问题描述
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;}
问题解答
回答1：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：
fnA 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
