<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;
}
a.cp_btn:before {
  position: absolute;
  top: 0px;
  left: -90%;
  content:'';
  height: 100%;
  width: 100%;
  background : #fff;
  opacity: 0.3;
  transition: .4s ease-in-out;
}
a.cp_btn:hover:before {
  left:0;
}