The way some classes (e.g. NumericVector have been implemented in various R/C++ versions gives us automatic coercion.

For example passing an integer vector to a C++ function that has a NumericVector as an argument will coerce the integer vector into a numeric vector automatically.

// [[export]]
double foo( NumericVector x){  
   return sum(x) ;
}

will give us:

x <- c(1,2)  
foo(x)  
# 3

x <- c(1L, 2L)  
foo(x)  
# 3

Sometimes, we would like to restrict x to just be a numeric vector. For this we would typically have to use SEXP as the argument class and then test conformity manually, something like this:

// [[export]]
double foo( SEXP x_ ){  
   if( !is<NumericVector>( x_ ) ) stop( "not a numeric vector" ) ;
   NumericVector x(x_) ;
   return sum(x) ;
}

Which is boring boiler plate code, so I’ve added the Strict class into Rcpp11 and Rcpp11. The class is pretty simple, it has two things:

• a constructor taking a SEXP, which makes it a perfect candidate for an attributes generated function. The constructor stores the SEXP and checks if it is compatible using the appropriate is<> function, if not it throws an exception.
• a get member function that returns a new object of the target class.

With this, we can write foo :

// [[export]]
double foo( Strict<NumericVector> x){  
   return sum(x.get()) ;
}