pimpl idiom

The PImpl Idiom (Pointer to IMPLementation) is a technique used for separating implementation from the interface. It minimizes header exposure and helps programmers to reduce build dependencies by moving the private data members in a separate class and accessing them through an opaque pointer.
Here's how the pimpl idiom can improve the software development lifecycle:
* Minimization of compilation dependencies.
* Separation of interface and implementation.
* Portability.

Pimpl header, 
```
class my_class {
   //  ... all public and protected stuff goes here ...
private:
   class impl; unique_ptr<impl> pimpl; // opaque type here
};
```
Pimpl implementation,
```
class my_class::impl {  // defined privately here
  // ... all private data and functions: all of these
  //     can now change without recompiling callers ...
};
my_class::my_class(): pimpl( new impl )
{
  // ... set impl values ...
}
```
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