Simple experiments contrasting Boost.Python vs pybind11 error_already_set.

Author: rwgk

error_already_set/error_fetch_restore.cc

@@ -0,0 +1,37 @@
+#include <stdexcept>
+
+std::string error_string() {
+  return "Implementation without side-effects here.";
+}
+
+struct error_already_set : std::exception {
+  error_already_set() : m_py_error_indicator("Fetch here.") {}
+  ~error_already_set() { /* GIL-safe dec_ref. */
+  }
+
+  void restore() { /* Restore here. */
+  }
+
+  const char* what() const noexcept override {
+    m_what = m_py_error_indicator + ": " + error_string();
+    return m_what.c_str();
+  }
+
+  std::string m_py_error_indicator;
+  mutable std::string m_what;
+};
+
+void use_what() {
+  try {
+    // Now error_already_set is not a fitting name anymore.
+    // throw fetched_python_error(); would be a more accurate description.
+    throw error_already_set();
+  } catch (error_already_set& e) {
+    e.restore();
+    // But we could also Fetch or Clear the error here, as needed.
+    // The reason for Fetch in the error_already_set constructor is
+    // something rwgk@ does not know.
+  }
+}
+
+int main() { use_what(); }

error_already_set/error_string_or_what.cc

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+#include <iostream>
+#include <stdexcept>
+
+std::string error_string() {
+  return "Implementation without side-effects here.";
+}
+
+struct error_already_set : std::exception {
+  const char* what() const noexcept override {
+    m_what = error_string();
+    return m_what.c_str();
+  }
+  mutable std::string m_what;
+};
+
+void use_error_string() {
+  try {
+    throw error_already_set();
+  } catch (const error_already_set&) {
+    std::cout << error_string() << std::endl;
+  }
+}
+
+void use_what() {
+  try {
+    throw error_already_set();
+  } catch (const error_already_set& e) {
+    // Is enabling e.what() worth the m_what overhead?
+    std::cout << e.what() << std::endl;
+  }
+}
+
+int main() {
+  use_error_string();
+  use_what();
+}

error_already_set/sizeof_std_exception.cc

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+#include <iostream>
+#include <stdexcept>
+
+// Actual & complete Boost.Python implementation (except for the EXPORT macro).
+struct error_already_set {
+  virtual ~error_already_set();
+};
+
+int main() {
+  std::cout << sizeof(error_already_set) << std::endl;
+  std::cout << sizeof(std::exception) << std::endl;
+  std::cout << sizeof(std::runtime_error) << std::endl;
+}