[英]Allocating class member with std::shared_ptr
Is my assumption, that in following example, memory referenced by b
will be deallocated once instance of A
goes out of scope at end of func()
, correct? 我的假设是,在下面的示例中,一旦A
实例超出func()
的范围,对b
引用的内存将进行重新分配,对吗?
class A{
public:
A() {
b = std::shared_ptr<char>(new char[100] { 0 } );
}
char* b;
}
void func {
A a;
}
No, not correct. 不,不正确。 b
is of type char *
and you assign to it a shared_ptr<char>
. b
的类型为char *
并为其分配了shared_ptr<char>
。 You should get a compilation error. 您应该得到一个编译错误。
Furthermore, the constructor is private
, another compilation error. 此外,构造函数是private
,这是另一个编译错误。
And how do you access b
in func()
? 以及如何在func()
访问b
? It is private in A
. 它在A
是私有A
。
Obviously your exercise is incomplete... so I just go from what you provided. 显然您的锻炼还不完整...所以我只是从您提供的内容出发。
Also I suggest to use unique_ptr
in case you can say it is a unique ownership (which it appears to be in my opinion). 另外,我建议使用unique_ptr
,以防您可以说这是唯一所有权(在我看来这是唯一的)。
This compiles: 这样编译:
#include <memory>
#include <iostream>
class A {
public:
A() {
std::cout << "A created with unique pointer" << std::endl;
b = std::unique_ptr<char>(new char[100] {
0
});
}
~A() {
std::cout << "A destroyed" << std::endl;
}
private:
std::unique_ptr<char> b;
};
void func() {
A a;
}
int main() {
std::cout << "Call func()" << std::endl;
func();
std::cout << "func() called" << std::endl;
return 0;
}
And at the end of func()
A
is destroyed and with it the unique_ptr
. 并且在func()
的末尾销毁了A
并且销毁了unique_ptr
。
However, ask yourself if you really need to use pointer? 但是,问问自己是否真的需要使用指针? In your case an automatic variable should be just fine; 在您的情况下,自动变量应该就可以了; it does the same (ie being destroyed when func()
exits. 它执行相同的操作(即当func()
退出时被销毁。
You're assigning to a char*
member, so I'm pretty sure the shared_ptr
is cleaned up as soon as the assignment completes (because it wasn't stored to another shared_ptr
that would maintain the reference count). 您正在分配给char*
成员,因此,我很确定在分配完成后就将对shared_ptr
进行清理(因为它没有存储到其他保持引用计数的shared_ptr
中)。
If you want to use shared_ptr
s, you have to use them consistently; 如果要使用shared_ptr
,则必须始终使用它们。 a raw pointer class member can't manage anything for you. 原始指针类成员无法为您管理任何事情。 The pointer is still initialized to where the memory was allocated, but it's not allocated anymore; 指针仍然初始化为分配内存的位置,但是不再分配。 accessing it even a line later in the constructor would be undefined behavior/a use-after-free bug. 即使在构造函数中稍后访问它,也将是未定义的行为/释放后使用的错误。
In any event, not safe because default shared_ptr
uses delete
, not delete[]
. 无论如何,这是不安全的, 因为默认的shared_ptr
使用delete
而不是delete[]
。 Is there some reason a std::shared_ptr<std::array<char, 100>>
couldn't be used? 是否有某种原因不能使用std::shared_ptr<std::array<char, 100>>
?
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