试图将一个重载的 cout function 调用放入另一个重载的 cout function

Question

我的代码中有两个对象。 一个在另一个中使用。 我正在尝试编写一个重载的 cout function，它将打印来自每个 class 的项目。 尝试运行我的程序时出现错误

error: invalid use of non-static data member 'Glip::brain'
         outs << brain;
                 ^~~~~

friend ostream &operator<<(ostream &outs, const Glip &glip){
    outs << std::right << setw(10) << glip.name << "\n";
            outs << "----------------- \n";
            outs << "Height: " << right << setw(1) << glip.height << "\n";
            outs << "Weight: " << setw(1) << glip.weight << "\n";
            outs << "Color: " << setw(1) << glip.color << "\n";
    if(glip.horn == true) {
        outs << left << setw(4) << "Horn";
    }
    else
        outs << setw(4) << "No Horn";

    outs << brain;

    return outs;
}

我究竟做错了什么？