如何创建const boost :: iterator_range [英] How do I create a const boost::iterator_range

问题描述

上的评论为什么boost :: find_first对其输入采用非const引用？建议调用者使用const_iterator模板参数创建非const iterator_range以证明迭代对象具有足够的生命周期。

The comment at Why does boost::find_first take a non-const reference to its input? suggests "the caller to create a non-const iterator_range with const_iterator template parameter to "prove" that the iterated object has a sufficient lifetime."

这是什么意思，我该怎么做？

What does this mean and how do I do it?

特别是，我如何实现const-这段代码的正确性？

In particular, how do I achieve const-correctness with this code?

typedef std::map<int, double> tMyMap;
tMyMap::const_iterator subrange_begin = my_map.lower_bound(123);
tMyMap::const_iterator subrange_end = my_map.upper_bound(456);

// I'd like to return a subrange that can't modify my_map
// but this vomits template errors complaining about const_iterators
return boost::iterator_range<tMyMap::const_iterator>(subrange_begin, subrange_end);  

推荐答案

在范围内使用非const引用 避免绑定到临时工具¹

Having non-const references to the range avoids binding to temporaries ¹

我通过让编译器完成你的工作来避免你的难题：

I'd avoid your conundrum² by letting the compiler do your work:

tMyMap const& my_map; // NOTE const
// ...

return boost::make_iterator_range(my_map.lower_bound(123), mymap.upper_bound(456));

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¹标准C ++延长了绑定到const的临时生命周期-reference变量，但这不适用于绑定到对象成员的引用。因此，通过引用汇总范围很容易出现这种错误。

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¹ Standard C++ extends lifetimes of temporaries bound to const-reference variables, but this doesn't apply to references bound to object members. Therefore, aggregating ranges by reference is very prone to this mistake.

/ OT：IMO甚至 注意事项/检查一些Boost Range功能（像适配器一样，使用起来经常太不安全;我经常陷入这些陷阱，而不是承认。

/OT: IMO even with the precautions/checks some Boost Range features (like adaptors) are frequently too unsafe to use; I've fallen into those traps more often than I care to admit.

²除了我们无法从您提供的样本中重现

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