Document duplicated map lookups
This pattern shows up a lot in code review. Being able to have it
documented will help with code review time, and hopefully will reduce
the instances of this showing up.
Change-Id: Ia3b1498866cde25526b537916e2ac54ff0818a17
Signed-off-by: Ed Tanous <etanous@nvidia.com>
diff --git a/docs/COMMON_ERRORS.md b/docs/COMMON_ERRORS.md
index 810066e..2305bf3 100644
--- a/docs/COMMON_ERRORS.md
+++ b/docs/COMMON_ERRORS.md
@@ -415,3 +415,31 @@
#include "generated/enums/sensor.hpp"
sensorJson["ReadingType"] = sensor::ReadingType::Frequency;
```
+
+## 17. Duplicated map lookups
+
+```cpp
+std::unordered_map<std::string, std::string> mymap;
+
+if (mymap.contains("mykey")){
+ std::string& myvalue = mymap["mykey"];
+}
+```
+
+While functionally correct doing the above results in more code generated, and
+duplicates the key lookup in the map. The above code also makes it more
+difficult to use const for "mymap", as operator[] is not const. Sometimes at()
+is used in place of operator[] to get around this, but also duplicates the
+lookup.
+
+As a minor consequence, the lookup key "mykey" is now duplicated, or needs to be
+loaded into a scope variable.
+
+```cpp
+std::unordered_map<std::string, std::string> mymap;
+
+auto it = mymap.find("mykey");
+if (it != mymap.end()){
+ std::string& myvalue = it->second;
+}
+```