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;
}
}
But this answer suggests not only that my definition of "singular" is wrong, but also that my comparison above is totally illegal.
Is it?
Introduction to Iterators in C++ Difficulty Level : Medium Last Updated : 26 May, 2020 An iterator is an object (like a pointer) that points to an element inside the container.
Dereferenceable iterators are always nonsingular, but the converse is not true. For example, a null pointer is nonsingular (there are well defined operations involving null pointers) even thought it is not dereferenceable. An iterator is valid if it is dereferenceable or past-the-end.
Below the iterator is the name of an object created by calling iterator () method of collection interface. “collection” is the collection object’s name. Iterators have 4 methods in Java that are used to traverse through collections and retrieve the required information. They are as follows:
Note: SplitIterator can also be considered as a cursor as it is a type of Iterator only. 1. Iterator Iterators in Java are used in the Collection framework to retrieve elements one by one. It is a universal iterator as we can apply it to any Collection object.
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:
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.
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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