c++ - STL 的 std::set 容器参数给重载的操作符 << ()

Question

我有以下代码，它在编译时给了我错误。问题发生在我在重载的插入运算符（即）运算符<<（）函数中创建一个 const_iterator 的地方。我猜 set 的模板参数版本不正确。任何人都可以请投入并使这段代码工作吗？

#include <iostream>
#include <set>

using namespace std;

class A {
public:
    int _i;
    A(int i) : _i(i) {}
    bool operator < (A a) const { return _i < a._i; }
};

class compA {
public:
    bool operator()(A a1, A a2) const { return (a1._i > a2._i); }
};

template<class T, class C>
std::ostream& operator << (std::ostream& os, const set<T, C>& s) {
    std::set<T, C>::const_iterator itr = s.begin();

    for (int i = 0; i < s.size(); i++) {
        os << *itr++ << " ";
    }

    return os;
}

int main() {
    set<A, less<A> > s1;
    set<A, compA> s2;

    s1.insert(A(9)); s1.insert(A(3)); s1.insert(A(-5)); s1.insert(A(3));
    s2.insert(A(2)); s2.insert(A(1)); s2.insert(A(7));

    cout << s1;
    cout << s2;
}

Answer 1

You forgot to declare and implement

std::ostream& operator << (std::ostream& os, const A& s)
{
    return os;
}

Also, the signature of the method in your comparison class should be:

bool operator()(const A& a1, const A& a2)

Answer 2

*itr将是 type const A&，因此当您os<<*itr++编译器不知道如何打印对象A时，您还需要提供operator <<for A。