c++中的标签联合体(tagged union)是什么_c++安全联合体实现与类型区分

enum class TypeTag { INT, DOUBLE, STRING };
<p>struct TaggedUnion {
TypeTag tag;
union {
int i_val;
double d_val;
std::string* str_ptr;
};</p><pre class="brush:php;toolbar:false;"><pre class="brush:php;toolbar:false;">// 构造函数
TaggedUnion(int i) : tag(TypeTag::INT), i_val(i) {}
TaggedUnion(double d) : tag(TypeTag::DOUBLE), d_val(d) {}
TaggedUnion(const std::string& s) : tag(TypeTag::STRING) {
    str_ptr = new std::string(s);
}

// 析构函数需处理资源释放
~TaggedUnion() {
    if (tag == TypeTag::STRING) {
        delete str_ptr;
    }
}

// 禁止拷贝（可进一步实现深拷贝）
TaggedUnion(const TaggedUnion&) = delete;
TaggedUnion& operator=(const TaggedUnion&) = delete;

// 访问函数
void print() const {
    switch (tag) {
        case TypeTag::INT:
            std::cout << "int: " << i_val << "\n";
            break;
        case TypeTag::DOUBLE:
            std::cout << "double: " << d_val << "\n";
            break;
        case TypeTag::STRING:
            std::cout << "string: " << *str_ptr << "\n";
            break;
    }
}

#include <variant>
#include <string>
#include <iostream>
<p>using Value = std::variant<int, double, std::string>;</p><p>void print_value(const Value& v) {
std::visit([](auto&& arg) {
std::cout << arg << "\n";
}, v);
}</p><p>int main() {
Value a = 42;
Value b = 3.14;
Value c = std::string("hello");</p><pre class="brush:php;toolbar:false;"><pre class="brush:php;toolbar:false;">print_value(a); // 输出: 42
print_value(b); // 输出: 3.14
print_value(c); // 输出: hello

return 0;

if (std::holds_alternative<std::string>(v)) {
    std::cout << std::get<std::string>(v);
}