How can I get a value from a map

Question

I have a map named valueMap as follows   typedef std  map lt std  string  std  string gt MAP  MAP valueMap         Entering data    Then I am passing this map to a function by reference   void function const MAP  amp map      std  string value   map  string         By doing so I am getting an error      How can I get the value from the map  which is passed as a reference to a function

User · Accepted Answer

Unfortunately std  map  operator   is a non-const member function  and you have a const reference   You either need to change the signature of function or do   MAP  const iterator pos   map find  string    if  pos    map end            handle the error   else       std  string value   pos- gt second              operator   handles the error by adding a default-constructed value to the map and returning a reference to it  This is no use when all you have is a const reference  so you will need to do something different   You could ignore the possibility and write string value   map find  string  - gt second   if your program logic somehow guarantees that  string  is already a key  The obvious problem is that if you re wrong then you get undefined behavior

User · Answer

The answer by Steve Jessop explains well  why you can t use std  map  operator   on a const std  map  Gabe Rainbow s answer suggests a nice alternative  I d just like to provide some example code on how to use map  at    So  here is an enhanced example of your function     void function const MAP  amp map  const std  string  amp findMe        try           const std  string amp  value   map at findMe           std  cout  lt  lt   Value of key      lt  lt  findMe c str    lt  lt          lt  lt  value c str    lt  lt  std  endl             TODO  Handle the element found            catch  const std  out of range amp             std  cout  lt  lt   Key      lt  lt  findMe c str    lt  lt      not found   lt  lt  std  endl             TODO  Deal with the missing element            And here is an example main   function   int main         MAP valueMap      valueMap  string      abc       function valueMap   string        function valueMap   strong        return 0      Output      Value of key  string   abc   Key  strong  not found   Code on Ideone

User · Answer

map at  key   throws exception if missing key     If k does not match the key of any element in the container  the   function throws an out of range exception    http   www cplusplus com reference map map at

User · Answer

How can I get the value from the map  which is passed as a reference to a function    Well  you can pass it as a reference  The standard reference wrapper that is    typedef std  map lt std  string  std  string gt  MAP     create your map reference type using map ref t   std  reference wrapper lt MAP gt       use it  void function map ref t map r           get to the map from inside the        std  reference wrapper        see the alternatives behind that link     MAP  amp  the map   map r         take the value from the map        by reference     auto  amp  value r   the map  key           change it   in place      value r    new        And the test        void test ref to map          MAP valueMap      valueMap  key      value          pass it by reference     function valueMap          check that the value has changed     assert   new      valueMap  key          I think this is nice and simple  Enjoy

User · Answer

The main problem is that operator    is used to insert and read a value into and from the map  so it cannot be const  If the key does not exist  it will create a new entry with a default value in it  incrementing the size of the map  that will contain a new key with an empty string  in this particular case  as a value if the key does not exist yet   You should avoid operator   when reading from a map and use  as was mention before   map at key   to ensure bound checking  This is one of the most common mistakes people often do with maps  You should use  insert  and  at  unless your code is aware of this fact  Check this talk about common bugs Curiously Recurring C   Bugs at Facebook

[c++] How can I get a value from a map?

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