这是一个单独的迭代器,如果是,我可以将它与另一个比较吗? [英] Is this a singular iterator and, if so, can I compare it to another one?
问题描述
我一直认为一个奇异迭代器是一个默认初始化的迭代器,它们可以作为可比较的 sentinel 值:
I always thought that a "singular" iterator was one that has been default-initialised, and these could serve as comparable sentinel values of sorts:
typedef std::vector<Elem>::iterator I;
I start = I();
std::vector<Elem> container = foo();
for (I it = container.begin(), end = container.end(); it != end; ++it) {
if ((start == I()) && bar(it)) {
// Does something only the first time bar(it) is satisfied
// ...
start = it;
}
}
但这个回答不仅表明我对单数的定义是错误的,而且我的上述比较是完全非法的。
But this answer suggests not only that my definition of "singular" is wrong, but also that my comparison above is totally illegal.
是吗?
推荐答案
显然,这将适用于一些迭代器 - T *
是一个明显的例子 - 但它绝对不能保证所有迭代器的正确行为。 C ++ 11 24.2.1 [iterator.requirements.general] p5:
Obviously this will work for some iterators - T*
being a clear example - but it's definitely not guaranteed correct behavior for all iterators. C++11 24.2.1 [iterator.requirements.general] p5:
with any sequence ...大多数表达式的结果未定义为奇异值;唯一的
例外是破坏一个拥有一个奇异值的迭代器,一个非奇异值的
赋值给一个拥有
奇异值的迭代器,对于满足
DefaultConstructible要求,使用值初始化的迭代器
作为复制或移动操作的源。
Singular values are not associated with any sequence ... Results of most expressions are undefined for singular values; the only exceptions are destroying an iterator that holds a singular value, the assignment of a non-singular value to an iterator that holds a singular value, and, for iterators that satisfy the DefaultConstructible requirements, using a value-initialized iterator as the source of a copy or move operation.
可以用一个简单的 bool
标志复制您所需的行为:
You can replicate your desired behavior with a simple bool
flag:
std::vector<Elem> container = foo();
bool did_it_already = false;
for (I it = container.begin(), end = container.end(); it != end; ++it) {
if (!did_it_already && bar(it)) {
// Does something only the first time bar(it) is satisfied
// ...
did_it_already = true;
}
}
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