c++ - hashmaps maps, unordered maps, with classes as keys -


this question has answer here:

i many different issues trying use stl maps, discouraging

so got many questions:

  • first had problem " < operator not defined", did not need ordering in maps(why need default anyway) found unordered_map have use c++11 or something, don't know means , not sure idea, standard c? portable ?

  • then got weird error:

    /usr/include/c++/4.8/bits/hashtable_policy.h:1070:12: error: invalid use of incomplete type ‘struct std::hash< folvariable>’

so why template intrusive? mean should act container, not minding object nor inside of it

anyone can me out error?

thanks

first had problem " < operator not defined", did not need ordering in maps(why need default anyway)

from documentation:

std::map sorted associative container contains key-value pairs unique keys. keys sorted using comparison function compare.

emphasis mine. default, comparison function operator <.

so found unordered_map have use c++11 or something, don't know means , not sure idea, standard c? portable ?

c++11 name of latest c++ standard. modern compilers support major features of standard now. if compiler supports it, it's idea use features require. it's not standard c, standard c++.

then got weird error:

/usr/include/c++/4.8/bits/hashtable_policy.h:1070:12: error: invalid use of incomplete type ‘struct std::hash< folvariable>’

std::unordered_map hash table. default, hashing function used std::unordered_map<key, value> specialization of std::hash<key>. error because no such specialization exists type folvariable.


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