[英]c++ class variables and default construction
class ClassB
{
int option;
public:
ClassB(void){} //without the default constructor, the example does not compile.
ClassB(int option)
{
this->option = option;
}
};
class ClassA
{
ClassB objB; //initialize objB using default constructor. When does this actually occur?
public:
ClassA(int option)
{
objB = ClassB(option); //initialize objB again using another constructor.
}
};
int main(void)
{
ClassA objA (2);
return 0;
}
I'm new to c++ (coming from c#), and i'm a bit confused about how class variables are initialized. 我是c ++的新手(来自c#),并且对如何初始化类变量有些困惑。 In the above example, ClassA declares a class object of type ClassB, stored by value. 在上面的示例中,ClassA声明了一个ClassB类型的类对象,按值存储。 In order to do this, ClassB must have a default constructor, which implies that ClassA first creates a ClassB using the default constructor. 为此,ClassB必须具有默认构造函数,这意味着ClassA首先使用默认构造函数创建ClassB。 But ClassA never uses this default objB because it's immediately overwritten in ClassA's constructor. 但是ClassA从未使用此默认objB,因为它会立即在ClassA的构造函数中被覆盖。
So my question: Is objB actually initialized twice? 所以我的问题是:objB实际上被初始化了两次吗?
If so, isn't that an unnecessary step? 如果是这样,那不是不必要的步骤吗? Would it be faster to declare objB as a pointer? 将objB声明为指针会更快吗?
If not, why does ClassB need to have a default constructor? 如果没有,为什么ClassB需要一个默认的构造函数?
The reason for this is that you are not initializing the objB
data member, but assigning to it after it has been default constructed. 这样做的原因是您没有初始化objB
数据成员,而是在默认构造它之后对其进行分配 。
ClassA(int option)
{
// By the time you get here, objB has already been constructed
// This requires that ClassB be default constructable.
objB = ClassB(option); // this is an assignment, not an initialization
}
To initialize it, use the constructor member initialization list: 要初始化它,请使用构造函数成员初始化列表:
ClassA(int option) : objB(option) {}
This initializes objB
with the right constructor, and does not require ClassB
to be default constructable. 这将使用正确的构造函数初始化objB
,并且不需要ClassB
是默认可构造的。 Note that the same applies to ClassB
, whose constructors should be 请注意,这同样适用于ClassB
,其构造函数应为
ClassB() : option() {} // initializes option with value 0
ClassB(int option) : option(option) {}
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