c++ - 基于范围的循环和范围类型

Question

引用以下在 Range-based for loop 下提到的文本片段

If range_expression is an expression of a class type C that has both a member named begin and a member named end (regardless of the type or accessibility of such member), then begin_expr is __range.begin() and end_expr is __range.end();

The member interpretation is used if the range type has a member named begin and a member named end. This is done regardless of whether the member is a type, data member, function, or enumerator, and regardless of its accessibility. Thus a class like class meow { enum { begin = 1, end = 2}; /* rest of class */ }; cannot be used with the range-based for loop even if the namespace-scope begin/end functions are present.

begin 和end 应该只是范围类型(​​可能返回迭代器类型)的成员函数名称的理解是否正确？很想了解此约束没有任何解决方法背后的技术原因。

Answer 1

您需要提供非成员begin 和end 函数(或其他可调用对象)，这些函数可以通过argument-dependent lookup 找到。 ，或者具有显式命名为 begin 和 end 的成员函数(或可调用对象)，它们不带参数，并且返回类似迭代器的对象(即它们应该支持后缀 ++ 和取消引用 *，当然可以使用 == 进行比较。

至于“技术原因”，这是因为基于范围的 for 循环只是普通迭代器 for 循环的语法糖。并且编译器必须知道为“开始”和“结束”迭代器调用哪些函数。