helper mongo::ErrorMsg class that builds error strings. lighter weigh…

…t than a StringBuilder, albeit less flexible.

util/assert_util.cpp

@@ -154,5 +154,22 @@ namespace mongo {
 #endif
     }
 
+    NOINLINE_DECL ErrorMsg::ErrorMsg(const char *msg, char ch) {
+        int l = strlen(msg);
+        assert( l < 128);
+        memcpy(buf, msg, l);
+        char *p = buf + l;
+        p[0] = ch;
+        p[1] = 0;
+    }
+
+    NOINLINE_DECL ErrorMsg::ErrorMsg(const char *msg, unsigned val) {
+        int l = strlen(msg);
+        assert( l < 128);
+        memcpy(buf, msg, l);
+        char *p = buf + l;
+        sprintf(p, "%u", val);
+    }
+
 }
 

util/assert_util.h

@@ -67,6 +67,21 @@ namespace mongo {
         int code;
     };
 
+    /** helper class that builds error strings.  lighter weight than a StringBuilder, albeit less flexible.
+        NOINLINE_DECL used in the constructor implementations as we are assuming this is a cold code path when used.
+
+        example: 
+          throw UserException(123, ErrorMsg("blah", num_val));
+    */
+    class ErrorMsg { 
+    public:
+        ErrorMsg(const char *msg, char ch);
+        ErrorMsg(const char *msg, unsigned val);
+        operator string() const { return buf; }
+    private:
+        char buf[256];
+    };
+
     class DBException : public std::exception {
     public:
         DBException( const ExceptionInfo& ei ) : _ei(ei) {}