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1.前言

本篇目的：理解C++之std::get用法。

1.在C++中，std::holds_alternative是一个模板函数，用于检查给定的std::variant对象是否包含指定类型的值。它返回一个bool值，指示给定类型是否存在于std::variant中。
2.在C++中，std::get是用于从std::tuple和std::variant等类型中获取值的函数模板。它接受一个索引作为参数，并返回相应位置的值。

2.应用实例

<1>.v1.0：std::holds_alternative用法

#include <iostream>
#include <variant>
#include <string>

int main() {
    std::variant<int, double, std::string> myVariant = "Hello";

    // 使用std::holds_alternative检查指定类型是否存在于variant中
    bool isInt = std::holds_alternative<int>(myVariant);
    bool isDouble = std::holds_alternative<double>(myVariant);
    bool isString = std::holds_alternative<std::string>(myVariant);

    std::cout << "isInt: " << isInt << std::endl;            // 输出：isInt: 0
    std::cout << "isDouble: " << isDouble << std::endl;      // 输出：isDouble: 0
    std::cout << "isString: " << isString << std::endl;      // 输出：isString: 1

    return 0;
}

<2>.v2.0: 对于std::tuple，std::get的用法。

#include <iostream>
#include <tuple>

int main() {
  std::tuple<int, std::string, double> myTuple(42, "Hello", 3.14);

  // 使用std::get获取tuple中的值
  int intValue = std::get<0>(myTuple);
  std::string stringValue = std::get<1>(myTuple);
  double doubleValue = std::get<2>(myTuple);

  std::cout << intValue << std::endl;        // 输出：42
  std::cout << stringValue << std::endl;     // 输出："Hello"
  std::cout << doubleValue << std::endl;     // 输出：3.14

  return 0;
}

<3>.v3.0: 对于std::variant，std::get的用法。

#include <iostream>
#include <variant>
#include <string>

int main() {
  std::variant<int, std::string, double> myVariant = 42;

  // 使用std::get获取variant中存储的值
  int intValue = std::get<int>(myVariant);
  std::cout << intValue << std::endl;        // 输出：42

  // 尝试使用std::get获取不匹配的类型，会抛出std::bad_variant_access异常
  try {
    std::string stringValue = std::get<std::string>(myVariant);
  } catch(const std::bad_variant_access& e) {
    std::cout << "Error: " << e.what() << std::endl;  // 输出：Error: std::bad_variant_access
  }

  return 0;
}

<4>.v4.0

#include <iostream>
#include <variant>
#include <string>
#include <utility>

using namespace std;

using Elem = std::variant<std::monostate, int32_t, int64_t, double, std::string, std::pair<int64_t, int64_t>>;

int main() {
    Elem e1 = 42;
    Elem e2 = "Hello, world!";
    Elem e3 = 3.14;
    Elem e4 = std::make_pair(10, 20);

    if (std::holds_alternative<int32_t>(e1)) {
        int32_t value = std::get<int32_t>(e1);
        cout << "e1 is an int32_t: " << value << endl;
    }

    if (std::holds_alternative<std::string>(e2)) {
        std::string value = std::get<std::string>(e2);
        cout << "e2 is a string: " << value << endl;
    }

    if (std::holds_alternative<double>(e3)) {
        double value = std::get<double>(e3);
        cout << "e3 is a double: " << value << endl;
    }

    if (std::holds_alternative<std::pair<int64_t, int64_t>>(e4)) {
        std::pair<int64_t, int64_t> value = std::get<std::pair<int64_t, int64_t>>(e4);
        cout << "e4 is a pair: (" << value.first << ", " << value.second << ")" << endl;
    }

    return 0;
}

<5>.v5.0

#include <iostream>
#include <string>
#include<typeinfo>
#include <variant>
#include <map>

using namespace std;

class Item {
public:

  template<typename S, typename T>
  Item &set(S key, T value) {
    printf("xxx---------->%s(), line = %d, key = %s, vlaue = %d\n",__FUNCTION__,__LINE__,key,value);
    findOrAllocateProp(key).set(value);
    return *this;
  }

  Item &setInt32(const char *key, int32_t value) {
    printf("xxx---------->%s(), line = %d, key = %s, vlaue = %d\n",__FUNCTION__,__LINE__,key,value);
    return set(key, value);
  }

  class Prop {
  public:
    using Elem = std::variant<std::monostate, int32_t, int64_t, double, std::string, std::pair<int64_t, int64_t>>;

    //Prop() = default;
    void setName(const char *name) {
      printf("xxx---------->%s(), line = %d, name = %s\n",__FUNCTION__,__LINE__,name);
      mName = name;
    }

    template <typename T> void set(const T& value) {
      //printf("xxx---------->%s(), line = %d,typeid(value) = %s\n",__FUNCTION__,__LINE__,typeid(value).name());
      printf("xxx---------->%s(), line = %d, value = %d\n",__FUNCTION__,__LINE__,value);
      mElem = value;

      //获取mElem中的int类型的值.
      int32_t val = std::get<int32_t>(mElem);
      printf("xxx---------->%s(), line = %d,val = %d\n",__FUNCTION__,__LINE__,val);
    }

  private:
    std::string mName;
    Elem mElem;
  };

  Prop &findOrAllocateProp(const char *key) {
    auto it = mProps.find(key);
    //printf("xxx---------->%s(), line = %d, key = %s, vlaue = %d\n",__FUNCTION__,__LINE__,key,it->first);
    printf("xxx---------->%s(), line = %d, key = %s\n",__FUNCTION__,__LINE__,key);
    if (it != mProps.end())
      return it->second;

    Prop &prop = mProps[key];
    prop.setName(key);
    return prop;
  }

  std::map<std::string, Prop> mProps;
};

int main(){
  Item it;
  it.setInt32("Tom", 18);


}