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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: 0std::cout << "isDouble: " << isDouble << std::endl; // 输出：isDouble: 0std::cout << "isString: " << isString << std::endl; // 输出：isString: 1return 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：42std::cout << stringValue << std::endl; // 输出："Hello"std::cout << doubleValue << std::endl; // 输出：3.14return 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);}
