9 79 D:\dev c++\cifa2.cpp [Error] in C++98 'grammar' must be initialized by constructor, not by '{...}'

这个错误提示是因为在 C++98 标准中，类的成员变量不能使用花括号来初始化，而必须使用构造函数。比如下面的代码就会导致这个错误：

class Grammar {
public:
    std::vector<std::string> nonterminals {"S", "A", "B", "C"};
    std::vector<std::string> terminals   {"a", "b", "c", "d", "e", "f"};

    std::unordered_map<std::string, std::vector<std::vector<std::string>>> rules {
        {"S", {{"A", "B", "C"}}},
        {"A", {{"a", "A"}, {"b"}}},
        {"B", {{"c", "B"}, {"d"}}},
        {"C", {{"e", "C"}, {"f"}}}
    };
};

解决这个问题的方法是，使用构造函数来初始化成员变量，例如：

class Grammar {
public:
    Grammar()
        : nonterminals{"S", "A", "B", "C"},
          terminals{"a", "b", "c", "d", "e", "f"},
          rules{
              {"S", {{"A", "B", "C"}}},
              {"A", {{"a", "A"}, {"b"}}},
              {"B", {{"c", "B"}, {"d"}}},
              {"C", {{"e", "C"}, {"f"}}}
          }
    {}

    std::vector<std::string> nonterminals;
    std::vector<std::string> terminals;
    std::unordered_map<std::string, std::vector<std::vector<std::string>>> rules;
};

这样就可以在 C++98 标准下正常编译了。当然，在 C++11 之后，使用花括号初始化成员变量是被允许的，所以如果你使用的是较新的编译器，也可以直接使用花括号初始化成员变量。