下面有关友员函数的描述中，正确的说法是( )。
A、一个友员函数可以同时定义为两个类的友员函数
B、友员函数一定是普通函数
C、在类的外部定义友员函数时必须加上friend关键字D、友员函数是是类的成员函数
可以参考：
https://blog.csdn.net/qq_28351609/article/details/84870156?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522162535716416780265441280%2522%252C%2522scm%2522%253A%252220140713.130102334..%2522%257D&request_id=162535716416780265441280&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2alltop_positive~default-1-84870156.first_rank_v2_pc_rank_v29&utm_term=c%2B%2B%E5%8F%8B%E5%85%83%E5%87%BD%E6%95%B0&spm=1018.2226.3001.4449
也可以参考：
https://blog.csdn.net/adriano119/article/details/5914443?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522162535716416780265441280%2522%252C%2522scm%2522%253A%252220140713.130102334..%2522%257D&request_id=162535716416780265441280&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2allsobaiduend~default-4-5914443.first_rank_v2_pc_rank_v29&utm_term=c%2B%2B%E5%8F%8B%E5%85%83%E5%87%BD%E6%95%B0&spm=1018.2226.3001.4449
望采纳！！