In STL maps is it better to use map insert than

Question

A while ago  I had a discussion with a colleague about how to insert values in STL maps  I preferred map key    value  because it feels natural and is clear to read whereas he preferred map insert std  make pair key  value    I just asked him and neither of us can remember the reason why insert is better  but I am sure it was not just a style preference rather there was a technical reason such as efficiency  The SGI STL reference simply says   quot Strictly speaking  this member function is unnecessary  it exists only for convenience  quot  Can anybody tell me that reason  or am I just dreaming that there is one

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A gotcha with map  insert   is that it won t replace a value if the key already exists in the map  I ve seen C   code written by Java programmers where they have expected insert   to behave the same way as Map put   in Java where values are replaced

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insert is better from the point of exception safety   The expression map key    value is actually two operations    map key  - creating a map element with default value    value - copying the value into that element    An exception may happen at the second step  As result the operation will be only partially done  a new element was added into map  but that element was not initialized with value   The situation when an operation is not complete  but the system state is modified  is called the operation with  side effect    insert operation gives a strong guarantee  means it doesn t have side effects  https   en wikipedia org wiki Exception safety   insert is either completely done or it leaves the map in unmodified state   http   www cplusplus com reference map map insert       If a single element is to be inserted  there are no changes in the container in case of exception  strong guarantee

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Now in c  11 I think that the best way to insert a pair in a STL map is   typedef std  map lt int  std  string gt  MyMap  MyMap map   auto amp  result   map emplace 3  Hello      The result will be a pair with    First element  result first   points to the pair inserted or point to the pair with this key if the key already exist  Second element  result second   true if the insertion was correct or false it something went wrong    PS  If you don  t case about the order you can use std  unordered map     Thanks

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insert is better from the point of exception safety   The expression map key    value is actually two operations    map key  - creating a map element with default value    value - copying the value into that element    An exception may happen at the second step  As result the operation will be only partially done  a new element was added into map  but that element was not initialized with value   The situation when an operation is not complete  but the system state is modified  is called the operation with  side effect    insert operation gives a strong guarantee  means it doesn t have side effects  https   en wikipedia org wiki Exception safety   insert is either completely done or it leaves the map in unmodified state   http   www cplusplus com reference map map insert       If a single element is to be inserted  there are no changes in the container in case of exception  strong guarantee

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The two have different semantics when it comes to the key already existing in the map  So they aren t really directly comparable   But the operator   version requires default constructing the value  and then assigning  so if this is more expensive then copy construction  then it will be more expensive  Sometimes default construction doesn t make sense  and then it would be impossible to use the operator   version

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Now in c  11 I think that the best way to insert a pair in a STL map is   typedef std  map lt int  std  string gt  MyMap  MyMap map   auto amp  result   map emplace 3  Hello      The result will be a pair with    First element  result first   points to the pair inserted or point to the pair with this key if the key already exist  Second element  result second   true if the insertion was correct or false it something went wrong    PS  If you don  t case about the order you can use std  unordered map     Thanks

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One note is that you can also use Boost Assign   using namespace std  using namespace boost  assign     bring  map list of    into scope  void something         map lt int int gt  my map   map list of 1 2  2 3  3 4  4 5  5 6

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The two have different semantics when it comes to the key already existing in the map  So they aren t really directly comparable   But the operator   version requires default constructing the value  and then assigning  so if this is more expensive then copy construction  then it will be more expensive  Sometimes default construction doesn t make sense  and then it would be impossible to use the operator   version

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One note is that you can also use Boost Assign   using namespace std  using namespace boost  assign     bring  map list of    into scope  void something         map lt int int gt  my map   map list of 1 2  2 3  3 4  4 5  5 6

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If the performance hit of the default constructor isn t an issue  the please  for the love of god  go with the more readable version

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The difference between insert   and operator   has already been well explained in the other answers  However  new insertion methods for std  map were introduced with C  11 and C  17 respectively   C  11 offers emplace   as also mentioned in einpoklum s comment and GutiMac s answer  C  17 offers insert or assign   and try emplace     Let me give a brief summary of the  quot new quot  insertion methods   emplace    When used correctly  this method can avoid unnecessary copy or move operations by constructing the element to be inserted in place  Similar to insert    an element is only inserted if there is no element with the same key in the container  insert or assign    This method is an  quot improved quot  version of operator    Unlike operator    insert or assign   doesn t require the map s value type to be default constructible  This overcomes the disadvantage mentioned e g  in Greg Rogers  answer  try emplace    This method is an  quot improved quot  version of emplace    Unlike emplace    try emplace   doesn t modify its arguments  due to move operations  if insertion fails due to a key already existing in the map   For more details on insert or assign   and try emplace   please see my answer here  Simple example code on Coliru

