# Regular type vs view type¶

A fundamental distinction in TRIQS is the difference between the regular type and the corresponding view types.

• The regular types

• Following the definition of A. Stepanov, a regular type is a value, much like an int, a double, or a std::vector. Cf http://www.stepanovpapers.com/DeSt98.pdf (I added move semantics to the definition).

• It has value semantics, more precisely

T a;               // default constructible
T a = b; T a (b);  // copy constructible
a = b;             // assignment
a == b; a != b;    // Equality.


with the following properties

T a = b; assert(a==b);
T a; a=b; is equivalent to  T a=b;
T a=c; T b=c; a=d; assert(b==c);

• When copied, assigned, it makes deep copy of the data and resizes itself if necessary (like e.g. std::vector).

• Examples:

• std::vector<T>
• triqs::arrays::array<T,R>, triqs::arrays::matrix<T>, triqs::arrays::vector<T>
• The view types

• They have a reference semantics.
• They are just partial or complete views of the corresponding regular type.
• They never make deep copy, either in copying, constructing, transforming to python,
• They cannot be resized.
• They are useful to work on arrays, matrices, or on some part thereof, without making copies (e.g. on a slice of an array, a column of a matrix).
• Examples:
• triqs::arrays::array_view<T,R>, triqs::arrays::matrix_view, triqs::arrays::vector_view<T>

Many triqs containers (array, matrix, green functions) is therefore provided in two flavors, and it is crucial for the user to learn and understand their difference in order to use them appropriately.

## Behaviour comparison table between regular and view types¶

A detailed comparison of the behaviour of the two type of types is provided in the following table.

Topics regular type (e.g. array, matrix, vector) view type (e.g. array_view, matrix_view, vector_view)
Contructors
• Constructors create a new fresh memory block.
• Constructors only make another view of the data.
Default Contructor
• Default constructor creates an empty array
• No default constructor (what would it view ?).
Copy contructors
• Create a new fresh memory block.
• Make a true copy of the data.
• Make another view of the data (shallow copy).
• Never copy data, so they are quite quick.
Assignment (=)
• The assignment operator creates a new datablock if size mismatchs.
• Hence, assignment never fail for size reason (unless there is a memory allocation exception of course)
• The assignment operator just copy data into the view. Behaviour is undefined if the size of the view is too small. (it throws if the array has option Tag::BoundCheck or if TRIQS_ARRAYS_ENFORCE_BOUNDCHECK is defined, Cf below).
Resizing
• Can be resized, invalidating all references/pointers to the data.
• Can be not be resized.
Invalidation
• References/pointers to the data may become invalid after resize, or assignment.
• References/pointers to the data are still valid after assignment.
Data Contiguity
• The data are contiguous in memory.
• The data may not be contiguous in memory (e.g. a slice).