c++ - 错误 : passing ‘constas ‘this’ argument discards qualifiers [-fpermissive]

Question

我有一个单例类:

class ObjectModel {
public:
    virtual ~ObjectModel();
    static ObjectModel& Get();

    const AttributeStruct operator [] (const std::string &symbol_path) const {
        auto path = split(symbol_path, '.');

        if (path.size() != 2) {
            throw std::runtime_error(Formatter() << "Path '" 
               << symbol_path << "' has a wrong format.");
        }

        const CipAttributeStruct attr = this->dic[path[0]][path[1]];
        if (attr.data == nullptr) {
            throw std::runtime_error(Formatter() << "Attribute '" 
               << symbol_path << "' isn't found.");
        }

        return attr;
    }
private:
    ObjectModel();
    ObjectModel( const ObjectModel& );
    ObjectModel& operator=( ObjectModel& );

    std::map<std::string, std::map<std::string, CipAttributeStruct> > dic = {};
};

用法:

void main() {
    auto &m = ObjectModel::Get();

    std::cout << *static_cast<uint32_t*>(m["Object1.Attribute"].data);
}

由于错误，我无法编译此代码:

error: passing ‘const std::map, std::map, lynx::cip::CipAttributeStruct> >’ as ‘this’ argument discards qualifiers [-fpermissive] const CipAttributeStruct attr = this->dic[path[0]][path[1]];

我在这里看到很多此类问题，但没有一个对我有帮助。我知道我可以删除 const 限定符来避免这个问题。但我想知道是否有另一种方法。

Answer 1

因为 operator[] 具有潜在的破坏性，所以它不是 const 操作。如果您只想观察 map 的内容，请使用at:

const CipAttributeStruct attr = this->dic.at(path[0]).at(path[1]);