Assign one struct to another in C

Question

Can you assign one instance of a struct to another  like so   struct Test t1  struct Test t2  t2   t1    I have seen it work for simple structures  bu does it work for complex structures  How does the compiler know how to copy data items depending on their type  i e  differentiating between an int and string

User · Answer

Yes  assignment is supported for structs  However  there are problems   struct S      char   p      struct S s1  s2  s1 p   malloc 100   s2   s1    Now the pointers of both structs point to the same block of memory - the compiler does not copy the pointed to data  It is now difficult to know which struct instance owns the data  This is why C   invented the concept of user-definable assignment operators - you can write specific code to handle this case

User · Answer

First Look at this example    The C code for a simple C program is given below  struct Foo       char a      int b      double c        foo1 foo2   void foo assign void        foo1   foo2    int main   char  argv   int argc          foo assign    return 0      The Equivalent ASM Code for foo assign   is   00401050  lt  foo assign gt     401050    55                      push    ebp   401051    89 e5                   mov     esp  ebp   401053    a1 20 20 40 00          mov    0x402020  eax   401058    a3 30 20 40 00          mov     eax 0x402030   40105d    a1 24 20 40 00          mov    0x402024  eax   401062    a3 34 20 40 00          mov     eax 0x402034   401067    a1 28 20 40 00          mov    0x402028  eax   40106c    a3 38 20 40 00          mov     eax 0x402038   401071    a1 2c 20 40 00          mov    0x40202c  eax   401076    a3 3c 20 40 00          mov     eax 0x40203c   40107b    5d                      pop     ebp   40107c    c3                      ret       As you can see that a assignment is simply replaced by a  mov  instruction in assembly  the assignment operator simply means  moving data from one memory location to another memory location   The assignment will only do it for immediate members of a structures and will fail to copy when you have Complex datatypes in a structure  Here COMPLEX means that you cant have array of pointers  pointing to lists     An array of characters within a structure will itself not work on most compilers  this is because assignment will simply try to copy without even looking at the datatype to be of complex type

User · Answer

Yes if the structure is of the same type  Think it as a memory copy

User · Answer

Yes  you can assign one instance of a struct to another using a simple assignment statement   In the case of non-pointer or non pointer containing struct members  assignment means copy   In the case of pointer struct members  assignment means pointer will point to the same address of the other pointer    Let us see this first hand   include  lt stdio h gt   struct Test      int foo      char  bar      int main        struct Test t1      struct Test t2      t1 foo   1      t1 bar   malloc 100   sizeof char        strcpy t1 bar   quot t1 bar value quot        t2 foo   2      t2 bar   malloc 100   sizeof char        strcpy t2 bar   quot t2 bar value quot        printf  quot t2 foo and bar before copy   d  s n quot   t2 foo  t2 bar       t2   t1     lt ---- ASSIGNMENT     printf  quot t2 foo and bar after copy   d  s n quot   t2 foo  t2 bar         The following 3 lines of code demonstrate that foo is deep copied and bar is shallow copied     strcpy t1 bar   quot t1 bar value changed quot        t1 foo   3      printf  quot t2 foo and bar after t1 is altered   d  s n quot   t2 foo  t2 bar       return 0

User · Answer

Did you mean  Complex  as in complex number with real and imaginary parts   This seems unlikely  so if not you d have to give an example since  complex  means nothing specific in terms of the C language   You will get a direct memory copy of the structure  whether that is what you want depends on the structure   For example if the structure contains a pointer  both copies will point to the same data   This may or may not be what you want  that is down to your program design   To perform a  smart  copy  or a  deep  copy   you will need to implement a function to perform the copy   This can be very difficult to achieve if the structure itself contains pointers and structures that also contain pointers  and perhaps pointers to such structures  perhaps that s what you mean by  complex    and it is hard to maintain   The simple solution is to use C   and implement copy constructors and assignment operators for each structure or class  then each one becomes responsible for its own copy semantics  you can use assignment syntax  and it is more easily maintained

User · Answer

This is a simple copy  just like you would do with memcpy    indeed  some compilers actually produce a call to memcpy   for that code   There is no  string  in C  only pointers to a bunch a chars  If your source structure contains such a pointer  then the pointer gets copied  not the chars themselves

[c] Assign one struct to another in C

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