如何在C++中检入已声明的标识符？

#include <iostream>

int main() {
    int x; // 声明变量x
    std::cout << x; // 使用变量x，编译器会检查x是否已声明
    return 0;
}

#include <iostream>
#include <typeinfo>

class Base { virtual void foo() {} };
class Derived : public Base {};

int main() {
    Base* b = new Derived;
    if (typeid(*b) == typeid(Derived)) {
        std::cout << "b is pointing to a Derived object" << std::endl;
    }
    delete b;
    return 0;
}

#include <iostream>
#include <typeinfo>

// 编译时检查
void compileTimeCheck() {
    int declaredVar = 10;
    // int undeclaredVar; // 如果取消注释，会导致编译错误
    std::cout << declaredVar << std::endl;
}

// 运行时检查
class Base { virtual void foo() {} };
class Derived : public Base {};

void runtimeCheck() {
    Base* b = new Derived;
    if (typeid(*b) == typeid(Derived)) {
        std::cout << "Runtime check passed: b is a Derived object." << std::endl;
    }
    delete b;
}

int main() {
    compileTimeCheck();
    runtimeCheck();
    return 0;
}