As you may know, Ubuntu 14.04 comes with GCC 4.8.4 which is sometimes not up to date enough to compile some project. You may want to install 4.9 or higher but "apt-get install gcc-4.9" will report that the gcc-4.9-base is already up-to-date.
Here are the steps to install the real compiler on Ubuntu 14.04.

First, you need to ensure you have "add-apt-repository" command installed. If not, install it by
apt-get install software-properties-common

PS: for Ubuntu version lower than 12, you need:
sudo apt-get install python-software-properties

Second, add test ppa repo:
add-apt-repository ppa:ubuntu-toolchain-r/test and update your library apt-get update

Third, install the new gcc:
apt-get install gcc-4.9 g++-4.9 cpp-4.9

Lastly, update the symbolic links or you will not able to compile with the typical gcc command.
There are mainly two ways to do it.

  1. Create links manually

    cd /usr/bin
    rm gcc g++ cpp
    ln -s gcc-4.9 gcc
    ln -s g++-4.9 g++
    ln -s cpp-4.9 cpp

  2. Use update alternatives command

    update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 48 \
    --slave /usr/bin/g++ g++ /usr/bin/g++-4.8 \
    --slave /usr/bin/gcc-ar gcc-ar /usr/bin/gcc-ar-4.8 \
    --slave /usr/bin/gcc-nm gcc-nm /usr/bin/gcc-nm-4.8 \
    --slave /usr/bin/gcc-ranlib gcc-ranlib /usr/bin/gcc-ranlib-4.8

    update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.9 49 \
    --slave /usr/bin/g++ g++ /usr/bin/g++-4.9 \
    --slave /usr/bin/gcc-ar gcc-ar /usr/bin/gcc-ar-4.9 \
    --slave /usr/bin/gcc-nm gcc-nm /usr/bin/gcc-nm-4.9 \
    --slave /usr/bin/gcc-ranlib gcc-ranlib /usr/bin/gcc-ranlib-4.9

    update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5 53 \
    --slave /usr/bin/g++ g++ /usr/bin/g++-5 \
    --slave /usr/bin/gcc-ar gcc-ar /usr/bin/gcc-ar-5 \
    --slave /usr/bin/gcc-nm gcc-nm /usr/bin/gcc-nm-5 \
    --slave /usr/bin/gcc-ranlib gcc-ranlib /usr/bin/gcc-ranlib-5

Now, try gcc -v and see it reports the correct version.