From bf1ca6460da45526ea928fd20e53b3b4b2a22bf0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ciro=20Santilli=20=E5=85=AD=E5=9B=9B=E4=BA=8B=E4=BB=B6=20?=
 =?UTF-8?q?=E6=B3=95=E8=BD=AE=E5=8A=9F?= <ciro.santilli@gmail.com>
Date: Wed, 13 Nov 2019 00:00:00 +0000
Subject: [PATCH] c++: copyfmt

---
 README.adoc              |  2 ++
 userland/cpp/copyfmt.cpp | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)
 create mode 100644 userland/cpp/copyfmt.cpp

diff --git a/README.adoc b/README.adoc
index 410cf41..dd0334a 100644
--- a/README.adoc
+++ b/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
diff --git a/userland/cpp/copyfmt.cpp b/userland/cpp/copyfmt.cpp
new file mode 100644
index 0000000..1dd040b
--- /dev/null
+++ b/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("");
+}