具有未知数据类型的C ++结构 [英] C++ Structure with unknown data types

问题描述

我的程序读取用户的输入并创建简单的表。用户开始指定列的数据类型和行数。

用户输入：

 创建表
添加属性字符串名称
添加属性int年龄
行3 
  

我需要从用户的输入准备一个结构。我有这样的东西：

  CTable 
 {
 unsigned attributesCnt; 
 string * attributesNames; 
 
 void **属性; 
}; 
  

所以，从用户的输入，程序执行这些步骤：

  CTable myTable; 
 
 myTable.attributesCnt = 2; // stringName，intAge
 
 myTable.attributesNames = new string [2]; 
 myTable.attributesNames [0] =名称; 
 myTable.attributesNames [1] =年龄; 
 
属性= new void [2]; // 2属性
 attributes [0] =（void *）new string [3]; //会有3行
属性[1] =（void *）new int [3];我需要记住，attributes [0]是字符串，attributes [1]是int 
 
 
 
这是正确的方式吗？
 
 
 
我只想使用标准库。 > 
 
解决方案
你正在寻找的是一个标记的联盟，也称为变体。它允许您在同一位置存储多个数据类型，就像常规联合，但包括一个附加但单独的数据成员，表示它的类型。 C ++标准库不包括变体，但它们很容易实现。
 
 
 
一旦你有一个变体，你可以将它应用到你的例子如下。
  myTable.attributesNames [0] =名称; 
 myTable.attributesNames [1] =年龄; 
 
 //我建议在这里使用std :: vector，而不是使用new / delete自己
 attributes = new Variant * [2]; // 2 attributes 
 attributes [0] = new Variant（player name）; 
 attributes [1] = new Variant（player_age）; 
  
以下示例显示了该变体可能如何实现。
  struct Variant 
 {
枚举类型
 {
 INT，
 STRINGPTR 
}; 
 
类型type_; 
 union 
 {
 int int_; 
 const char * stringptr_; 
} data_; 
 
显式变量（int data）：type_（INT）
 {
 data_.int_ = data; 
} 
 
显式变量（const char * data）：type_（STRINGPTR）
 {
 data_.stringptr_ = data; 
} 
 
输入getType（）const {return type_; 
 int getIntValue（）const 
 {
 if（type_！= INT）
 throw std :: runtime_error（Variant is not an int）; 
 return data_.int_; 
} 
 
 const char * getStringPtr（）const 
 {
 if（type_！= STRINGPTR）
 throw std :: runtime_error（Variane not not一个字符串）; 
 return data_.stringptr_; 
} 
}; 
 
 int main（）
 {
 Variant intval（1）; 
 Variant stringval（hello）; 
 
 std :: cout<<< intval.getIntValue（）<< std :: endl; 
 std :: cout<<< stringval.getStringPtr（）<< std :: endl; 
} 
  
 
My program reads user's input and create easy "table". User on the start specifies data types of columns and the number of rows.

User's input:
create table
add attribute string Name
add attribute int    Age
rows                   3
I need prepare a structure from user's input now. I have something like this:
CTable
{
    unsigned   attributesCnt;
    string   * attributesNames;

    void    ** attributes;
};
So, from the user's input, the progam does these steps:
CTable myTable;

myTable.attributesCnt = 2; // string "Name", int "Age"

myTable.attributesNames = new string[2];
myTable.attributesNames[0] = "Name";
myTable.attributesNames[1] = "Age";

attributes = new void[2]; // 2 attributes
attributes[0] = (void*) new string[3]; // there will be 3 rows
attributes[1] = (void*) new int[3];
I need remember that "attributes[0]" is string and "attributes[1]" is int too.

Is this "right" way?

I would like use only standard libraries.
解决方案
What you are looking for is a tagged union also called a variant. It allows you to store multiple data types at the same location just like a regular union but includes an additional but separate data member that indicates it's type. The C++ Standard Library does not include variants but they are easy enough to implement.

Once you have a variant you can apply it to your example like below.
myTable.attributesNames[0] = "Name";
myTable.attributesNames[1] = "Age";

// I recommend using std::vector here instead of using new/delete yourself
attributes = new Variant*[2]; // 2 attributes
attributes[0] = new Variant("player name");
attributes[1] = new Variant(player_age);
The following example shows how the variant might be implemented.
struct Variant
{
    enum Type
    {
        INT,
        STRINGPTR
    };

    Type    type_;
    union
    {
        int         int_;
        const char* stringptr_;
    }       data_;

    explicit Variant(int data) : type_(INT)
    {
        data_.int_ = data;
    }

    explicit Variant(const char *data) : type_(STRINGPTR)
    {
        data_.stringptr_ = data;
    }

    Type getType() const { return type_; }
    int getIntValue() const
    {
        if(type_ != INT)
            throw std::runtime_error("Variant is not an int");
        return data_.int_;
    }

    const char *getStringPtr() const
    {
        if(type_ != STRINGPTR)
            throw std::runtime_error("Variane is not a string");
        return data_.stringptr_;
    }
};

int main()
{
    Variant intval(1);
    Variant stringval("hello");

    std::cout << intval.getIntValue() << std::endl;
    std::cout << stringval.getStringPtr() << std::endl;
}


                        
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