No operator found while comparing structs in C

Question

Comparing two instances of the following struct  I receive an error   struct MyStruct1       MyStruct1 const MyStruct2  amp  my struct 2  const int  an int   -1            my struct 2  my struct 2           an int  an int              std  string toString   const       MyStruct2 my struct 2      int an int       The error is      error C2678  binary        no operator   found which takes a left-hand operand   of type  myproj  MyStruct1   or there   is no acceptable conversion    Why

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Because you did not write a comparison operator for your struct  The compiler does not generate it for you  so if you want comparison  you have to write it yourself

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Starting in C  20  it should be possible to add a full set of default comparison operators       lt    etc   to a class by declaring a default three-way comparison operator   spaceship  operator   like this   struct Point       int x      int y      auto operator lt   gt  const Point amp   const   default       With a compliant C  20 compiler  adding that line to MyStruct1 and MyStruct2 may be enough to allow equality comparisons  assuming the definition of MyStruct2 is compatible

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By default structs do not have a    operator  You ll have to write your own implementation   bool MyStruct1  operator   const MyStruct1  amp other  const               Compare the values  and return a bool result

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C  20 introduced default comparisons  aka the  spaceship  operator lt   gt   which allows you to request compiler-generated  lt   lt          gt    and or  gt  operators with the obvious naive    implementation     auto operator lt   gt  const MyClass amp   const   default       but you can customise that for more complicated situations  discussed below    See here for the language proposal  which contains justifications and discussion   This answer remains relevant for C  17 and earlier  and for insight in to when you should customise the implementation of operator lt   gt       It may seem a bit unhelpful of C   not to have already Standardised this earlier  but often structs classes have some data members to exclude from comparison  e g  counters  cached results  container capacity  last operation success error code  cursors   as well as decisions to make about myriad things including but not limited to    which fields to compare first  e g  comparing a particular int member might eliminate 99  of unequal objects very quickly  while a map lt string string gt  member might often have identical entries and be relatively expensive to compare - if the values are loaded at runtime  the programmer may have insights the compiler can t possibly in comparing strings  case sensitivity  equivalence of whitespace and separators  escaping conventions    precision when comparing floats doubles whether NaN floating point values should be considered equal comparing pointers or pointed-to-data  and if the latter  how to know how whether the pointers are to arrays and of how many objects bytes needing comparison  whether order matters when comparing unsorted containers  e g  vector  list   and if so whether it s ok to sort them in-place before comparing vs  using extra memory to sort temporaries each time a comparison is done how many array elements currently hold valid values that should be compared  is there a size somewhere or a sentinel   which member of a union to compare normalisation  for example  date types may allow out-of-range day-of-month or month-of-year  or a rational fraction object may have 6 8ths while another has 3 4ers  which for performance reasons they correct lazily with a separate normalisation step  you may have to decide whether to trigger a normalisation before comparison what to do when weak pointers aren t valid how to handle members and bases that don t implement operator   themselves  but might have compare   or operator lt  or str   or getters     what locks must be taken while reading comparing data that other threads may want to update   So  it s kind of nice to have an error until you ve explicitly thought about what comparison should mean for your specific structure  rather than letting it compile but not give you a meaningful result at run-time   All that said  it d be good if C   let you say bool operator     const   default  when you d decided a  naive  member-by-member    test was ok   Same for      Given multiple members bases   default   lt    lt     gt   and  gt   implementations seem hopeless though - cascading on the basis of order of declaration s possible but very unlikely to be what s wanted  given conflicting imperatives for member ordering  bases being necessarily before members  grouping by accessibility  construction destruction before dependent use    To be more widely useful  C   would need a new data member base annotation system to guide choices - that would be a great thing to have in the Standard though  ideally coupled with AST-based user-defined code generation    I expect it ll happen one day   Typical implementation of equality operators  A plausible implementation  It s likely that a reasonable and efficient implementation would be   inline bool operator   const MyStruct1 amp  lhs  const MyStruct1 amp  