I have followed SO questions and answers from each search query. But they all are related with specific one.
At the basic, I mean before you are going to write down a format (may be a simple one) it will gives you a warnings.
From iOS 8.0 by default views are size classes. Even if you disable size classes it will still contains some auto layout constraints.
So if you are planning to set constrains via code using VFL. Then you must take care of one below line.
// Remove constraints if any.
[self.view removeConstraints:self.view.constraints];
I had search a lot in SO, but the solution was lies in Apple Sample Code.
So you must have to remove default constraints before planning to add new one.