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diff --git a/examples/extend-c-api-impl.cc b/examples/extend-c-api-impl.cc
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+// This is an example of how to write C++ functions and make them usable with the qpdf C API. It
+// consists of three files:
+// - extend-c-api.h -- a plain C header file
+// - extend-c-api.c -- a C program that calls the function
+// - extend-c-api.cc -- a C++ file that implements the function
+
+#include "extend-c-api.h"
+
+// Here, we add a function to get the number of pages in a PDF file and make it callable through the
+// C API.
+
+// This is a normal C++ function that works with QPDF in a normal way. It doesn't do anything
+// special to be callable from C.
+int
+numPages(std::shared_ptr<QPDF> qpdf)
+{
+    return qpdf->getRoot().getKey("/Pages").getKey("/Count").getIntValueAsInt();
+}
+
+// Now we define the glue that makes our function callable using the C API.
+
+// This is the C++ implementation of the C function.
+QPDF_ERROR_CODE
+num_pages(qpdf_data qc, int* npages)
+{
+    // Call qpdf_c_wrap to convert any exception our function might through to a QPDF_ERROR_CODE
+    // and attach it to the qpdf_data object in the same way as other functions in the C API.
+    return qpdf_c_wrap(qc, [&qc, &npages]() { *npages = numPages(qpdf_c_get_qpdf(qc)); });
+}