问c++编程原则和实践第7章练习4 Symbol_table::get_value始终显示错误未定义

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class Variable {
public:
    string name;
    double value;
    bool var;   // variable (true) or constant (false)
    Variable(string s, double d, bool b) :name(s), value(d), var(b) { }
private:
};
class Symbol_table {
public:
    double get(string s);
    void set(string s, double d);
    bool is_declared(string var);
    double declare(string s, double val, bool var = true); //default is variable
private:
    vector<Variable> var_table;
};
double Symbol_table::get_value(string s) {
    // return the value of the Variable names s
    for (Variable& v : var_table) {
        cout << "getting variable: " << v.name << " " << v.value << " " << v.var << endl;
        if (v.name == s) {
            return v.value;
        }
    }
    error("get: undefined variable ", s);
}