c++ - 断线异常？

Question

我正在抛出一些解析异常。但是异常需要破损的字符串..？

//Parse exception
class ParseException : public exception {
public:
    //Constructor
    //Arguments:
    //  Str: Message
    //  Pos: Position
    ParseException(string Str, int Pos) {
        msg = Str;
        pos = Pos;
    }

    //Get what happened(From exception)
    //Returns:
    //  Message with position
    virtual const char* what() const throw() {
        string str = msg;
        str += " at " + to_string(pos);
        return str.c_str();
    }
private:
    string msg; //Message
    int pos;    //Position
};

这是异常类。我像这样抛出这个异常:

throw ParseException("Mismatched bracket", P.Pos);

抛出此异常并转到:

try {
    Parse(p);
}
catch (ParseException e) { // <<< Here
    cerr << "ParseException: " << e.what() << endl;
}

我得到的是:

ParseException: ▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌

我的代码有问题吗？还是Visual Studio(或Compiler)的问题？

Answer 1

As noted in the comments ，如果不创建未定义的行为，则不能返回本地 string 的 c_str。将 what 值的 string 缓存存储在异常本身上可能是有意义的； the char* returned by what needs to live as long as the exception object ，所以缓存异常是合理的。

class ParseException : public exception {
public:
    //Constructor
    //Arguments:
    //  Str: Message
    //  Pos: Position
    ParseException(string Str, int Pos) : msg(std::move(Str)), pos(Pos) {}

    //Get what happened(From exception)
    //Returns:
    //  Message with position
    virtual const char* what() const throw() {
        // Lazily populate what so it's not populated until called
        if (_what.empty()) {
            _what = msg + " at " + to_string(pos);
        }
        return _what.c_str();
    }
private:
    string msg; //Message
    int pos;    //Position
    string _what;
};

或者，您可以在构造函数中计算值，这样 what 保持 nothrow 兼容(匹配 C++ 标准库异常)。