c++: copyfmt

README.adoc

@@ -13834,6 +13834,8 @@ Programs under link:userland/cpp/[] are examples of https://en.wikipedia.org/wik
 ** link:userland/cpp/template.cpp[]: basic example
 ** link:userland/cpp/template_class_with_static_member.cpp[]: https://stackoverflow.com/questions/3229883/static-member-initialization-in-a-class-template
 ** link:userland/cpp/if_constexpr.cpp[]: C++17 `if constexpr`
+* iostream
+** link:userland/cpp/copyfmt.cpp[]: `std::copyfmt` restores stream state, see also: https://stackoverflow.com/questions/12560291/set-back-default-floating-point-print-precision-in-c/53673686#53673686
 * fstream
 ** link:userland/cpp/file_write_read.cpp[]
 ** link:userland/cpp/temporary_directory.cpp[]: illustrates `std::filesystem::temp_directory_path` and answers https://stackoverflow.com/questions/3379956/how-to-create-a-temporary-directory-in-c/58454949#58454949

userland/cpp/copyfmt.cpp

@@ -0,0 +1,33 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+
+#include <cassert>
+#include <iomanip>
+#include <iostream>
+#include <sstream>
+
+int main() {
+    constexpr float pi = 3.14159265359;
+
+    std::stringstream ss;
+
+    // Sanity check default print.
+    ss << pi;
+    assert(ss.str() == "3.14159");
+    ss.str("");
+
+    // Change precision format to scientific,
+    // and restore default afterwards.
+    std::ios ss_state(nullptr);
+    ss_state.copyfmt(ss);
+    ss << std::setprecision(2);
+    ss << std::scientific;
+    ss << pi;
+    assert(ss.str() == "3.14e+00");
+    ss.str("");
+    ss.copyfmt(ss_state);
+
+    // Check that cout state was restored.
+    ss << pi;
+    assert(ss.str() == "3.14159");
+    ss.str("");
+}