blocks|key|2572260|text|这是C%2B%2B20之前的aggregate+initialization的一个微妙问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2572261|在C%2B%2B20之前，B+(和A)是aggregate+types|style|CODE|2572262|(强调我的)|2572263|2572264|没有用户提供的、继承的或显式的构造函数(显式默认或删除的构造函数被允许)(从C%2B%2B17开始)(直到C%2B%2B20)|blockquote|BOLD|2572265|2572266|2572267|然后|2572268|2572269|如果初始化器子句的数目小于成员数目and+bases+(since+C%2B%2B17)或者初始化器列表完全为空，则按照通常的列表初始化规则(对于非类类型和使用默认构造函数的非聚合类执行值初始化，对于聚合体执行聚合初始化)，剩余的成员and+bases+(since+C%2B%2B17)通过空列表进行by+their+default+member+initializers,+if+provided+in+the+class+definition,+and+otherwise+(since+C%2B%2B14)初始化。|2572270|2572271|2572272|因此，B{}通过聚合初始化来构造一个临时对象，它将直接用空表初始化基对象，即进行聚合初始化来构造A基子对象。请注意，绕过了B的构造函数。问题是在这样的上下文中，不能调用protected描述器来销毁直接构造的A类型的基子对象。(它不会抱怨protected构造函数，因为A的聚合初始化也绕过了它。)|2572273|您可以将其更改为foo(B());以避免聚合初始化；B()执行value-initialization，临时对象将由B的构造函数初始化，然后一切正常。|2572274|顺便说一句，既然使用了C%2B%2B20，你的代码就会工作得很好。|2572275|2572276|没有用户声明或继承的构造函数(从C%2B%2B20开始)|2572277|2572278|2572279|B+(和A)不再是聚合类型。B{}先执行list+initialization，然后用B的构造函数初始化临时对象，效果和B()一样。|2572280|entityMap|0|LINK|mutability|MUTABLE|url|https://en.cppreference.com/w/cpp/language/aggregate_initialization|1|https://en.cppreference.com/w/cpp/language/aggregate_initialization#Explanation|2|https://en.cppreference.com/w/cpp/language/value_initialization|3|https://en.cppreference.com/w/cpp/language/list_initialization^0|A|O|0|0|9|1|D|1|G|F|1|0|0|0|K|F|0|0|0|0|0|H|N|35|N|3Z|2U|0|0|0|3|3|1C|1|1P|1|2C|9|2W|1|3B|9|3R|1|0|8|9|Q|3|1M|1|V|K|2|0|0|0|0|0|0|0|1|4|1|E|3|17|1|1O|3|K|J|3|0^^$0|@$1|2|3|4|5|6|7|1O|8|@]|9|@$A|1P|B|1Q|1|1R]]|C|$]]|$1|D|3|E|5|6|7|1S|8|@$A|1T|B|1U|F|G]|$A|1V|B|1W|F|G]]|9|@$A|1X|B|1Y|1|1Z]]|C|$]]|$1|H|3|I|5|6|7|20|8|@]|9|@]|C|$]]|$1|J|3|-4|5|6|7|21|8|@]|9|@]|C|$]]|$1|K|3|L|5|M|7|22|8|@$A|23|B|24|F|N]]|9|@]|C|$]]|$1|O|3|-4|5|6|7|25|8|@]|9|@]|C|$]]|$1|P|3|-4|5|6|7|26|8|@]|9|@]|C|$]]|$1|Q|3|R|5|6|7|27|8|@]|9|@]|C|$]]|$1|S|3|-4|5|6|7|28|8|@]|9|@]|C|$]]|$1|T|3|U|5|M|7|29|8|@$A|2A|B|2B|F|G]|$A|2C|B|2D|F|G]|$A|2E|B|2F|F|G]]|9|@]|C|$]]|$1|V|3|-4|5|6|7|2G|8|@]|9|@]|C|$]]|$1|W|3|-4|5|6|7|2H|8|@]|9|@]|C|$]]|$1|X|3|Y|5|6|7|2I|8|@$A|2J|B|2K|F|G]|$A|2L|B|2M|F|G]|$A|2N|B|2O|F|G]|$A|2P|B|2Q|F|G]|$A|2R|B|2S|F|G]|$A|2T|B|2U|F|G]|$A|2V|B|2W|F|G]]|9|@]|C|$]]|$1|Z|3|10|5|6|7|2X|8|@$A|2Y|B|2Z|F|G]|$A|30|B|31|F|G]|$A|32|B|33|F|G]]|9|@$A|34|B|35|1|36]]|C|$]]|$1|11|3|12|5|6|7|37|8|@]|9|@]|C|$]]|$1|13|3|-4|5|6|7|38|8|@]|9|@]|C|$]]|$1|14|3|15|5|M|7|39|8|@]|9|@]|C|$]]|$1|16|3|-4|5|6|7|3A|8|@]|9|@]|C|$]]|$1|17|3|-4|5|6|7|3B|8|@]|9|@]|C|$]]|$1|18|3|19|5|6|7|3C|8|@$A|3D|B|3E|F|G]|$A|3F|B|3G|F|G]|$A|3H|B|3I|F|G]|$A|3J|B|3K|F|G]|$A|3L|B|3M|F|G]]|9|@$A|3N|B|3O|1|3P]]|C|$]]|$1|1A|3|-4|5|6|7|3Q|8|@]|9|@]|C|$]]]|1B|$1C|$5|1D|1E|1F|C|$1G|1H]]|1I|$5|1D|1E|1F|C|$1G|1J]]|1K|$5|1D|1E|1F|C|$1G|1L]]|1M|$5|1D|1E|1F|C|$1G|1N]]]]

