public member function
<set>
std::set::count
size_type count (const value_type& val) const;
Count elements with a specific value
Searches the container for elements equivalent to val and returns the number of matches.
Because all elements in a set container are unique, the function can only return 1 (if the element is found) or zero (otherwise).
set 的两个元素被认为是等效的，如果容器的 比较对象 返回falsereflexively (i.e., no matter the order in which the elements are passed as arguments).
参数
- val
- Value to search for.
成员类型value_type是容器中元素的类型，在 set 中定义为其第一个模板参数的别名（T).
返回值
1if the container contains an element equivalent to val, or zero otherwise.
成员类型size_type是一种无符号整型类型。
示例
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// set::count
#include <iostream>
#include <set>
int main ()
{
std::set<int> myset;
// set some initial values:
for (int i=1; i<5; ++i) myset.insert(i*3); // set: 3 6 9 12
for (int i=0; i<10; ++i)
{
std::cout << i;
if (myset.count(i)!=0)
std::cout << " is an element of myset.\n";
else
std::cout << " is not an element of myset.\n";
}
return 0;
}
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输出
0 is not an element of myset.
1 is not an element of myset.
2 is not an element of myset.
3 is an element of myset.
4 is not an element of myset.
5 is not an element of myset.
6 is an element of myset.
7 is not an element of myset.
8 is not an element of myset.
9 is an element of myset.
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异常安全
强保证：如果抛出异常，容器没有发生变化。