          [
1>              _Kty=std::string
1>  ,            _Ty=int
1>          ]

Answer 1

有两个错误。

• 第一个在这一行:

  DBIT_Type it = std::find(phoneDb.begin(), phoneDb.end(), name);
  

  映射中元素的类型是 std::pair，其中包含键(姓名)和值(电话号码)。如果你想使用 std::find()，你需要给它你正在寻找的值对，而不仅仅是键。

  可以使用 std::map::find() 代替搜索 key :

  DBIT_Type it = phoneDb.find(name);
  

• 第二个在这一行:

  DBIT_Type it = std::find(phoneDb.begin(), phoneDb.end(), ph);
  

  这会失败，因为 ph 是一个 int，但是映射的键是一个 std::string。您显然是在尝试 search an element by its value (电话号码)而不是按键(姓名)。