先安装GCC.
yum install gcc-c++

yum install cmake

yum install centos-release-scl

yum install devtoolset-8-gcc devtoolset-8-gcc-c++

scl enable devtoolset-8 -- bash

软连接GCC.
find / -name 'gcc'
得到
/opt/rh/devtoolset-7/root/usr/lib/gcc
/opt/rh/devtoolset-7/root/usr/bin/gcc
/opt/rh/devtoolset-7/root/usr/libexec/gcc
/opt/rh/devtoolset-8/root/usr/lib/gcc
/opt/rh/devtoolset-8/root/usr/bin/gcc
/opt/rh/devtoolset-8/root/usr/libexec/gcc
/usr/lib/gcc
/usr/bin/gcc
/usr/libexec/gcc
红色是我们想要的。
查看版本
/opt/rh/devtoolset-8/root/usr/bin/gcc -v
得到：
Using built-in specs.
COLLECT_GCC=/opt/rh/devtoolset-8/root/usr/bin/gcc
COLLECT_LTO_WRAPPER=/opt/rh/devtoolset-8/root/usr/libexec/gcc/x86_64-redhat-linux/8/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,fortran,lto --prefix=/opt/rh/devtoolset-8/root/usr --mandir=/opt/rh/devtoolset-8/root/usr/share/man --infodir=/opt/rh/devtoolset-8/root/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-gcc-major-version-only --with-linker-hash-style=gnu --with-default-libstdcxx-abi=gcc4-compatible --enable-plugin --enable-initfini-array --with-isl=/builddir/build/BUILD/gcc-8.3.1-20190311/obj-x86_64-redhat-linux/isl-install --disable-libmpx --enable-gnu-indirect-function --with-tune=generic --with-arch_32=x86-64 --build=x86_64-redhat-linux
Thread model: posix
gcc version 8.3.1 20190311 (Red Hat 8.3.1-3) (GCC) 
软连gcc版本。
cp /usr/bin/gcc /usr/bin/gcc_back
ln -s /opt/rh/devtoolset-8/root/usr/bin/gcc /usr/bin/gcc -f
cp /usr/bin/gcc /usr/bin/g++_back
ln -s /opt/rh/devtoolset-8/root/usr/bin/g++ /usr/bin/g++ -f
验证
gcc -v
得到：
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/opt/rh/devtoolset-8/root/usr/libexec/gcc/x86_64-redhat-linux/8/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,fortran,lto --prefix=/opt/rh/devtoolset-8/root/usr --mandir=/opt/rh/devtoolset-8/root/usr/share/man --infodir=/opt/rh/devtoolset-8/root/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-gcc-major-version-only --with-linker-hash-style=gnu --with-default-libstdcxx-abi=gcc4-compatible --enable-plugin --enable-initfini-array --with-isl=/builddir/build/BUILD/gcc-8.3.1-20190311/obj-x86_64-redhat-linux/isl-install --disable-libmpx --enable-gnu-indirect-function --with-tune=generic --with-arch_32=x86-64 --build=x86_64-redhat-linux
Thread model: posix
gcc version 8.3.1 20190311 (Red Hat 8.3.1-3) (GCC)

安装完成！
 echo "scl enable devtoolset-8 -- bash">>~/.bashrc