初始化成员函数字段

Question

在阅读名为 Abominable functions 的 C++1z 论文时，我发现了以下代码：

class rectangle {
    public:
    using int_property = int() const;  // common signature for several methods

    int_property top;
    int_property left;
    int_property bottom;
    int_property right;
    int_property width;
    int_property height;

    // Remaining details elided
};

我以前从未见过这样的代码（论文本身说找到这样的代码很奇怪）所以我想尝试一下这种方法并给那些 int_property:

class rectangle {
    int f() const { return 0; }
    public:
    using int_property = int() const;  // common signature for several methods

    int_property top = f; // <--- error!
    int_property left;
    int_property bottom;
    int_property right;
    int_property width;
    int_property height;

    // Remaining details elided
};

在我上面的修改中，编译器抱怨 f 说 (only '= 0' is allowed) before ';' token；我的其他尝试是：

class rectangle {
    int f() const { return 0; }
    public:
    using int_property = int() const;  // common signature for several methods

        // invalid initializer for member function 'int rectangle::top() const'
        int_property top{f};
        int_property left{&f};

        // invalid pure specifier (only '= 0' is allowed) before ';' token
        int_property bottom = f;
        int_property right = &f;

        int_property width;
        int_property height;

        // class 'rectangle' does not have any field named 'width'
        // class 'rectangle' does not have any field named 'height'
        rectangle() : width{f}, height{&rectangle::f} {}
};

所以问题是：

我应该怎么做才能使所有int_property“字段”都指向一个函数？
如何为所有int_property“字段”赋值？

Answer 1

该论文在 2015 年 11 月 10 日作为提案 #P0172R0 引入 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/ . I believe you are using compiler that is not supporting this functionality currently, but you can check later :). Also it might be interesting for you to read current standard http://open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf，或者至少检查您的编译器当前支持的功能。

Answer 2

int() const 是一个带有 cv-qualifier 的函数类型。声明 int_property top; 声明了一个函数，而不是一个变量。此声明与 int top() const;.

具有相同的效果

与其他成员函数一样，您可以通过提供函数定义来定义它们。

int rectangle::top() const {
    return 0;
}

初始化成员函数字段

Initializing member functions fields

c++

compiler-errors

member-functions