boost的operators库:
官方文档: http://www.boost.org/doc/libs/1_58_0/libs/utility/operators.htm
中文文档: http://stlchina.huhoo.net/boost/libs/utility/operators.htm
继承boost::addable, 提供+=,自动实现+
已知 a += b;就是 a = a+b;
得
A operator+(A &a1,A &a2)
{
A temp = a1;
temp +=a2.data;
return temp;
}