g ++：const丢弃限定符 [英] g++: const discards qualifiers

问题描述

为什么我得到放弃限定符错误：

why do I get a discard qualifiers error:

customExc.cpp: In member function ‘virtual const char* CustomException::what() const’:
customExc.cpp: error: passing ‘const CustomException’ as ‘this’ argument of ‘char customException::code()’ discards qualifiers

关于以下代码示例

#include <iostream>


class CustomException: public std::exception {

public:

    virtual const char* what() const throw() {
        static std::string msg;
        msg  = "Error: ";
        msg += code();  // <---------- this is the line with the compile error 
        return msg.c_str();
    }

    char code() { return 'F'; }
};

在讨论类似问题之前，我已经在SOF上搜索过。

I have searched around on SOF before regarding simular issues.

我已经在每一个可能的地方添加了一个 const 。

I have already added a const on every possible place.

请赐教 - 我不要't明白了......

Please enlighten me - I don't get the point...

编辑：
这里是在Ubuntu-Carmic-32bit（g ++ v4.4.1）

EDIT: here are the steps to reproduce on Ubuntu-Carmic-32bit (g++ v4.4.1)

1. 将示例另存为 customExc.cpp


2. 输入 make customExc.o

1. save example as customExc.cpp
2. type make customExc.o

编辑：错误与 CustomException 有关。类 Foo 与它无关。所以我删除了它。

EDIT: The error is related to CustomException. The class Foo has nothing to do with it. So I have deleted it.

推荐答案

CustomException :: what code> CustomException ::代码。 CustomException :: what 是一个const方法，由const 在 后指明what（）。由于它是一个常量方法，它不能做任何可能会修改的东西。 CustomException :: code 不是一个const方法，这意味着它不承诺不修改自己。因此 CustomException :: what 无法调用 CustomException :: code 。

CustomException::what calls CustomException::code. CustomException::what is a const method, as signified by the const after what(). Since it is a const method, it cannot do anything that may modify itself. CustomException::code is not a const method, which means that it does not promise to not modify itself. So CustomException::what can't call CustomException::code.

请注意，const方法不一定与const实例相关。 Foo :: bar 可以将 exc 变量声明为非const并调用const方法，如 CustomException ::什么;这只是意味着 CustomException :: what 承诺不会修改 exc ，但其他代码可能会是。

Note that const methods are not necessarily related to const instances. Foo::bar can declare its exc variable as non-const and call const methods like CustomException::what; this simply means that CustomException::what promises not to modify exc, but other code might.

C ++常见问题有关于 const方法。

The C++ FAQ has a bit more information on const methods.

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