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If the performance hit of the default constructor isn t an issue  the please  for the love of god  go with the more readable version

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This is a rather restricted case  but judging from the comments I ve received I think it s worth noting   I ve seen people in the past use maps in the form of   map lt  const key  const val gt  Map    to evade cases of accidental value overwriting  but then go ahead writing in some other bits of code   const cast lt  T  gt Map   val    Their reason for doing this as I recall was because they were sure that in these certain bits of code they were not going to be overwriting map values  hence  going ahead with the more  readable  method      I ve never actually had any direct trouble from the code that was written by these people  but I strongly feel up until today that risks - however small - should not be taken when they can be easily avoided   In cases where you re dealing with map values that absolutely must not be overwritten  use insert  Don t make exceptions merely for readability

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The fact that std  map insert   function doesn t overwrite value associated with the key allows us to write object enumeration code like this    string word  map lt string  size t gt  dict  while getline cin  word         dict insert make pair word  dict size          It s a pretty common problem when we need to map different non-unique objects to some id s in range 0  N  Those id s can be later used  for example  in graph algorithms  Alternative with operator   would look less readable in my opinion   string word  map lt string  size t gt  dict  while getline cin  word         size t sz   dict size        if   dict count word           dict word    sz

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If your application is speed critical i will advice using    operator because it creates total 3 copies of the original object out of which 2 are temporary objects and sooner or later destroyed as   But in insert    4 copies of the original object are created out of which 3 are temporary objects  not necessarily  temporaries   and are destroyed   Which means extra time for  1  One objects memory allocation 2  One extra constructor call 3  One extra destructor call 4  One objects memory deallocation  If your objects are large  constructors are typical  destructors do a lot of resource freeing  above points count even more  Regarding readability  i think both are fair enough   The same question came into my mind but not over readability but speed  Here is a sample code through which I came to know about the point i mentioned   class Sample       static int  noOfObjects       int  objectNo  public      Sample              objectNo   noOfObjects                   std  cout lt  lt  Inside default constructor of object   lt  lt  objectNo lt  lt std  endl             Sample  const Sample amp  sample         objectNo   noOfObjects                   std  cout lt  lt  Inside copy constructor of object   lt  lt  objectNo lt  lt std  endl              Sample                 std  cout lt  lt  Destroying object   lt  lt  objectNo lt  lt std  endl           int Sample   noOfObjects   0    int main int argc  char  argv          Sample sample      std  map lt int Sample gt  map       map insert  std  make pair lt int Sample gt   1  sample           map 1    sample      return 0

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If the performance hit of the default constructor isn t an issue  the please  for the love of god  go with the more readable version

User · Answer

A gotcha with map  insert   is that it won t replace a value if the key already exists in the map  I ve seen C   code written by Java programmers where they have expected insert   to behave the same way as Map put   in Java where values are replaced

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When you write  map key    value    there s no way to tell if you replaced the value for key  or if you created a new key with value   map  insert   will only create   using std  cout  using std  endl  typedef std  map lt int  std  string gt  MyMap  MyMap map         std  pair lt MyMap  iterator  bool gt  res   map insert MyMap  value type key value    if     res second         cout  lt  lt   key    lt  lt   key  lt  lt    already exists             lt  lt    with value    lt  lt   res first - gt second  lt  lt  endl    else       cout  lt  lt   created key    lt  lt  key  lt  lt    with value    lt  lt  value  lt  lt  endl      For most of my apps  I usually don t care if I m creating or replacing  so I use the easier to read map key    value

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Another thing to note with std  map   myMap nonExistingKey   will create a new entry in the map  keyed to nonExistingKey initialized to a default value   This scared the hell out of me the first time I saw it  while banging my head against a nastly legacy bug    Wouldn t have expected it   To me  that looks like a get operation  and I didn t expect the  side-effect    Prefer map find   when getting from your map

