Proper way to initialize C structs

Question

Our code involves a POD  Plain Old Datastructure  struct  it is a basic c   struct that has other structs and POD variables in it that needs to get initialized in the beginning     Based one what I ve read  it seems that   myStruct    MyStruct  calloc 1  sizeof MyStruct      should initialize all the values to zero  as does   myStruct   new MyStruct      However  when the struct is initialized in the second way  Valgrind later complains  conditional jump or move depends on uninitialised value s   when those variables are used  Is my understanding flawed here  or is Valgrind throwing false positives

User · Answer

I write some test code    include  lt string gt   include  lt iostream gt   include  lt stdio h gt   using namespace std   struct sc       int x      string y      int  z      int main int argc  char   argv       int  r   new int 128      for int i   0  i  lt  128  i               r i    i 32          cout  lt  lt  r 100   lt  lt  endl     delete r      sc  a   new sc     sc  aa   new sc 2      sc  b   new sc       sc  ba   new sc 2         cout  lt  lt   az    lt  lt  a- gt z  lt  lt  endl     cout  lt  lt   bz    lt  lt  b- gt z  lt  lt  endl     cout  lt  lt   a    lt  lt  a- gt x  lt  lt    y   lt  lt  a- gt y  lt  lt   end   lt  lt  endl     cout  lt  lt   b    lt  lt  b- gt x  lt  lt    y   lt  lt  b- gt y  lt  lt    end   lt  lt endl     cout  lt  lt   aa    lt  lt  aa- gt x  lt  lt    y   lt  lt  aa- gt y  lt  lt    end   lt  lt endl     cout  lt  lt   ba    lt  lt  ba- gt x  lt  lt    y   lt  lt  ba- gt y  lt  lt    end   lt  lt endl      g   compile and run     a out  132 az 0x2b0000002a bz 0 a 854191480 yend b 0 yend aa 854190968 yend ba 0 yend

User · Answer

You need to initialize whatever members you have in your struct  e g    struct MyStruct     private      int someInt       float someFloat      public      MyStruct    someInt  0   someFloat  1 0        Initializer list will set appropriate values

User · Answer

Since it s a POD struct  you could always memset it to 0 - this might be the easiest way to get the fields initialized  assuming that is appropriate

User · Answer

From what you ve told us it does appear to be a false positive in valgrind  The new syntax with    should value-initialize the object  assuming it is POD   Is it possible that some subpart of your struct isn t actually POD and that s preventing the expected initialization  Are you able to simplify your code into a postable example that still flags the valgrind error   Alternately perhaps your compiler doesn t actually value-initialize POD structures   In any case probably the simplest solution is to write constructor s  as needed for the struct subparts

User · Answer

That seems to me the easiest way  Structure members can be initialized using curly braces           For example  following is a valid initialization   struct Point        int x  y         int main             A valid initialization  member x gets value 0 and y        gets value 1   The order of declaration is followed      struct Point p1    0  1         There is good information about structs in c   - https   www geeksforgeeks org structures-in-cpp

User · Answer

In C   classes structs are identical  in terms of initialization    A non POD struct may as well have a constructor so it can initialize members  If your struct is a POD then you can use an initializer   struct C       int x       int y      C  c    0      Zero initialize POD   Alternatively you can use the default constructor   C  c   C            Zero initialize using default constructor C  c                Latest versions accept this syntax  C  c   new C        Zero initialize a dynamically allocated object      Note the difference between the above and the initialize version of the constructor     Note  All above comments apply to POD structures  C  c                members are random C  c   new C        members are random  more officially undefined     I believe valgrind is complaining because that is how C   used to work   I am not exactly sure when C   was upgraded with the zero initialization default construction   Your best bet is to add a constructor that initializes the object  structs are allowed constructors    As a side note  A lot of beginners try to value init   C c       Unfortunately this is not a variable declaration  C c       This syntax was added to overcome this confusion      The correct way to do this is  C c   C      A quick search for the  Most Vexing Parse  will provide a better explanation than I can

[c++] Proper way to initialize C++ structs

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