Understanding the RcppEigen Compilation Error

When interfacing R with C++ using Rcpp and RcppEigen, passing sparse matrices (specifically R's S4 dgCMatrix class) into Eigen sparse matrix structures is a common high-performance workflow. However, developers frequently encounter confusing compiler errors such as error: expected ']' before '{' token when compiling functions using sourceCpp().

Root Causes of the Error

This error is typically triggered by two underlying issues in how Rcpp parses C++ attributes and template signatures:

  • Attribute Placement Order: The // [[Rcpp::depends(RcppEigen)]] dependency declaration must appear at the top of the file or before any // [[Rcpp::export]] tags. Placing depends directly between export and the function header disrupts Rcpp's code generator.
  • Complex Template Signatures: Rcpp's code parser struggles to automatically wrap complex inline template types like const Eigen::Map<Eigen::SparseMatrix<double> >& directly inside the exported function parameter list.

The Correct Approach

To solve this, use RcppEigen's built-in Eigen::MappedSparseMatrix<double> class, which is explicitly designed to map R's S4 dgCMatrix without performing deep memory copies. Creating a typedef or type alias for this class keeps your function signature clean and easily parseable by Rcpp attributes.

Updated C++ Code Solution

#include <RcppEigen.h>

// [[Rcpp::depends(RcppEigen)]]

using namespace Rcpp;

// Define a clean type alias for mapped sparse matrices
typedef Eigen::MappedSparseMatrix<double> MappedSpMat;

// [[Rcpp::export]]
NumericVector dispcore(const MappedSpMat& M) {
    std::vector<double> lik(M.rows(), 0.0);
    return wrap(lik);
}

Testing the Solution in R

You can compile and test the updated function directly in R using the Matrix package:

library(Rcpp)
library(RcppEigen)
library(Matrix)

# Source the C++ file
sourceCpp("micro.cpp")

# Create a sample sparse dgCMatrix
mat <- Matrix(c(1, 0, 0, 2, 0, 3), nrow = 3, sparse = TRUE)

# Call the C++ function
result <- dispcore(mat)
print(result)
# Output: [1] 0 0 0

Key Best Practices

  • Dependency Order: Place all // [[Rcpp::depends(...)]] declarations above exported function signatures.
  • Use Mapped Matrix Types: Prefer Eigen::MappedSparseMatrix<double> over manually wrapping standard Eigen::SparseMatrix instances to ensure zero-copy read-only access.
  • Use Type Aliases: Simplify template-heavy parameter lists with typedef or using aliases so the Rcpp attribute parser can generate wrapper code reliably.