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Another thing to note with std  map   myMap nonExistingKey   will create a new entry in the map  keyed to nonExistingKey initialized to a default value   This scared the hell out of me the first time I saw it  while banging my head against a nastly legacy bug    Wouldn t have expected it   To me  that looks like a get operation  and I didn t expect the  side-effect    Prefer map find   when getting from your map

User · Answer

The two have different semantics when it comes to the key already existing in the map  So they aren t really directly comparable   But the operator   version requires default constructing the value  and then assigning  so if this is more expensive then copy construction  then it will be more expensive  Sometimes default construction doesn t make sense  and then it would be impossible to use the operator   version

User · Answer

When you write  map key    value    there s no way to tell if you replaced the value for key  or if you created a new key with value   map  insert   will only create   using std  cout  using std  endl  typedef std  map lt int  std  string gt  MyMap  MyMap map         std  pair lt MyMap  iterator  bool gt  res   map insert MyMap  value type key value    if     res second         cout  lt  lt   key    lt  lt   key  lt  lt    already exists             lt  lt    with value    lt  lt   res first - gt second  lt  lt  endl    else       cout  lt  lt   created key    lt  lt  key  lt  lt    with value    lt  lt  value  lt  lt  endl      For most of my apps  I usually don t care if I m creating or replacing  so I use the easier to read map key    value

User · Answer

This is a rather restricted case  but judging from the comments I ve received I think it s worth noting   I ve seen people in the past use maps in the form of   map lt  const key  const val gt  Map    to evade cases of accidental value overwriting  but then go ahead writing in some other bits of code   const cast lt  T  gt Map   val    Their reason for doing this as I recall was because they were sure that in these certain bits of code they were not going to be overwriting map values  hence  going ahead with the more  readable  method      I ve never actually had any direct trouble from the code that was written by these people  but I strongly feel up until today that risks - however small - should not be taken when they can be easily avoided   In cases where you re dealing with map values that absolutely must not be overwritten  use insert  Don t make exceptions merely for readability

User · Answer

When you write  map key    value    there s no way to tell if you replaced the value for key  or if you created a new key with value   map  insert   will only create   using std  cout  using std  endl  typedef std  map lt int  std  string gt  MyMap  MyMap map         std  pair lt MyMap  iterator  bool gt  res   map insert MyMap  value type key value    if     res second         cout  lt  lt   key    lt  lt   key  lt  lt    already exists             lt  lt    with value    lt  lt   res first - gt second  lt  lt  endl    else       cout  lt  lt   created key    lt  lt  key  lt  lt    with value    lt  lt  value  lt  lt  endl      For most of my apps  I usually don t care if I m creating or replacing  so I use the easier to read map key    value

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Here s another example  showing that operator   overwrites the value for the key if it exists  but  insert does not overwrite the value if it exists   void mapTest       map lt int float gt  m      for  int i   0   i   lt    2   i             pair lt map lt int float gt   iterator bool gt  result   m insert  make pair  5   float i            if  result second         printf    d  gt value  f successfully inserted as brand new value n   result first- gt first  result first- gt second         else       printf     The map already contained  d  gt value  f  nothing changed n   result first- gt first  result first- gt second            puts   All map values         for  map lt int float gt   iterator iter   m begin     iter   m end       iter       printf    d  gt  f n   iter- gt first  iter- gt second            now watch this      m 5  900 f     using operator   OVERWRITES map values   puts   All map values         for  map lt int float gt   iterator iter   m begin     iter   m end       iter       printf    d  gt  f n   iter- gt first  iter- gt second

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Here s another example  showing that operator   overwrites the value for the key if it exists  but  insert does not overwrite the value if it exists   void mapTest       map lt int float gt  m      for  int i   0   i   lt    2   i             pair lt map lt int float gt   iterator bool gt  result   m insert  make pair  5   float i            if  result second         printf    d  gt value  f successfully inserted as brand new value n   result first- gt first  result first- gt second         else       printf     The map already contained  d  gt value  f  nothing changed n   result first- gt first  result first- gt second            puts   All map values         for  map lt int float gt   iterator iter   m begin     iter   m end       iter       printf    d  gt  f n   iter- gt first  iter- gt second            now watch this      m 5  900 f     using operator   OVERWRITES map values   puts   All map values         for  map lt int float gt   iterator iter   m begin     iter   m end       iter       printf    d  gt  f n   iter- gt first  iter- gt second

