class A {
public:
    //explicit A(int i){}
    A(int i){}
};

void fun(const A& a) {}

int main() {
    // If I use explicit for A constructor,I can prevent this mistake.
    // (Or shall I call it as feature?)
    fun(10);
}

或者我应该允许隐式转换,以允许用户以较少的输入调用我的API？