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c++ - Deleted implicitly-declared copy assignment operator

问题描述

According to the C++ reference on Copy assignment operator:

A defaulted copy assignment operator for class T is defined as deleted if any of the following is true

T has a non-static data member of non-class type (or array thereof) that is const ...

I was hoping to create a case where I had a const class-type data member and a defaulted copy assignment operator not defined as deleted. In doing so, I found a discrepancy between clang and gcc. Consider the following code:

struct B {
  void operator=(const B&) const {}
};

struct A {
  const B b{};
  A& operator=(const A&) = default;
};

int main() {
  A a1{}, a2{};
  a1 = a2;       //only works with clang
  B b1{}, b2{};
  b1 = b2;       //works in both
}

When I compile this with g++ -std=c++14 I get the following errors:

In member function ‘A& A::operator=(const A&)’:
error: non-static const member ‘const B A::b’, can’t use default assignment operator
note: synthesized method ‘A& A::operator=(const A&)’ first required here

This does, comma, however, compile with clang, as the reference seems to indicate that it should. Am I in error? Which compiler is correct?

I'm using gcc version 7.3.0 (Ubuntu 7.3.0-27ubuntu1~18.04) and clang version 6.0.0-1ubuntu2.

标签： c++g++c++14clang++assignment-operator

解决方案

看来clang是对的，

尽管尚未确认，但有一份关于 gcc 主题的报告，正如有人指出的那样，与本案相关的两条规则不适用

[class.copy.assign]/7

(7.2) const 非类类型（或其数组）的非静态数据成员，或

[...]

(7.4) 类类型 M（或其数组）的直接非静态数据成员或由于重载决议（[over.match]）而无法复制/移动的直接基类 M，用于查找 M 的相应赋值运算符，导致歧义或从默认赋值运算符中删除或无法访问的函数。

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