rhs        return lhs my struct2    rhs my struct2  amp  amp             lhs an int        rhs an int      Note that this needs an operator   for MyStruct2 too   Implications of this implementation  and alternatives  are discussed under the heading Discussion of specifics of your MyStruct1 below   A consistent approach to      lt    gt   lt   etc  It s easy to leverage std  tuple s comparison operators to compare your own class instances - just use std  tie to create tuples of references to fields in the desired order of comparison   Generalising my example from here   inline bool operator   const MyStruct1 amp  lhs  const MyStruct1 amp  rhs        return std  tie lhs my struct2  lhs an int                std  tie rhs my struct2  rhs an int      inline bool operator lt  const MyStruct1 amp  lhs  const MyStruct1 amp  rhs        return std  tie lhs my struct2  lhs an int   lt             std  tie rhs my struct2  rhs an int            etc      When you  own   i e  can edit  a factor with corporate and 3rd party libs  the class you want to compare  and especially with C  14 s preparedness to deduce function return type from the return statement  it s often nicer to add a  tie  member function to the class you want to be able to compare   auto tie   const   return std  tie my struct1  an int       Then the comparisons above simplify to   inline bool operator   const MyStruct1 amp  lhs  const MyStruct1 amp  rhs        return lhs tie      rhs tie        If you want a fuller set of comparison operators  I suggest boost operators  search for less than comparable    If it s unsuitable for some reason  you may or may not like the idea of support macros  online     define TIED OP STRUCT  OP  GET FIELDS        inline bool operator OP const STRUCT amp  lhs  const STRUCT amp  rhs                    return std  tie GET FIELDS lhs   OP std  tie GET FIELDS rhs              define TIED COMPARISONS STRUCT  GET FIELDS        TIED OP STRUCT      GET FIELDS        TIED OP STRUCT      GET FIELDS        TIED OP STRUCT   lt   GET FIELDS        TIED OP STRUCT   lt    GET FIELDS        TIED OP STRUCT   gt    GET FIELDS        TIED OP STRUCT   gt   GET FIELDS       that can then be used a la      define MY STRUCT FIELDS X  X my struct2  X an int TIED COMPARISONS MyStruct1  MY STRUCT FIELDS     C  14 member-tie version here   Discussion of specifics of your MyStruct1  There are implications to the choice to provide a free-standing versus member operator         Freestanding implementation  You have an interesting decision to make   As your class can be implicitly constructed from a MyStruct2  a free-standing   non-member bool operator   const MyStruct2 amp  lhs  const MyStruct2 amp  rhs  function would support     my MyStruct2    my MyStruct1      by first creating a temporary MyStruct1 from my myStruct2  then doing the comparison   This would definitely leave MyStruct1  an int set to the constructor s default parameter value of -1   Depending on whether you include an int comparison in the implementation of your operator    a MyStruct1 might or might not compare equal to a MyStruct2 that itself compares equal to the MyStruct1 s my struct 2 member   Further  creating a temporary MyStruct1 can be a very inefficient operation  as it involves copying the existing my struct2 member to a temporary  only to throw it away after the comparison    Of course  you could prevent this implicit construction of MyStruct1s for comparison by making that constructor explicit or removing the default value for an int    Member implementation  If you want to avoid implicit construction of a MyStruct1 from a MyStruct2  make the comparison operator a member function   struct MyStruct1               bool operator   const MyStruct1 amp  rhs  const               return tie      rhs tie       or another approach as above            Note the const keyword - only needed for the member implementation - advises the compiler that comparing objects doesn t modify them  so can be allowed on const objects   Comparing the visible representations  Sometimes the easiest way to get the kind of comparison you want can be         return lhs to string      rhs to string         which is often very expensive too - those strings painfully created just to be thrown away   For types with floating point values  comparing visible representations means the number of displayed digits determines the tolerance within which nearly-equal values are treated as equal during comparison

User · Answer

In C    structs do not have a comparison operator generated by default  You need to write your own   bool operator   const MyStruct1 amp  lhs  const MyStruct1 amp  rhs        return    your comparison code goes here

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Comparison doesn t work on structs in C or C    Compare by fields instead

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You need to explicitly define operator    for MyStruct1   struct MyStruct1     bool operator     const MyStruct1  amp rhs  const        your logic for comparision between   this  and  rhs            Now the    comparison is legal for 2 such objects

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Out of the box  the    operator only works for primitives  To get your code to work  you need to overload the    operator for your struct

[c++] No == operator found while comparing structs in C++

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