错误C2582：'B'中没有'operator ='功能 [英] error C2582: 'operator =' function is unavailable in 'B'

问题描述

以下代码在main（）中的 e = f行给出了编译错误（至少在使用MS VS 2008时）：

Following code gives a compilation error (at least when using MS VS 2008) for line "e=f" in main():

错误C2582：'运算符='函数在'B'中不可用

error C2582: 'operator =' function is unavailable in 'B'

class A {
public:
    A() { }
    static const double x;
};
const double A::x = 0.0;

class B {
public:
    B() : x(0.0) { }
    const double x;
};

int main( int argc, char *argv[] )
{
    A c,d;
    B e,f;

    c = d;
    e = f;

    return 0;
}

应同时为A和B两个类生成默认赋值运算符！

The default assignment operator should be generated for both classes, A and B !?

在12.8.10中：如果类定义未显式声明
复制赋值运算符，则将隐式声明一个。

in 12.8.10: "If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly."

推荐答案

隐式生成的运算符将递归地分配每个非静态成员。但是， x 是 const ，因此无法分配给它。这样可以防止隐式运算符的生成（特别是，它导致将其定义为已删除）。

The implicitly generated operator would recursively assign each non-static member. However, x is const, so it can't be assigned to. This prevents the implicit operator from being generated (specifically, it causes it to be defined as deleted).

在C ++ 11 12.8 / 23中指定：

This is specified in C++11 12.8/23:

如果X具有：

A defaulted copy/move assignment operator for class X is defined as deleted if X has:

• ，则将类别X的默认复制/移动赋值运算符定义为已删除。


• const非类类型（或其数组）的非静态数据成员，或


• ...

（尽管我刚刚注意到您的编译器早于C ++ 11；规则相似，但是用较老的方言指定

(Although I just noticed that your compiler predates C++11; the rules are similar, but specified in different language, in older dialects with no concept of "deleted" functions).

如果您要为其成员（或基类）无法重新分配的类分配赋值运算符，则必须定义

If you want an assignment operator for a class whose members (or base classes) can't be reassigned, you'll have to define it yourself.

在 A类中，常量成员是静态的，因此不构成对象的一部分。因此，它不会阻止将（空）对象分配给该对象。

In class A, the constant member is static, so doesn't form part of an object. Therefore, it doesn't prevent an (empty) object from being assigned to.

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