Convert DefaultArg to string

Hi all,

I want to get a function’s default parameter value as string. That’s to say, if a function “void func(int a = 5);”, I want to get the string “5”.

I checked ParmVarDecl, but found it only returns an Expr as the default arg.

So my question is, how to get a default parameter value as string, or how to convert an Expr to string?