how to do a map to ofstreams?-CodePudding

There is a similar question to this one but it was not answered.

I am trying to create a map of std::ofstream objects but I cannot build the code.

So far I have tried

//std::map<std::string, std::ofstream> my_files;  //this fails
std::map<std::string, std::ofstream& > my_files;

for(auto & member: members){
   filename=GetFileName();
   //create the file
   std::ofstream thefile(filename);

   my_files[member.first]=thefile;
}
//Here I use the ofstreams in the map to write etc

I tried the first (commented) line and I got this error. Use of deleted function. So I change it to the line above, but I still get the same error error: use of deleted function ‘std::basic_ofstream<

How can I build a map of ofstream objects?

Note: In the similar question, someone suggested to use a map of strings but the reason to use a map of ofstream is that I am not going to be opening and closing the file everytime I want to do a minimal change to each.

CodePudding user response：

The problem is that a reference is not object by itself. A reference refers to some other object. So you cannot store a reference in a container.

You can solve this by replacing my_files[member.first]=thefile; with:

my_files.emplace(member.first, std::ofstream(filename));

Also, replace std::map<std::string, std::ofstream& > my_files; with:

std::map<std::string, std::ofstream > my_files; //removed the &

Modified Code

std::map<std::string, std::ofstream > my_files; //removed the &

for(auto & member: members){
   filename=GetFileName();

   my_files.emplace(member.first, std::ofstream(filename)); //ADDED THIS
}

CodePudding user response：

First of all, references cannot be stored in containers. But you seem to want to store the std::ofstream itself, not a reference to it, anyway, since your std::ofstream object is otherwise going to be destroyed at the end of the loop body leaving you with a dangling reference in the map, so:

std::map<std::string, std::ofstream > my_files;

Secondly, stream objects such as std::ofstream are not copyable, so you cannot simply copy them into the map.

However, they are movable, so you can move them into the map:

my_files[member.first] = std::move(thefile);

(requires #include<utility>) or alternatively, by constructing it directly in the assignment:

my_files[member.first] = std::ofstream(filename);

For explanation of std::move if you haven't seen it before, see this question.