我已经安装了gcc和gcc-5，但是无法安装g ++。
sudo apt install g++ g++-5 libstdc++-5-dev libc6-dev libc6=2.23-0ubuntu11 libc-dev-bin
将失败，因为
The following packages have unmet dependencies:
g++ : Depends: gcc (>= 4:5.3.1-1ubuntu1) but it is not going to be installed
Depends: gcc-5 (>= 5.3.1-3~) but it is not going to be installed
g++-5 : Depends: gcc-5 (= 5.4.0-6ubuntu1~16.04.11) but it is not going to be installed
但是
sudo apt install gcc gcc-5
Reading package lists... Done
Building dependency tree
Reading state information... Done
gcc is already the newest version (4:5.3.1-1ubuntu1).
gcc-5 is already the newest version (5.4.0-6ubuntu1~16.04.11).
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.