Correct way of initializing a struct in a class constructor

Question

So I want to add a struct from a c header file as a class member to a c++ class. But I get a compiler error for the cpp file: bar was not declared inn thi

Answer 1

What you are doing there is assignment, not initialization. Initialization happens in the initialization list of a constructor, before the constructor body, or in C++11 in an initializer right after the member variable declaration:

myClass.hpp, general case:

/** you might want to do this if you are linking 
 * against the C lib or object file of that header:
 */
extern "C" { 
  #include fileWithStruct.h
}

class myClass
{
public:
  foo bar; //no need for "struct" in C++ here
};

C++11:

myClass.cpp

#include "myClass.hpp"

//Initialize structure in Constrcutor
myClass::myClass(  )
  : bar{1, 0, "someString", 0x4}
{}

Antoher option is to provide the initial value of foo with an brace-or-equal-initializer at the member variable declaration:

myClass.hpp

extern "C" { 
  #include fileWithStruct.h
}

class myClass
{
public:
  foo bar{1, 0, "someString", 0x4};
};

In this case, you need not define a constructor, since it's generated implicitly by the compiler (if needed), correctly initializing bar.

C++03:

Here aggregate initialization in init lists is not available, so you have to use workarounds, e.g.:

myClass.cpp

#include "myClass.hpp"

//Initialize structure in Constrcutor
myClass::myClass(  )
  : bar() //initialization with 0
{
  const static foo barInit = {1, 0, "someString", 0x4}; //assignment
  bar = barInit;
}

Or:

#include "myClass.hpp"
namespace {
  foo const& initFoo() {
    const static foo f = {1, 0, "someString", 0x4};
    return f;
  }
}

//Initialize structure in Constrcutor
myClass::myClass(  )
  : bar(initFoo()) //initialization
{ }

Answer 2

You need to specify that the foo struct should have "C-linkage". The following is a complete example.

// fileWithStruct.h
#ifdef __cplusplus
extern "C" { // Declare as extern "C" if used from C++
#endif

typedef struct _foo
{
  int a;
  int b;
  const char* c;
  int d;
} foo;


#ifdef __cplusplus
}
#endif

The myClass header file:

// myClass.hpp
#include "fileWithStruct.h"

class myClass
{
public:
  myClass();

  foo bar;
};

The C+11 implementation of myClass which uses extended initializer lists:

// myClass.cpp
#include "myClass.hpp"

myClass::myClass(  )
  : bar({1, 0, "someString", 0x4})
{
}

... and the C++03 version if you haven't moved to C++11 yet:

#include "myClass.hpp"

myClass::myClass(  )
  : bar()
{
  bar.a = 1;
  bar.b = 0;
  bar.c = "someString";
  bar.d = 0x4;
}

Answer 3

C++03 Style

#include "fileWithStruct.h"
/* say the contents were
struct foo
{
   int foo1;
   float foo2;
};
*/

class myClass
{
    public:
        int val;
        foo bar;
        // since foo is a POD-struct (a.k.a C struct), no constructor would be present
        // however bar() will zero-initialize everything in the struct
        myClass() : val(), bar()
        {
        }
};

The parentheses following bar matters. Refer value and zero-initialization to understand why this works. It is to be noted that by adding a constructor to myClass, we've made it a non-POD type. To work around this, one can retain myClass as an aggregate and write:

class myClass
{
    public:
        int val;
        foo bar;
};

int main()
{
   myClass zeroed_obj = { };
   myClass inited_obj = { 2, {0, 1.0f} };
   myClass partially_inited_obj = { 2 };    // equivalent to {2, {}}; which would zero all of myClass::bar
   myClass garbage_obj;    // warning: when left uninitialized, every member without a constructor will end up with garbage value
}

C++11 Style

class myClass
{
public:
   // default member initializations
   int val = { };         // zero-initialization
   foo bar = { 0, 0.0f }; // aggregate-initializing foo here, just giving { } will zero all of myClass::bar

   // should you want to receive an element from the constructor, this can be done too
   // aggregate initializing a struct in constructor initialization list is allowed from C++11 onwards
   // in C++03, we would've resorted to just setting the member of bar inside the constructor body
   myClass(int _foo1) : val{}, bar{_foo1, 0.f}
   {
   }

   // since we've a non-default constructor, we've to re-introduce the default constructor
   // if we need the above in-class initialization to work
   myClass() = default;
};

Here we use C++11's uniform initialization syntax. However, by doing this myClass becomes a non-POD type; member initialization is akin to adding a constructor to the class, thereby rendering myClass a non-trivial but standard-layout class. As per C++11 for a class to be POD it should be both trivial and standard-layout. Instead doing

#include "fileWithStruct.h"
#include <type_traits>
#include <iostream>

class myClass
{
public:
   int val;
   foo bar;
};

int main()
{
    myClass obj { }; // initializes val, bar.foo1 and bar.foo2 to 0
    myClass m { 0, {1, 2.0f} }; // initilizes each member separately
    std::cout << std::is_pod<myClass>::value << std::endl; // will return 1
}

will retain myClass as a POD.

Refer to this excellent post to know more about aggregates and PODs.

Answer 4

Let us consider an example. Consider a Linked List in which each node is represented by:

struct ListNode {
     int val;
     ListNode *next;
     ListNode(int x){
        val = x;
        next = NULL;
     }
};

To initialize a node with value 5 and next pointer pointing to NULL you can write the code snippet as:

ListNode node = new ListNode();
node.val = 5;
node.next = NULL;

or

ListNode node = new ListNode(5);

Another fancy declaration can be made as

struct ListNode {
     int val;
     ListNode *next;
     ListNode(int x) : val(x), next(NULL) {}
};

Here ListNode(int x): val(x), next(NULL) is a constructor which initializes the value of the struct ListNode.

Hope this make things more clear and easy. :)

Answer 5

Initialization should be done this way (C++11):

myClass::myClass(  )
: bar{1, 0, "someString", 0x4}
{

}

Also, do not forget to declare your constructor in your class definition.