Question

在Kenny Kerr先生的this column中，他定义了一个结构和类似于这样的typedef：

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

那么这个typedef是什么意思？

另一个问题是关于以下代码：

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

“＆amp; boolean_struct :: member”是什么意思？

Answer 1

在Kenny Kerr先生的本专栏中，他定义了一个struct和一个typedef 这样：
struct boolean_struct { int member; };    
typedef int boolean_struct::* boolean_type;    
那么这个typedef是什么意思？

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

指向int的指针不一样。区别在于boolean_type的对象需要boolean_struct个对象才能取消引用它。指向int的普通指针不会。查看其不同之处的最佳方法是通过一些代码示例。

仅考虑int的正常指针：

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;
}

现在考虑我们是否使用int内的boolean_struct成员指针：

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，语法及其行为是不同的。

另一个问题是关于以下代码：
operator boolean_type() const throw()
{    
    return Traits::invalid() != m_value ? &boolean_struct::member : nullptr;  
}
“＆amp; boolean_struct :: member”是什么意思？

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

这个typedef是什么意思？

1 个答案: