C++“错误:传递'const std::map<int, std::basic_string<char>>'作为......的“这个"论点 [英] C++ "error: passing 'const std::map<int, std::basic_string<char> >' as 'this' argument of ..."

问题描述

使用以下代码(为简洁起见):

With the following code (excerpted for brevity):

颜色.h:

class color {
public:
    color();

    enum colorType {
        black, blue, green, cyan, red,
        magenta, brown, lightgray, nocolor
    };

    colorType getColorType();
    void setColorType(colorType cColortype);

    string getColorText() const;

private:
    colorType cColortype = nocolor;
    map<int, string> colors = {
        {black, "black"},
        {blue, "blue"},
        {green, "green"},
        {cyan, "cyan"},
        {red, "red"},
        {magenta, "magenta"},
        {brown, "brown"},
        {lightgray, "lightgray"},
        {nocolor, "nocolor"}};
};

颜色.cpp:

color::color() {
}

color::colorType color::getColorType() {
    return cColortype;
}

void color::setColorType(colorType cColortype) {
    this->cColortype = cColortype;
}

string color::getColorText() const {
    return colors[cColortype];
}

我收到以下错误:

color.cpp:16:29: 错误:将 'const std::map >' 作为 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& 的 'this' 参数传递std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = int;_Tp = std::basic_string;_比较 = std::less;_Alloc = std::allocator >>;std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = std::basic_string;std::map<_Key, _Tp, _Compare, _Alloc>::key_type = int]' 丢弃限定符 [-fpermissive]

color.cpp:16:29: error: passing 'const std::map >' as 'this' argument of 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = int; _Tp = std::basic_string; _Compare = std::less; _Alloc = std::allocator > >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = std::basic_string; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = int]' discards qualifiers [-fpermissive]

错误是指返回颜色[cColortype];"在 getColorText 中.

The error refers to "return colors[cColortype];" in getColorText.

我正在为一个班级项目编写此内容，我可以通过删除 getColorText 签名中的 const 声明来使其工作，但我正在尝试学习/采用良好做法并遵守建议将 const 用于不修改数据的成员函数，所以我想知道如何处理这个问题.

I'm writing this for a class project and I can get it to work for the sake of the assignment by removing the const declaration in the getColorText signature but I'm trying to learn/adopt good practices and adhere to the recommendation to use const for member functions that don't modify data so I want to know how to deal with this going forward.

我通常非常擅长调试/故障排除，但错误消息太复杂以至于没有多大帮助.

I'm usually really good at debugging/troubleshooting but the error message is so convoluted that it's not much help.

感谢任何帮助.

推荐答案

string color::getColorText() const {
    return colors[cColortype];
}

问题是您已将函数标记为 const.std::map 上的 operator[] 被标记为非常量，并且不能在这样的 const 函数中使用.您需要手动使用 std::map::find (或其他机制)搜索输入类型并处理未找到的情况.

The issue is that you've marked the function as const. The operator[] on std::map is marked as non-const, and cannot be used in a const function like this. You need to manually use std::map::find (or other mechanism) to search for the input type and handle the case where it's not found.

如果您使用的是 C++11，则可以改用 std::map::at，它允许在常量映射上使用，如果请求则抛出异常元素不存在.

If you're using C++11, you can instead use std::map::at, which IS allowed to be used on a constant map, and throws an exception if the requested element is not present.

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