std::set is an associative container that contains a sorted set of unique objects of type Key. Sorting is done using the key comparison function Compare. Search, removal, and insertion operations have logarithmic complexity. Sets are usually implemented as Redâblack trees.
Everywhere the standard library uses the Compare requirements, uniqueness is determined by using the equivalence relation. In imprecise terms, two objects a and b are considered equivalent if neither compares less than the other: !comp(a, b) && !comp(b, a).
std::set meets the requirements of Container, AllocatorAwareContainer, AssociativeContainer and ReversibleContainer.
std::set are constexpr: it is possible to create and use std::set objects in the evaluation of a constant expression.
However, std::set objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.
set
set
value_type
The member types iterator and const_iterator may be aliases to the same type. This means defining a pair of function overloads using the two types as parameter types may violate the One Definition Rule. Since iterator is convertible to const_iterator, a single function with a const_iterator as parameter type will work instead.
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <set>
#include <string_view>
template<typename T>
std::ostream& operator<<(std::ostream& out, const std::set<T>& set)
{
if (set.empty())
return out << "{}";
out << "{ " << *set.begin();
std::for_each(std::next(set.begin()), set.end(), [&out](const T& element)
{
out << ", " << element;
});
return out << " }";
}
int main()
{
std::set<int> set{1, 5, 3};
std::cout << set << '\n';
set.insert(2);
std::cout << set << '\n';
set.erase(1);
std::cout << set << "\n\n";
std::set<int> keys{3, 4};
for (int key : keys)
{
if (set.contains(key))
std::cout << set << " does contain " << key << '\n';
else
std::cout << set << " doesn't contain " << key << '\n';
}
std::cout << '\n';
std::string_view word = "element";
std::set<char> characters(word.begin(), word.end());
std::cout << "There are " << characters.size() << " unique characters in "
<< std::quoted(word) << ":\n" << characters << '\n';
}
Output:
{ 1, 3, 5 }
{ 1, 2, 3, 5 }
{ 2, 3, 5 }
{ 2, 3, 5 } does contain 3
{ 2, 3, 5 } doesn't contain 4
There are 5 unique characters in "element":
{ e, l, m, n, t }[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior LWG 103 C++98 iterator allows modification of keys iterator made constant LWG 230 C++98Key was not required to be CopyConstructible
Key might not be able to be constructed) Key is also required to
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