c++: copyfmt

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("");
+}