Can someone please point me towards some nice resources for understanding and using nested classes? I have some material like Programming Principles and things like this IBM Knowledge Center - Nested Classes
But I'm still having trouble understanding their purpose. Could someone please help me?
This question is related to
c++
nested
inner-classes
I don't use nested classes much, but I do use them now and then. Especially when I define some kind of data type, and I then want to define a STL functor designed for that data type.
For example, consider a generic Field
class that has an ID number, a type code and a field name. If I want to search a vector
of these Field
s by either ID number or name, I might construct a functor to do so:
class Field
{
public:
unsigned id_;
string name_;
unsigned type_;
class match : public std::unary_function<bool, Field>
{
public:
match(const string& name) : name_(name), has_name_(true) {};
match(unsigned id) : id_(id), has_id_(true) {};
bool operator()(const Field& rhs) const
{
bool ret = true;
if( ret && has_id_ ) ret = id_ == rhs.id_;
if( ret && has_name_ ) ret = name_ == rhs.name_;
return ret;
};
private:
unsigned id_;
bool has_id_;
string name_;
bool has_name_;
};
};
Then code that needs to search for these Field
s can use the match
scoped within the Field
class itself:
vector<Field>::const_iterator it = find_if(fields.begin(), fields.end(), Field::match("FieldName"));
Nested classes are just like regular classes, but:
Some examples:
Assume you want to have a class SomeSpecificCollection
which would aggregate objects of class Element
. You can then either:
declare two classes: SomeSpecificCollection
and Element
- bad, because the name "Element" is general enough in order to cause a possible name clash
introduce a namespace someSpecificCollection
and declare classes someSpecificCollection::Collection
and someSpecificCollection::Element
. No risk of name clash, but can it get any more verbose?
declare two global classes SomeSpecificCollection
and SomeSpecificCollectionElement
- which has minor drawbacks, but is probably OK.
declare global class SomeSpecificCollection
and class Element
as its nested class. Then:
SomeSpecificCollection
you refer to just Element
, and everywhere else as SomeSpecificCollection::Element
- which looks +- the same as 3., but more clearSomeSpecificCollection
is also a class.In my opinion, the last variant is definitely the most intuitive and hence best design.
Let me stress - It's not a big difference from making two global classes with more verbose names. It just a tiny little detail, but imho it makes the code more clear.
This is especially useful for introducing typedefs or enums. I'll just post a code example here:
class Product {
public:
enum ProductType {
FANCY, AWESOME, USEFUL
};
enum ProductBoxType {
BOX, BAG, CRATE
};
Product(ProductType t, ProductBoxType b, String name);
// the rest of the class: fields, methods
};
One then will call:
Product p(Product::FANCY, Product::BOX);
But when looking at code completion proposals for Product::
, one will often get all the possible enum values (BOX, FANCY, CRATE) listed and it's easy to make a mistake here (C++0x's strongly typed enums kind of solve that, but never mind).
But if you introduce additional scope for those enums using nested classes, things could look like:
class Product {
public:
struct ProductType {
enum Enum { FANCY, AWESOME, USEFUL };
};
struct ProductBoxType {
enum Enum { BOX, BAG, CRATE };
};
Product(ProductType::Enum t, ProductBoxType::Enum b, String name);
// the rest of the class: fields, methods
};
Then the call looks like:
Product p(Product::ProductType::FANCY, Product::ProductBoxType::BOX);
Then by typing Product::ProductType::
in an IDE, one will get only the enums from the desired scope suggested. This also reduces the risk of making a mistake.
Of course this may not be needed for small classes, but if one has a lot of enums, then it makes things easier for the client programmers.
In the same way, you could "organise" a big bunch of typedefs in a template, if you ever had the need to. It's a useful pattern sometimes.
The PIMPL (short for Pointer to IMPLementation) is an idiom useful to remove the implementation details of a class from the header. This reduces the need of recompiling classes depending on the class' header whenever the "implementation" part of the header changes.
It's usually implemented using a nested class:
X.h:
class X {
public:
X();
virtual ~X();
void publicInterface();
void publicInterface2();
private:
struct Impl;
std::unique_ptr<Impl> impl;
}
X.cpp:
#include "X.h"
#include <windows.h>
struct X::Impl {
HWND hWnd; // this field is a part of the class, but no need to include windows.h in header
// all private fields, methods go here
void privateMethod(HWND wnd);
void privateMethod();
};
X::X() : impl(new Impl()) {
// ...
}
// and the rest of definitions go here
This is particularly useful if the full class definition needs the definition of types from some external library which has a heavy or just ugly header file (take WinAPI). If you use PIMPL, then you can enclose any WinAPI-specific functionality only in .cpp
and never include it in .h
.
One can implement a Builder pattern with nested class. Especially in C++, personally I find it semantically cleaner. For example:
class Product{
public:
class Builder;
}
class Product::Builder {
// Builder Implementation
}
Rather than:
class Product {}
class ProductBuilder {}
Source: Stackoverflow.com