Ubuntu18.04LTS安装g++错误以及解决方法

参考文献：
[1]/questions/35934227/cannot-install-g-on-ubuntu-14-04-4-lts-i-have-tried-commands-and-software-cen

[2]https://blog.csdn.net/xiangxianghehe/article/details/80112149

错误输出：

root@DESKTOP-2HT6OMN:/home/h3c# apt install g++
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 g++ : Depends: g++-5 (>= 5.3.1-3~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

原因：
软件源输出不对。
解决方法：
更新软件源。
替换为：

deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse

附加：
之前使用的软件源为阿里软件源：
版本一：

#deb包
deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse  
deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse  
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse  
deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse  
##测试版源  
deb http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse  
# 源码  
deb-src http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse  
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse  
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse  
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse  
##测试版源  
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse  
# Canonical 合作伙伴和附加  
##deb http://archive.canonical.com/ubuntu/ xenial partner  
##deb http://extras.ubuntu.com/ubuntu/ xenial main

版本二：

deb http://mirrors.aliyun.com/ubuntu/ trusty main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ trusty-backports main restricted universe multiverse

之后，发现了不对。
参考文献：
[3]https://blog.csdn.net/zhangjiahao14/article/details/80554616

Ubuntu 12.04 (LTS)代号为precise。

Ubuntu 14.04 (LTS)代号为trusty。

Ubuntu 15.04 代号为vivid。

Ubuntu 15.10 代号为wily。

Ubuntu 16.04 (LTS)代号为xenial。

所以，最新的软件源要对应系统版本。