## HolderGenerator

### Introduction

A HolderGenerator is a unary metafunction class which returns types suitable for holding instances of its argument in a wrapped C++ class instance.

### Concept Requirements

In the table below, `G` denotes an type which models `HolderGenerator`, and `X` denotes a class type.

Expression | Requirements
--- | ---
`G::apply<X>::type` | A concrete subclass of `instance_holder` which can hold objects of type `X`.