This is a subtle issue of <a href="https://en.cppreference.com/w/cpp/language/aggregate_initialization" rel="nofollow noreferrer">aggregate initialization</a> before C++20. 

Before C++20, <code>B</code> (and <code>A</code>) are <a href="https://en.cppreference.com/w/cpp/language/aggregate_initialization#Explanation" rel="nofollow noreferrer">aggregate types</a>:

(emphasis mine)

<blockquote>
 no user-provided, inherited, or explicit constructors (explicitly defaulted or deleted constructors are allowed) (since C++17) (until C++20)
</blockquote>

Then

<blockquote>
 If the number of initializer clauses is less than the number of members <code>and bases (since C++17)</code> or initializer list is completely empty, the remaining members <code>and bases (since C++17)</code> are initialized <code>by their default member initializers, if provided in the class definition, and otherwise (since C++14)</code> by empty lists, in accordance with the usual list-initialization rules (which performs value-initialization for non-class types and non-aggregate classes with default constructors, and aggregate initialization for aggregates).
</blockquote>

So <code>B{}</code> constructs a temporary object via aggregate initialization, which will initialize the base subobject directly with empty list, i.e. perform aggregate initialization to construct the <code>A</code> base subobject. Note that the constructor of <code>B</code> is bypassed. The problem is that in such context, the <code>protected</code> desctructor can't be called to destroy the directly constructed base subobject of type <code>A</code>. (It doesn't complain about the <code>protected</code> constructor because it's bypassed by the aggregate initialization of <code>A</code> too.)

You can change it to <code>foo(B());</code> to avoid aggregate initialization; <code>B()</code> performs <a href="https://en.cppreference.com/w/cpp/language/value_initialization" rel="nofollow noreferrer">value-initialization</a>, the temporary object will be initialized by <code>B</code>'s constructor, then anything is fine. 

BTW since C++20 you code will work fine.

<blockquote>
 no user-declared or inherited constructors (since C++20)
</blockquote>

<code>B</code> (and <code>A</code>) are not aggregate types again. <code>B{}</code> performes <a href="https://en.cppreference.com/w/cpp/language/list_initialization" rel="nofollow noreferrer">list initialization</a>, and then the temporary object is initialized by <code>B</code>'s constructor; the effect is just same as <code>B()</code>.

In the following code, compiled with Clang 8.0.0+ and <code>-std=c++17</code>, creating a derived class instance using <code>B{}</code> gives an error <code>error: temporary of type 'A' has protected destructor</code>. Why is <code>A</code> appearing in this message when the temporary has type <code>B</code> (and thus should have a public destructor)?

<a href="https://godbolt.org/z/uOzwYa" rel="nofollow noreferrer">https://godbolt.org/z/uOzwYa</a>

<pre><code>class A {
protected:
 A() = default;
 ~A() = default;
};

class B : public A {
// can also omit these 3 lines with the same result
public:
 B() = default;
 ~B() = default;
};

void foo(const B&amp;) {}

int main() {

 // error: temporary of type 'A' has protected destructor
 foo(B{});
 // ^

 return 0;
}
</code></pre>

"temporary of type 'A' has protected destructor", but its type is B

在使用Clang 8.0.0+和-std=c++17编译的以下代码中，使用B{}创建派生类实例会产生错误error: temporary of type 'A' has protected destructor。当临时的类型为B (因此应该有一个公共析构函数)时，为什么A会出现在此消息中？class A {protec...

问“类型为'A‘的临时函数具有受保护的析构函数”，但其类型为B
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问“类型为'A‘的临时函数具有受保护的析构函数”，但其类型为BEN

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问“类型为'A‘的临时函数具有受保护的析构函数”，但其类型为B
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