In this package vignette, we introduce how to use the C++ header-only library that **splines2** contains with the **Rcpp** package (Eddelbuettel 2013) for constructing spline basis functions. The introduction is intended for package developers who would like to use **splines2** package at C++ level.

Different with the procedure based functions at R level, **splines2** provides several spline classes in its C++ interface for ease of usage and maintenance. The implementations use the **Armadillo** (Sanderson 2016) library with help of **RcppArmadillo** (Eddelbuettel and Sanderson 2014) and require C++11. We may include the header file named `splines2Armadillo.h`

to get the access to all the classes and implementations in the name space `splines2`

.

```
#include <RcppArmadillo.h>
// [[Rcpp::plugins(cpp11)]]
// include header file from splines2 package
#include <splines2Armadillo.h>
// for ease of demonstration
using arma
using splines2
```

The `BSpline`

class is for creating B-spline basis functions.

There are four constructors in addition to the default constructor: `BSpline()`

.

The first non-default constructor is called when internal knots are explicitly specified.

```
// 1. specified internal_knots
const vec& x,
BSpline(const vec& internal_knots,
const unsigned int degree = 3,
const vec& boundary_knots = vec())
```

The second non-default constructor is called when an unsigned integer representing the degree of freedom of the *complete spline basis functions* (different with `df`

in the R interface) is specified. Then the number of internal knots is computed as `spline_df - degree - 1`

and the placement of internal knots uses quantiles of specified `x`

.

```
// 2. specified spline degree of freedom (df)
const vec& x,
BSpline(const unsigned int spline_df,
const unsigned int degree = 3,
const vec& boundary_knots = vec())
```

The third non-default constructor is intended for basis functions with an extended knot sequence. The multiplicities of knots in the sequence can be different but should not be greater than `degree + 1`

.

```
// 3. specified degree and (extended) knot sequence
const rvec& x,
BSpline(const unsigned int degree,
const rvec& knot_sequence)
```

The fourth non-default constructor is explicit and takes a pointer to a base class object. It can be useful when we want to create a new object using the same specification (degree, internal_knots, boundary_knots, etc.) of an existing object.

```
// 4. create a new object from a base class pointer
const SplineBase* pSplineBase) BSpline(
```

The main methods are

`basis()`

for spline basis matrix`derivative()`

for derivatives of spline basis`integral()`

for integrals of spline basis

The specific function signatures are as follows:

```
const bool complete_basis = true)
mat basis(const unsigned int derivs = 1,
mat derivative(const bool complete_basis = true)
const bool complete_basis = true) mat integral(
```

In addition, we may set and get the spline specifications through the following *setter* and *getter* functions, respectively.

```
// setter functions
const vec&);
SplineBase* set_x(const double);
SplineBase* set_x(const vec&);
SplineBase* set_internal_knots(const vec&);
SplineBase* set_boundary_knots(const vec&);
SplineBase* set_knot_sequence(const unsigned int);
SplineBase* set_degree(const unsigned int);
SplineBase* set_order(
// getter functions
vec get_x();
vec get_internal_knots();
vec get_boundary_knots();
vec get_knot_sequence();unsigned int get_degree();
unsigned int get_order();
unsigned int get_spline_df();
```

The *setter* function returns a pointer to the current object so that the specification can be chained for convenience. For example,

```
0, 0.1, 1) }; // 0, 0.1, ..., 1
vec x { arma::regspace(5 }; // df = 5 (and degree = 3, by default)
BSpline obj { x, // change degree to 2 and get basis
2)->basis() }; mat basis_mat { obj.set_degree(
```

The class `MSpline`

for M-splines, `ISpline`

for I-splines, `CSpline`

for C-splines, and `NaturalSpline`

have the exactly same constructors and function members with `BSpline`

except there is no available `integral()`

method for `CSpline`

and no `degree`

related methods for `NaturalSpline`

. The class `PeriodicMSpline`

for periodic M-splines also have the same non-default constructors and function members except that it does not have provide constructor and methods for extended knot sequence.

The `BernsteinPoly`

class is implemented for the generalized Bernstein polynomials.

The main non-default constructor is as follows:

```
const vec& x,
BernsteinPoly(const unsigned int degree,
const vec& boundary_knots = vec())
```

Same with `BSpline`

, the main methods are

`basis()`

for basis matrix`derivative()`

for derivatives of basis functions`integral()`

for integrals of basis functions

The specific function signatures are as follows:

```
const bool complete_basis = true)
mat basis(const unsigned int derivs = 1,
mat derivative(const bool complete_basis = true)
const bool complete_basis = true) mat integral(
```

In addition, we may similarly set and get the specifications through the following *setter* and *getter* functions, respectively.

```
// setter functions
const vec&);
SplineBase* set_x(const double);
SplineBase* set_x(const unsigned int);
SplineBase* set_degree(const unsigned int);
SplineBase* set_order(const vec&);
SplineBase* set_boundary_knots(
// getter functions
vec get_x();unsigned int get_degree();
unsigned int get_order();
vec get_boundary_knots();
```

The *setter* function also returns a pointer to the current object.

Eddelbuettel, Dirk. 2013. *Seamless R and C++ Integration with Rcpp*. Springer.

Eddelbuettel, Dirk, and Conrad Sanderson. 2014. “RcppArmadillo: Accelerating R with High-Performance C++ Linear Algebra.” *Computational Statistics and Data Analysis* 71: 1054–63. http://dx.doi.org/10.1016/j.csda.2013.02.005.

Sanderson, Conrad. 2016. “Armadillo: An Open Source C++ Linear Algebra Library for Fast Prototyping and Computationally Intensive Experiments.” *Journal of Open Source Software* 1: 26.