<a href="#" class="cp_btn">button</a>
a.cp_btn {
position: relative;
display: block;
width: 160px;
padding: 0.8em;
text-align: center;
text-decoration: none;
color: #fff;
background: #EC407A;
border:1px solid #fff;
overflow: hidden;
a.cp_btn:before {
content:"";
position: absolute;
top: 0px;
left: -150px;
height: 100%;
width: 160px;
background : #fff;
opacity: 0.3;
transition: .4s;
}
a.cp_btn:hover:before {
left:0;
}