load std::map from text file

Question

This is a very simple thing, so I want to keep it as simple as it sounds. All I want is to load a bunch of key-value paires from a file, and populate them in to a map. I do not really care how the text is structured, as long as it is easy to read.

What i have now is:

• xml with xsd generated code (overkill)
• Protocol buffer (also overkill)
• INI style text file

I like the syntax of the INI file, but I not want to write a parser for that. It sounds to me like I would be doing something lots of people have done before me. Is there not some sort of library to read simple structured files like this?

Solution

Since you seem to want the simplest thing humanly possible, I'm going to suggest something incredibly simple that may or may not work based on your map contents. If your map data values are strings that include spaces, this wont work. If they're strings without spaces, or numeric, you're set.

This isn't tested code, but it's close and simple so you should be fine even if it doesn't quite compile. Just change KeyType and ValueType to int, string, float, or whatever you're actually using in the file.

Set up file like:

key value
key2 value2
key3 value3
key4 value4

Read like:

KeyType key;
ValueType value;

std::map<KeyType, ValueType> myMap;
while (infile >> key >> value)
    myMap[key] = value;

Comments

[W. Goeman]

That is very simple indeed! Would work perfectly for what I am doing. Still, not INI format, but should work

[Jonathan Wakely]

-1, the loop condition should be while (infile >> key >> value) and for the loop body myMap[key] = value; is simpler

[Tony Delroy]

Basic idea's great, but just to emphasise and explain what Jonathan said: this is not simply a matter of style - the eof() condition is not set until after a failed attempt to read past the end (thought experiment: even if not at the last character in infile, how could it know at the while() whether it had enough characters for both key and value?). Separately, and FWIW, this can work for std::string keys and values with spaces with a simply change to while (getline(infile, key) && getline(infile, value)).

[John Humphreys]

Agreed, didn't think it through far enough - thanks for the suggestion, have modified answer. And I like Tony's idea if you're actually going to use this for multiword string values.