As Foaly sad there is a lot going wrong there but your app is not responding due to this line of code:
a.draw (a);
You are probably looking for this:
a.draw (b);
But the text won't draw anything because you don't specify a font (At least I think SFML 2.x doesn't have a default font). Another thing is you shouldn't name your variables like a,b and c. Name them after what they are for.