[英]Can you write a copy constructor for a union with const members?
Suppose I have a struct that contains a union with const
members, like so:假设我有一个包含
const
成员联合的结构,如下所示:
struct S
{
// Members
const enum { NUM, STR } type;
union
{
const int a;
const std::string s;
};
// Constructors
S(int t_a) : type(NUM), a(t_a);
S(const std::string & t_s) : type(STR), s(t_s);
};
So far, so good.到现在为止还挺好。 But now say I want to write a copy-constructor for this type.
但是现在说我想为这种类型编写一个复制构造函数。
It doesn't seem like this involves doing anything nefarious, but since I need to initialize the const members in member initializers I don't see how to do this based on logic that depends on the type
member.这似乎并不涉及做任何邪恶的事情,但是由于我需要在成员初始值设定项中初始化 const 成员,因此我不知道如何根据取决于
type
成员的逻辑来执行此操作。
Questions:问题:
Is it possible to write this constructor?是否可以编写此构造函数?
If not, is this essentially a syntactic oversight, or is there some fundamental reason that the language can't support such a thing?如果不是,这本质上是一种语法上的疏忽,还是存在某种语言不能支持这样的事情的根本原因?
Yes, it is possible to write copy constructor here.是的,可以在这里编写复制构造函数。 Actually it is already done inside
std::variant
implementation, which shall support const
-types among others.实际上它已经在
std::variant
实现中完成,它应该支持const
类型等。 So your class S
can be replaced with所以你的类
S
可以替换为
using S = std::variant<const int, const std::string>;
But if for dome reason you cannot use std::variant
then copy-constructor can be written using std::construct_at
function as follows:但是,如果由于圆顶原因您不能使用
std::variant
则可以使用std::construct_at
函数编写复制std::construct_at
函数,如下所示:
#include <string>
struct S {
const enum { NUM, STR } type;
union {
const int a;
const std::string s;
};
S(int t_a) : type(NUM), a(t_a) {}
S(const std::string & t_s) : type(STR), s(t_s) {}
S(const S & rhs) : type(rhs.type) {
if ( type == NUM ) std::construct_at( &a, rhs.a );
if ( type == STR ) std::construct_at( &s, rhs.s );
}
~S() {
if ( type == STR ) s.~basic_string();
}
};
int main() {
S s(1);
S u = s;
S v("abc");
S w = v;
}
Demo: https://gcc.godbolt.org/z/TPe8onhWs演示: https : //gcc.godbolt.org/z/TPe8onhWs
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