How to insert into a protobuf map in c++?

Question

For the following Maps unable to insert into the Map

map<string, double> doubleProps = 4;
map<string, int32> intProps = 5;
map<string,string> stringProps=6;

Here are the following which I have tried, but its not working

doubleProps["key"]="value" //doesn't work
*(doubleProps["key"]).insert("value") //doesn't work
doubleProps.insert("key","value")

Answer 1

This is the way to insert into a map in protobuf object in c++

(*mutable_stringprops())["key"]=value