= delete的一些用途是什么？ [英] What are some uses for =delete?
问题描述
今天上午，我问了一个问题导致另一个：我应该什么时候使用 = delete ？我不认为有专门的专门针对 = delete 在SO，所以我查了一本书叫C ++编程语言。
如果还有更多话要说，或者我错了，请评论或回答。
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= delete 是非常有用的！这里有几个例子： 基本上我们可以防止复制基类，因为它可能经常导致切片：
struct Base {
Base（）{}
Base& operator =（const Base&）= delete; // disallow copying
Base（const Base&）= delete;
基本& operator =（Base&&）= delete; // disallow moving
Base（Base&&）= delete;
};
struct Der：public Base {};
void func（）{
Der d;
Base base = d; //这将不工作，因为复制构造函数被删除！
//这种行为是需要的 - 否则会发生切片
}
当模板函数无法以某种类型运行时，它也很有用：
template< class T>
void fn（T p）{/ * ... * /}; // do something with T
void fn（int）= delete; //不允许使用int
void fun（）{
fn（4）; //啊哈！不能使用fn（）与int！
fn（-4.5）; // fine
fn（hello）; // fine
}
= delete 也可以禁止不必要的转换：
struct Z {
Z（double）; //可以用double
初始化Z（int）= delete; //但不是整数
};
void f（）{
Z z1 {1}; //错误！不能使用int
Z z2 {1.0}; // double is ok
}
$ b b
= delete 的一些更高级用途包括禁止堆栈或免费商店分配：
class FS_Only {
〜FS_Only（）= delete; // disallow stack allocation
};
类Stack_Only {
void * operator new（size_t）= delete; // disallow heap allocation
};
...你的想法。希望这有助于某人！ = delete 可以帮助编写可读，无bug和优雅的代码。
编辑：
正如在注释中正确指出的，现在不可能删除 FS_Only 所以这一个不是很好地使用 = delete 毕竟。
Earlier today I asked a question that led to another one: When should I use =delete? I don't think there is a post dedicated solely to =delete on SO, so I looked it up in a book called "The C++ Programming Language". I will list my findings in my answer below.
Please comment or answer if there's more to say or if I'm mistaken.
It turns out that =delete is extremely useful! Here are a few examples:
Basically we can prevent copying base classes because it might often lead to slicing:
struct Base {
Base(){}
Base& operator=(const Base&) = delete; // disallow copying
Base(const Base&) = delete;
Base& operator=(Base && ) = delete; // disallow moving
Base(Base && ) = delete;
};
struct Der : public Base {};
void func() {
Der d;
Base base = d; // this won't work because the copy constructor is deleted!
// this behavior is desired - otherwise slicing would occur
}
It's also useful when a template function cannot run with a certain type:
template<class T>
void fn(T p) { /* ... */ }; // do something with T
void fn(int) = delete; // disallow use with int
void fun() {
fn(4); // aha! cannot use fn() with int!
fn(-4.5); // fine
fn("hello");// fine
}
=delete can also disallow undesired conversions:
struct Z {
Z(double); // can initialize with a double
Z(int) = delete; // but not with an integer
};
void f() {
Z z1 { 1 }; // error! can't use int
Z z2 { 1.0 }; // double is ok
}
Some more advanced uses of =delete include prohibiting stack or free store allocation:
class FS_Only {
~FS_Only() = delete; // disallow stack allocation
};
class Stack_Only {
void* operator new(size_t) = delete; // disallow heap allocation
};
... You get the idea. Hope this helps someone! =delete can help write readable, bugless and elegant code.
Edit:
As it was correctly pointed out in the comments, it is now impossible to delete FS_Only objects, so this one isn't such a good use of =delete after all.
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