R: Create an Rcpp Module to Expose a C++ Class in R

exposeClass {Rcpp} | R Documentation

## Create an Rcpp Module to Expose a C++ Class in R

### Description

The arguments specify a C++ class and some combination of constructors, fields and methods to be shared with R by creating a corresponding reference class in R. The information needed in the call to `exposeClass()` is the simplest possible in order to create a C++ module for the class; for example, fields and methods in this class need only be identified by their name. Inherited fields and methods can also be included, but more information is needed. The function writes a C++ source file, containing a module definition to expose the class to R, plus one line of R source to create the corresponding reference class.

### Usage

    exposeClass(class, constructors = , fields = , methods = , file = ,
        header = , module = , CppClass = class, readOnly = , rename = ,
        Rfile = TRUE)

### Arguments

`class` | The name of the class in R. By default, this will be the same as the name of the class in C++, unless argument `CppClass` is supplied.
`constructors` | A list of the signatures for any of the class constructors to be called from R. Each element of the list gives the data types in C++ for the arguments to the corresponding constructor. See Details and the example.
`fields, methods` | The vector of names for the fields and for the methods to be exposed in R. For inherited fields and methods, type information needs to be supplied; see the section “Inherited Fields and Methods”.
`file` | Usually, the name for the file on which to write the C++ code, by default `paste0(CppClass, "Module.cpp")`. If the current working directory in R is the top-level directory for a package, the function writes the file in the `"src"` subdirectory. Otherwise the file is written in the working directory. The argument may also be a connection, already open for writing.
`header` | Whatever lines of C++ header information are needed to include the definition of the class. Typically this includes a file from the package where we are writing the module definition, as in the example below.
`module` | The name for the Rcpp module, by default `paste0("class_",CppClass)`.
`CppClass` | The name for the class in C++. By default and usually, the intended class name in R.
`readOnly` | Optional vector of field names. These fields will be created as read-only in the interface.
`rename` | Optional named character vector, used to name fields or methods differently in R from their C++ name. The elements of the vector are the C++ names and the corresponding elements of `names(rename)` the desired names in R. So `c(.age = "age")` renames the C++ field or method `age` as `.age`.
`Rfile` | Controls the writing of a one-line R command to create the reference class corresponding to the C++ module information. By default, this will be a file `paste0(class, "Class.R")`. If the working directory is an R package source directory, the file will be written in the `R` subdirectory, otherwise in the working directory itself. Supplying a character string substitutes that file name for the default. The argument may also be a connection open for writing or `FALSE` to suppress writing the R source altogether.

### Details

The file created by the call to these functions only depends on the information in the C++ class supplied. This file is intended to be part of the C++ source for an R package. The file only needs to modified when the information changes, either because the class has changed or because you want to expose different information to R. In that case you can either recall `exposeClass()` or edit the C++ file created.

The Rcpp Module mechanism has a number of other optional techniques, not covered by `exposeClass()`. These should be entered into the C++ file created. See the “rcpp-modules” vignette with the package for current possibilities.

For fields and methods specified directly in the C++ class, the fields and method arguments to `exposeClass()` are character vectors naming the corresponding members of the class. For module construction, the data types of directly specified fields and of the arguments for the methods are not needed.

For inherited fields or methods, data type information is needed. See the section “Inherited Fields and Methods”.

For exposing class constructors, the module needs to know the signatures of the constructors to be exposed; each signature is a character vector of the corresponding C++ data types.

### Value

Nothing, called for its side effect.

### Inherited Fields and Methods

If the C++ class inherits from one or more other classes, the standard Rcpp `Module` mechanism can not be used to expose inherited fields or methods. An indirect mechanism is used, generating free functions in C++ to expose the inherited members in R.

This mechanism requires data type information in the call to `exposeClass()`. This is provided by naming the corresponding element of the `fields` or `methods` argument with the name of the member. The actual element of the `fields` argument is then the single data type of the field.

For the `methods` argument the argument will generally need to be a named list. The corresponding element of the list is the vector of data types for the return value and for the arguments, if any, to the method. For example, if C++ method `foo()` took a single argument of type `NumericVector` and returned a value of type `long`, the `methods` argument would be `list(foo = c("long", "NumericVector"))`.

See the second example below.

### Author(s)

John Chambers

### See Also

`setRcppClass`, which must be called from some R source in the package.

### Examples

    ## Not run:
    ### Given the following C++ class, defined in file PopBD.h,
    ### the call to exposeClass() shown below will write a file
    ### src/PopBDModule.cpp containing a corresponding module definition.
    ###   class PopBD {
    ###     public:
    ###       PopBD(void);
    ###       PopBD(NumericVector initBirth, NumericVector initDeath);
    ###
    ###       std::vector<double> birth;
    ###       std::vector<double> death;
    ###       std::vector<int> lineage;
    ###       std::vector<long> size;
    ###       void evolve(int);
    ###
    ###   };
    ### A file R/PopBDClass.R will be written containing the one line:
    ###   PopBD <- setRcppClass("PopBD")
    ###
    ### The call below exposes the lineage and size fields, read-only,
    ### and the evolve() method.

    exposeClass("PopBD",
          constructors =
            list("", c("NumericVector", "NumericVector")),
          fields = c("lineage", "size"),
          methods = "evolve",
          header = '#include "PopBD.h"',
          readOnly = c("lineage", "size"))

    ### Example with inheritance:  the class PopCount inherits from
    ### the previous class, and adds a method table().  It has the same
    ### constructors as the previous class.
    ### To expose the table() method, and the inherited evolve() method and size field:

    exposeClass("PopCount",
          constructors =
            list("", c("NumericVector", "NumericVector")),
          fields = c(size = "std::vector<long>"),
          methods = list("table", evolve = c("void", "int")),
          header = '#include "PopCount.h"',
          readOnly = "size")

    ## End(Not run)

[Package Rcpp version 1.0.8 Index]