Add logic to capture stderr

Author: tharun571

test/test_interpreter.cpp

@@ -25,6 +25,26 @@
     #include <unistd.h>
 #endif
 
+// Object for capturing stderr
+class StdErrRedirect {
+public:
+    StdErrRedirect(std::ostream& new_stream) : old_cerr_buff(std::cerr.rdbuf()), ss(static_cast<std::stringstream&>(new_stream)) {
+        std::cerr.rdbuf(ss.rdbuf());
+    }
+
+    ~StdErrRedirect() {
+        std::cerr.rdbuf(old_cerr_buff);
+    }
+
+    std::string getCaptured() {
+        return ss.str();
+    }
+
+private:
+    std::streambuf* old_cerr_buff;
+    std::stringstream& ss;
+};
+
 TEST_SUITE("execute_request")
 {
     TEST_CASE("stl")
@@ -677,14 +697,12 @@ TEST_SUITE("xmagics_apply"){
         xcpp::xmagics_manager manager;
 
         std::stringstream ss;
-        auto cerr_buff = std::cerr.rdbuf();
-        std::cerr.rdbuf(ss.rdbuf());
+    StdErrRedirect redirect(ss);
 
-        manager.apply("test", "line", "");
+    manager.apply("test", "line", "");
 
-        std::cerr.rdbuf(cerr_buff);
-        REQUIRE(ss.str() == "UsageError: %%test is a cell magic, but the cell body is empty.\n"
-                      "If you only intend to display %%test help, please use a double line break to fill in the cell body.\n");
+    REQUIRE(redirect.getCaptured() == "UsageError: %%test is a cell magic, but the cell body is empty.\n"
+                  "If you only intend to display %%test help, please use a double line break to fill in the cell body.\n");
 
     }
 }