How do I deal with signed unsigned mismatch warnings C4018

Question

I work with a lot of calculation code written in C   with high performance and low memory overhead in mind  It uses STL containers  mostly vector  a lot  and iterates over that containers almost in every single function   The iterating code looks like this   for  int i   0  i  lt  things size      i                   but it produces the signed unsigned mismatch warning  C4018 in Visual Studio    Replacing int with some unsigned type is a problem because we frequently use OpenMP pragmas  and it requires the counter to be int   I m about to suppress the  hundreds of  warnings  but I m afraid I ve missed some elegant solution to the problem   On iterators  I think iterators are great when applied in appropriate places  The code I m working with will never change random-access containers into list or something  so iterating with int i is already container agnostic   and will always need the current index  And all the additional code you need to type  iterator itself and the index  just complicates matters and obfuscates the simplicity of the underlying code

User · Answer

I would just do  int pnSize   primeNumber size    for  int i   0  i  lt  pnSize  i        cout  lt  lt  primeNumber i   lt  lt

User · Answer

I had a similar problem  Using size t was not working  I tried the other one which worked for me   as below   for int i   things size  -1 i gt  0 i--

User · Answer

You can use    size t type  to remove warning messages iterators   distance  like are first hint  only iterators function object   For example      simple class who output his value class ConsoleOutput   public    ConsoleOutput int value  m value value        int Value   const   return m value    private    int m value         functional object class Predicat   public    void operator   ConsoleOutput const amp  item          std  cout  lt  lt  item Value    lt  lt  std  endl          void main          fill list   std  vector lt ConsoleOutput gt  list    list push back ConsoleOutput 1      list push back ConsoleOutput 8          1  using size t   for  size t i   0  i  lt  list size      i          std  cout  lt  lt  list at i  Value    lt  lt  std  endl            2  iterators   distance  for std  distance only non const iterators   std  vector lt ConsoleOutput gt   iterator itDistance   list begin    endDistance   list end      for     itDistance    endDistance    itDistance             int or size t     int const position   static cast lt int gt  std  distance list begin    itDistance        std  cout  lt  lt  list at position  Value    lt  lt  std  endl            3  iterators   std  vector lt ConsoleOutput gt   const iterator it   list begin    end   list end      for     it    end    it          std  cout  lt  lt    it  Value    lt  lt  std  endl           4  functional objects   std  for each list begin    list end    Predicat

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I will give you a better idea  for decltype things size    i   0  i  lt  things size    i                                  decltype is     Inspects the declared type of an entity or the type and value category   of an expression    So  It deduces type of things size   and i will be a type as same as things size    So   i  lt  things size   will be executed without any warning

User · Answer

I can also propose following solution for C  11    for  auto p   0U  p  lt  sys size    p            C   is not smart enough for auto p   0  so I have to put p   0U

User · Answer

Ideally  I would use a construct like this instead   for  std  vector lt your type gt   const iterator i   things begin    i    things end      i         if you ever need the distance  you may call std  distance      it won t cause any overhead because the compiler will likely optimize the call   size t distance   std  distance things begin    i       This a has the neat advantage that your code suddenly becomes container agnostic   And regarding your problem  if some library you use requires you to use int where an unsigned int would better fit  their API is messy  Anyway  if you are sure that those int are always positive  you may just do   int int distance   static cast lt int gt  distance     Which will specify clearly your intent to the compiler  it won t bug you with warnings anymore

User · Answer

If you can t won t use iterators and if you can t won t use std  size t for the loop index  make a  size   to int conversion function that documents the assumption and does the conversion explicitly to silence the compiler warning    include  lt cassert gt   include  lt cstddef gt   include  lt limits gt      When using int loop indexes  use size as int container  instead of    container size   in order to document the inherent assumption that the size    of the container can be represented by an int  template  lt typename ContainerType gt     constexpr    int size as int const ContainerType  amp c        const auto size   c size        if no auto  use  typename ContainerType  size type      assert size  lt   static cast lt std  size t gt  std  numeric limits lt int gt   max          return static cast lt int gt  size       Then you write your loops like this   for  int i   0  i  lt  size as int things     i            The instantiation of this function template will almost certainly be inlined   In debug builds  the assumption will be checked  In release builds  it won t be and the code will be as fast as if you called size   directly   Neither version will produce a compiler warning  and it s only a slight modification to the idiomatic loop   If you want to catch assumption failures in the release version as well  you can replace the assertion with an if statement that throws something like std  out of range  container size exceeds range of int     Note that this solves both the signed unsigned comparison as well as the potential sizeof int     sizeof Container  size type  problem   You can leave all your warnings enabled and use them to catch real bugs in other parts of your code

User · Answer

It s all in your things size   type  It isn t int  but size t  it exists in C    not in C  which equals to some  usual  unsigned type  i e  unsigned int for x86 32   Operator  less    lt   cannot be applied to two operands of different sign  There s just no such opcodes  and standard doesn t specify  whether compiler can make implicit sign conversion  So it just treats signed number as unsigned and emits that warning   It would be correct to write it like  for  size t i   0  i  lt  things size      i             or even faster  for  size t i   0  ilen   things size    i  lt  ilen    i

[c++] How do I deal with "signed/unsigned mismatch" warnings (C4018)?

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