为什么调用bind时，不能直接加&，而必须用ref

如题

#include <iostream>
#include <functional>
using namespace std;
void g(ostream &os)
{
    os << "1";
}
int main()
{
    //非法
    auto f = bind(g, &cout);
    f();
    //合法
    auto h = bind(g, ref(cout));
    h();
    return 0;
}

---

因为函数原型就是这样规定的，你写成&cout就是个指针了，当然非法；用std::ref包裹一下就符合原型要求了，相当于传个引用进去，同样还有不可变的std::cref，下面是官方对于参数类型的要求：

fn
A function object, pointer to function or pointer to member.
Fn shall have a decay type which is move-constructible from fn.

args...
List of arguments to bind: either values, or placeholders.
The types in Args... shall have decay types which are move-constructible from their respective arguments in args....
If for any argument, its decay type is a reference_wrapper, it bounds to its referenced value instead.

你看最后一行，说要求得是个reference_wrapper，std::ref和std::cref就是干这个的