[英]C++ How to Initialize member object?
Is there some way to initialize(?) member variable c of type C in the first part of the below example? 在下面的示例的第一部分中,是否有某种方法可以初始化C类型的成员变量c? Or must I use the new() method shown in the second part of the example? 还是必须使用示例第二部分中显示的new()方法?
Class B takes class A as an injected dependency. B类将A类作为注入依赖项。 So does class C. Class B is additionally composed of class C. C类也是如此。B类另外由C类组成。
How do I get the injected A to B's member C? 我如何获得注入的A给B的成员C?
Part 1 第1部分
class A { // ...; };
class C {
public:
C(A &a) : a(a) {} // constructor
};
// Does not work as is. Want to make compiler manage C lifetime.
class B {
public:
B(A &a); // constructor
C c(a); // member variable
};
// constructor
B::B(A &a) : a(a) {
}
Part 2 第2部分
// Works, but requires programmer to manage C's lifetime.
class B {
public:
B(A &a); // constructor
C *c; // member variable
};
// constructor
B::B(A &a) : a(a) {
c = new C(a);
}
Several good answers below! 以下是几个不错的答案! My apologies for the confusing example. 对于令人困惑的示例,我深表歉意。 I have up-voted all of the good answers and questions. 我对所有好的答案和问题都投票赞成。 Unfortunately I can only mark one answer as the accepted answer, so I am choosing the first one which gave me the "ah-ha" moment in which I saw the solution to my real problem, which was more complex than my lame example here. 不幸的是,我只能将一个答案标记为已接受的答案,因此我选择的第一个答案是“ ah-ha”,在那一刻中,我看到了实际问题的解决方案,这比此处的la脚示例更为复杂。
Member variables are initialized in constructor's initialization list (before body) so you need to do that: 成员变量在构造函数的初始化列表(在主体之前)中初始化,因此您需要执行以下操作:
B::B(A &a)
: c(a) // Calls constructor C(a) on member c
{}
You almost have it: 您几乎拥有它:
class B {
public:
B(A &a);
C c(a); //see note 1
};
B::B(A &a) : a(a) { //see note 2
}
There are two problems with C c(a);
C c(a);
有两个问题C c(a);
here: 这里:
a
is not in scope. a
不在范围内。 a
only exists within the scope of the constructor, so c
needs to be initialized from there. a
仅存在于构造函数的范围内,因此c
需要从那里初始化。 C c = value;
) or braces ( C c{value};
) when initializing an NSDMI. 但是,即使在C ++ 11中,初始化NSDMI时也必须使用等号( C c = value;
)或大括号( C c{value};
)。 You've almost got this right: 您几乎完全正确:
B::B(A &a) : a(a)
You're trying to initialize a data member called a
with the argument given to the constructor. 您正在尝试使用提供给构造函数的参数来初始化名为a
的数据成员。 You actually want to initialize c
like this, not a non-existent a
: 您实际上想要这样初始化c
,而不是不存在的a
:
B::B(A &a) : c(a)
The lifetime of c
will be that of the instance of the B
class. c
的生存期将是B
类实例的生存期。 Using dynamic memory management is certainly not necessary. 使用动态内存管理当然不是必需的。
"How do I get the injected A to B's member C?" “我如何将注入的A交给B的成员C?”
You can do so using B
's constructor member initializer list 您可以使用B
的构造函数成员初始化器列表进行操作
class B {
public:
B(A &a) : c(a) {
// ^^^^
}
C c; // <<< It's not possible to initialize members in their
// declaration.
};
The following: 下列:
C(A &a) : a(a) {}
will not compile since a
(the first a
in the initialization list) is not a member variable of C
. 将不会编译,因为a
(初始化列表中的第a
)不是C
的成员变量。
Same applies to the constructor of B
. 同样适用于B
的构造函数。
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