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
{ }