Difference between namespace and class

Classes are data types. They are an expanded concept of structures, they can contain data members, but they can also contain functions as members whereas a namespace is simply an abstract way of grouping items together. A namespace cannot be created as an object; think of it more as a naming convention. It is used as additional information to differentiate similar functions, classes, variables etc. with the same name available in different libraries. In essence, a namespace defines a scope.Following are some points to justify : 1. A namespace is a way of grouping identifiers so that they don't clash. Using a class implies that you can create an instance of that class, not true with namespaces. 2. You can use using-declarations with namespaces, and that's not possible with classes unless you derive from them. 3. You can reopen a namespace and add stuff across translation units. You cannot do this with classes.For example:- 

is legal, but: 

is not. 4.You can have unnamed namespaces but you can't have a unnamed class.For example: 

5. If length of a name makes code difficult to read, or is tedious to type in a header file where using directives can’t be used, we can make a namespace alias which serves as an abbreviation for the actual name. For example: 

Output :

 42 

In case of class we have to use typedef.