linux centos 装g++安装不了

今天需要编译一个项目的时候在装g++都装不上，

[root@master hadoop]# yum install g++
Loaded plugins: fastestmirror, refresh-packagekit, security
Loading mirror speeds from cached hostfile
 * base: mirrors.yun-idc.com
 * extras: mirrors.yun-idc.com
 * updates: mirrors.yun-idc.com
Setting up Install Process
No package g++ available.

 一直提示这个错误，后面参考网上资料用： yum install gcc-c++就可以，如下图

[root@master hadoop]# yum install gcc-c++
Loaded plugins: fastestmirror, refresh-packagekit, security
Loading mirror speeds from cached hostfile
 * base: mirrors.yun-idc.com
 * extras: mirrors.yun-idc.com
 * updates: mirrors.yun-idc.com
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package gcc-c++.x86_64 0:4.4.7-4.el6 will be installed
--> Processing Dependency: libstdc++-devel = 4.4.7-4.el6 for package: gcc-c++-4.4.7-4.el6.x86_64
--> Running transaction check
---> Package libstdc++-devel.x86_64 0:4.4.7-4.el6 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

 至于是什么原因就不清楚