User · Answer

The difference between insert   and operator   has already been well explained in the other answers  However  new insertion methods for std  map were introduced with C  11 and C  17 respectively   C  11 offers emplace   as also mentioned in einpoklum s comment and GutiMac s answer  C  17 offers insert or assign   and try emplace     Let me give a brief summary of the  quot new quot  insertion methods   emplace    When used correctly  this method can avoid unnecessary copy or move operations by constructing the element to be inserted in place  Similar to insert    an element is only inserted if there is no element with the same key in the container  insert or assign    This method is an  quot improved quot  version of operator    Unlike operator    insert or assign   doesn t require the map s value type to be default constructible  This overcomes the disadvantage mentioned e g  in Greg Rogers  answer  try emplace    This method is an  quot improved quot  version of emplace    Unlike emplace    try emplace   doesn t modify its arguments  due to move operations  if insertion fails due to a key already existing in the map   For more details on insert or assign   and try emplace   please see my answer here  Simple example code on Coliru

User · Answer

If your application is speed critical i will advice using    operator because it creates total 3 copies of the original object out of which 2 are temporary objects and sooner or later destroyed as   But in insert    4 copies of the original object are created out of which 3 are temporary objects  not necessarily  temporaries   and are destroyed   Which means extra time for  1  One objects memory allocation 2  One extra constructor call 3  One extra destructor call 4  One objects memory deallocation  If your objects are large  constructors are typical  destructors do a lot of resource freeing  above points count even more  Regarding readability  i think both are fair enough   The same question came into my mind but not over readability but speed  Here is a sample code through which I came to know about the point i mentioned   class Sample       static int  noOfObjects       int  objectNo  public      Sample              objectNo   noOfObjects                   std  cout lt  lt  Inside default constructor of object   lt  lt  objectNo lt  lt std  endl             Sample  const Sample amp  sample         objectNo   noOfObjects                   std  cout lt  lt  Inside copy constructor of object   lt  lt  objectNo lt  lt std  endl              Sample                 std  cout lt  lt  Destroying object   lt  lt  objectNo lt  lt std  endl           int Sample   noOfObjects   0    int main int argc  char  argv          Sample sample      std  map lt int Sample gt  map       map insert  std  make pair lt int Sample gt   1  sample           map 1    sample      return 0

User · Answer

A gotcha with map  insert   is that it won t replace a value if the key already exists in the map  I ve seen C   code written by Java programmers where they have expected insert   to behave the same way as Map put   in Java where values are replaced

User · Answer

The fact that std  map insert   function doesn t overwrite value associated with the key allows us to write object enumeration code like this    string word  map lt string  size t gt  dict  while getline cin  word         dict insert make pair word  dict size          It s a pretty common problem when we need to map different non-unique objects to some id s in range 0  N  Those id s can be later used  for example  in graph algorithms  Alternative with operator   would look less readable in my opinion   string word  map lt string  size t gt  dict  while getline cin  word         size t sz   dict size        if   dict count word           dict word    sz

User · Answer

When you write  map key    value    there s no way to tell if you replaced the value for key  or if you created a new key with value   map  insert   will only create   using std  cout  using std  endl  typedef std  map lt int  std  string gt  MyMap  MyMap map         std  pair lt MyMap  iterator  bool gt  res   map insert MyMap  value type key value    if     res second         cout  lt  lt   key    lt  lt   key  lt  lt    already exists             lt  lt    with value    lt  lt   res first - gt second  lt  lt  endl    else       cout  lt  lt   created key    lt  lt  key  lt  lt    with value    lt  lt  value  lt  lt  endl      For most of my apps  I usually don t care if I m creating or replacing  so I use the easier to read map key    value

User · Answer

The two have different semantics when it comes to the key already existing in the map  So they aren t really directly comparable   But the operator   version requires default constructing the value  and then assigning  so if this is more expensive then copy construction  then it will be more expensive  Sometimes default construction doesn t make sense  and then it would be impossible to use the operator   version

User · Answer

If the performance hit of the default constructor isn t an issue  the please  for the love of god  go with the more readable version

User · Answer

A gotcha with map  insert   is that it won t replace a value if the key already exists in the map  I ve seen C   code written by Java programmers where they have expected insert   to behave the same way as Map put   in Java where values are replaced

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