for log handler a string reference is sufficient, inside the log handler used string.append instead of + operator

docs/guides/logging.md

@@ -35,13 +35,13 @@ Writing a log is as simple as `#!cpp CROW_LOG_<LOG LEVEL> << "Hello";` (replace&
 
 
 Assuming you have an existing logger or Crow's default format just doesn't work for you. Crow allows you to use a custom logger for any log made using the `CROW_LOG_<LOG LEVEL>` macro.<br>
-All you need is a class extending `#!cpp crow::ILogHandler` containing the method `#!cpp void log(std::string, crow::LogLevel)`.<br>
+All you need is a class extending `#!cpp crow::ILogHandler` containing the method `#!cpp void log(const std::string&, crow::LogLevel)`.<br>
 Once you have your custom logger, you need to set it via `#!cpp crow::logger::setHandler(&MyLogger);`. Here's a full example:<br>
 ```cpp
 class CustomLogger : public crow::ILogHandler {
  public:
   CustomLogger() {}
-  void log(std::string message, crow::LogLevel /*level*/) {
+  void log(const std::string& message, crow::LogLevel /*level*/) {
     // "message" doesn't contain the timestamp and loglevel
     // prefix the default logger does and it doesn't end
     // in a newline.

examples/example.cpp

@@ -3,7 +3,7 @@
 class ExampleLogHandler : public crow::ILogHandler
 {
 public:
-    void log(std::string /*message*/, crow::LogLevel /*level*/) override
+    void log(const std::string& /*message*/, crow::LogLevel /*level*/) override
     {
         //            cerr << "ExampleLogHandler -> " << message;
     }

examples/example_vs.cpp

@@ -3,7 +3,7 @@
 class ExampleLogHandler : public crow::ILogHandler
 {
 public:
-    void log(std::string message, crow::LogLevel level) override
+    void log(const std::string & message, crow::LogLevel level) override
     {
         //            cerr << "ExampleLogHandler -> " << message;
     }

examples/example_with_all.cpp

@@ -3,7 +3,7 @@
 class ExampleLogHandler : public crow::ILogHandler
 {
 public:
-    void log(std::string /*message*/, crow::LogLevel /*level*/) override
+    void log(const std::string& /*message*/, crow::LogLevel /*level*/) override
     {
         //            cerr << "ExampleLogHandler -> " << message;
     }

include/crow/logging.h

@@ -35,34 +35,44 @@ namespace crow
     public:
         virtual ~ILogHandler() = default;
 
-        virtual void log(std::string message, LogLevel level) = 0;
+        virtual void log(const std::string& message, LogLevel level) = 0;
     };
 
     class CerrLogHandler : public ILogHandler
     {
     public:
-        void log(std::string message, LogLevel level) override
+        void log(const std::string &message, LogLevel level) override
         {
-            std::string prefix;
+            std::string log_msg;
+            log_msg.reserve(message.length() + 1+32+3+8+2);
+            log_msg
+                .append("(")
+                .append(timestamp())
+                .append(") [");
+
             switch (level)
             {
                 case LogLevel::Debug:
-                    prefix = "DEBUG   ";
+                    log_msg.append("DEBUG   ");
                     break;
                 case LogLevel::Info:
-                    prefix = "INFO    ";
+                    log_msg.append("INFO    ");
                     break;
                 case LogLevel::Warning:
-                    prefix = "WARNING ";
+                    log_msg.append("WARNING ");
                     break;
                 case LogLevel::Error:
-                    prefix = "ERROR   ";
+                    log_msg.append("ERROR   ");
                     break;
                 case LogLevel::Critical:
-                    prefix = "CRITICAL";
+                    log_msg.append("CRITICAL");
                     break;
             }
-            std::cerr << std::string("(") + timestamp() + std::string(") [") + prefix + std::string("] ") + message << std::endl;
+
+            log_msg.append("] ")
+            .append(message);
+
+            std::cerr << log_msg << std::endl;
         }
 
     private: