无法将模板类传递给cout

Question

当我尝试编译以下代码（在下面的小片段中将其截断）时，

#include <iostream>

using namespace std;

template <typename value_type>
class Tree {
public:
    Tree();
    ~Tree();
};

template <typename value_type>
const std::ostream& operator<<(const std::ostream& o, const Tree<value_type>& t) {
    return o;
}

int main() {
    Tree<int> tree;
    cout << tree << endl;
}

我收到以下错误：

铛在Mac

error: reference to overloaded function could not be resolved;
      did you mean to call it?
        cout << tree << endl;
                        ^~~~

Debian Linux上的gnu gcc

error: no match for 'operator<<'
(operand types are
    'const ostream {aka const std::basic_ostream<char>}'
    and '<unresolved overloaded function type>')
        cout << tree << endl;
        ~~~~~~~~~~~~~^~~~~~~

如果我从不实现运算符重载，则gnu g ++会给我以下错误：

error: no match for 'operator<<'
(operand types are
    'std::ostream {aka std::basic_ostream<char>}'
    and 'Tree<int>')
        cout << tree << endl;
        ~~~~~^~~~~~~

我实际上不明白我在这里做错了什么。我要做的就是能够ostream像您一样将我的模板类传递给。有任何想法吗？

Answer 1

删除consts- std::ostream您不能将const流用于任何内容。

template <typename value_type>
std::ostream& operator<<(std::ostream& o, const Tree<value_type>& t) {
    return o;
}