std set_union 是否总是从第一个开始获取公共元素

Question

当两个数组中有公共元素时，std set_union是否总是从第一个数组中取出那些公共元素？从下面的代码片段可以看出，它总是从第一个数组中选取共同的元素，但这是有保证的吗？如何让它从第二个开始挑选。

#include <algorithm>
#include <vector>
#include <string>
#include <iostream>

struct Foo
{
  Foo(int i, const std::string& n): id(i), name(n){}
  int id;
  std::string name;
};

bool comp(const Foo& first, const Foo& second)
{
  return first.id < second.id;
}

int main()
{
  Foo foo5A(5, "A");
  Foo foo10A(10, "A");
  Foo foo15A(15, "A");
  Foo foo20A(20, "A");
  Foo foo25A(25, "A");
  Foo fooA[] = {foo5A, foo10A, foo15A, foo20A, foo25A};

  Foo foo10B(10, "B");
  Foo foo20B(20, "B");
  Foo foo30B(30, "B");
  Foo foo40B(40, "B");
  Foo foo50B(50, "B");
  Foo fooB[] = {foo10B, foo20B, foo30B, foo40B, foo50B};

  std::vector<Foo> fooUnion;
  std::set_union(fooA, fooA+5, fooB, fooB+5, std::back_inserter(fooUnion), comp);

  for(const auto& f : fooUnion)
  {
    std::cout << f.id << ":" << f.name << std::endl;    
  }  
}

输出为：

5:A
10:A
15:A
20:A
25:A
30:B
40:B
50:B

Answer 1

是的，确实如此，来自参考 here:

The union of two sets is formed by the elements that are present in either one of the sets, or in both. Elements from the second range that have an equivalent element in the first range are not copied to the resulting range.

如果你想让它从第二个 (fooB) 中选择，你交换参数：

std::set_union(fooB, fooB+5, fooA, fooA+5, std::back_inserter(fooUnion), comp);

std set_union 是否总是从第一个开始获取公共元素

does std set_union always take common elements from first

c++

stl

set-union