Avoid footgun.

Factor temporary std::string instance to separate variable
as this prolongs its lifetime long enough for result of
.c_str to survive. Otherwise printf could obtain pointer
to already destructed string.

For more information consult GotW #88 at [1].

[1] https://herbsutter.com/2008/01/01/gotw-88-a-candidate-for-the-most-important-const/

Author: MarekKnapek

echo_server.cpp

@@ -22,7 +22,8 @@ void handle_request(const std::error_code& error, tcp::socket socket) {
     server.async_accept(io, &handle_request); // The Loop
 
     if (error) {
-        printf("Error while accepting: %s\n", error.message().c_str());
+        const auto &msg = error.message();
+        printf("Error while accepting: %s\n", msg.c_str());
         return;
     }
 
@@ -33,14 +34,16 @@ void handle_request(const std::error_code& error, tcp::socket socket) {
     asio::async_read_until(request->socket, request->buffer, '\n',
         [request](const auto &error, size_t bytes_transferred) {
             if (error) {
-                printf("Error while reading: %s\n", error.message().c_str());
+                const auto &msg = error.message();
+                printf("Error while reading: %s\n", msg.c_str());
                 return;
             }
             printf("Received %zu bytes\n", bytes_transferred);
             asio::async_write(request->socket, request->buffer,
                 [request](const auto &error, size_t bytes_transferred) {
                     if (error) {
-                        printf("Error while writing: %s\n", error.message().c_str());
+                        const auto &msg = error.message();
+                        printf("Error while writing: %s\n", msg.c_str());
                         return;
                     }
                     printf("Sent %zu bytes\n", bytes_transferred);