用c++上下文无关语言的代码

以下是一个简单的C++上下文无关语言的代码示例，实现了一个简单的语法分析器来解析带有加法和乘法的表达式：

#include <iostream>
#include <string>

using namespace std;

// 定义语法树节点
class Node {
public:
    virtual int eval() = 0;
};

// 定义数字节点
class NumNode : public Node {
public:
    NumNode(int num): num(num) {}
    int eval() { return num; }
private:
    int num;
};

// 定义加法节点
class AddNode : public Node {
public:
    AddNode(Node* left, Node* right): left(left), right(right) {}
    int eval() { return left->eval() + right->eval(); }
private:
    Node* left;
    Node* right;
};

// 定义乘法节点
class MulNode : public Node {
public:
    MulNode(Node* left, Node* right): left(left), right(right) {}
    int eval() { return left->eval() * right->eval(); }
private:
    Node* left;
    Node* right;
};

// 定义语法分析器
class Parser {
public:
    Parser(string expr): expr(expr), pos(0) {}
    Node* parse() {
        Node* left = parseTerm();
        while (pos < expr.length()) {
            char op = expr[pos];
            if (op != '+' && op != '*') {
                throw runtime_error("Invalid operator");
            }
            pos++;
            Node* right = parseTerm();
            if (op == '+') {
                left = new AddNode(left, right);
            } else {
                left = new MulNode(left, right);
            }
        }
        return left;
    }
private:
    Node* parseTerm() {
        if (pos >= expr.length()) {
            throw runtime_error("Unexpected end of expression");
        }
        if (isdigit(expr[pos])) {
            return parseNum();
        } else if (expr[pos] == '(') {
            pos++;
            Node* node = parse();
            if (pos >= expr.length() || expr[pos] != ')') {
                throw runtime_error("Missing closing parenthesis");
            }
            pos++;
            return node;
        } else {
            throw runtime_error("Invalid term");
        }
    }
    Node* parseNum() {
        int num = 0;
        while (pos < expr.length() && isdigit(expr[pos])) {
            num = num * 10 + (expr[pos] - '0');
            pos++;
        }
        return new NumNode(num);
    }
    string expr;
    int pos;
};

int main() {
    string expr = "2*3+4";
    Parser parser(expr);
    Node* root = parser.parse();
    int result = root->eval();
    cout << result << endl; // 输出 10
    return 0;
}

示例中的语法分析器能够解析一些简单的表达式，并且使用了上下文无关语言的思想来实现语法分析。