c++ - 这个typedef是什么意思？

Question

在 Kenny Kerr 先生的 this column ，他定义了一个结构和一个 typedef，如下所示:

struct boolean_struct { int member; };
typedef int boolean_struct::* boolean_type;

那么这个typedef是什么意思呢？

另一个问题是关于下面的代码:

operator boolean_type() const throw()
{
    return Traits::invalid() != m_value ? &boolean_struct::member : nullptr;
}

“&boolean_struct::member”是什么意思？

Answer 1

In Mr Kenny Kerr's this column, he defined a struct and a typedef like this:

struct boolean_struct { int member; };    
typedef int boolean_struct::* boolean_type;    

Then what is the meaning of this typedef?

typedef 创建一个名为 boolean_type 的类型，它相当于指向 boolean_struct 中的 int 成员的指针> 对象。

与指向 int 的指针不同。区别在于 boolean_type 的对象需要一个 boolean_struct 对象才能取消引用它。指向 int 的普通指针不会。了解这有何不同的最佳方式是通过一些代码示例。

只考虑指向 int 的普通指针:

struct boolean_struct { int member; }; 

int main()
{
    // Two boolean_struct objects called bs1 and bs2 respectively:
    boolean_struct bs1;
    boolean_struct bs2;
    // Initialize each to have a unique value for member:
    bs1.member = 7;
    bs2.member = 14;

    // Obtaining a pointer to an int, which happens to be inside a boolean_struct:
    int* pi1 = &(bs1.member);
    // I can dereference it simply like this:
    int value1 = *pi1;
    // value1 now has value 7.

    // Obtaining another pointer to an int, which happens to be inside
    // another boolean_struct:
    int* pi2 = &(bs2.member);
    // Again, I can dereference it simply like this:
    int value2 = *pi2; 
    // value2 now has value 14.

    return 0;
}

现在考虑如果我们在 boolean_struct 中使用指向 int 成员的指针:

struct boolean_struct { int member; }; 
typedef int boolean_struct::* boolean_type;   

int main()
{

    // Two boolean_struct objects called bs1 and bs2 respectively: 
    boolean_struct bs1; 
    boolean_struct bs2; 
    // Initialize each to have a unique value for member: 
    bs1.member = 7; 
    bs2.member = 14; 

    // Obtaining a pointer to an int member inside a boolean_struct
    boolean_type pibs = &boolean_struct::member;

    // Note that in order to dereference it I need a boolean_struct object (bs1):
    int value3 = bs1.*pibs;
    // value3 now has value 7.

    // I can use the same pibs variable to get the value of member from a
    // different boolean_struct (bs2):
    int value4 = bs2.*pibs;
    // value4 now has value 14.

    return 0;
} 

如您所见，语法和它们的行为是不同的。

Another question is concerning the following code:

operator boolean_type() const throw()
{    
    return Traits::invalid() != m_value ? &boolean_struct::member : nullptr;  
}

What is the meaning of "&boolean_struct::member" ?

这将返回 boolean_struct 中的 member 变量的地址。参见上面的代码示例。