Why can't you initialize data members in a structure in c or c plus plus?

Answer 1

You can initialise a structure's data members in both C and C++.

In C, you initialise the structure at the point of instantiation.

typedef struct foo

{

int m_data1;

double m_data2;

} foo;

int main()

{

foo f={.m_data1=42, .m_data2=1.99};

return 0;

}

You can also assign members separately from instantiation, but this is considered bad coding practice as all the members will be initialised with garbage (whatever happens to reside in memory at the point of instantiation). This raises the potential for accessing that garbage data before it is assigned.

foo f;

/*

any attempt to access f's members at this point

will result in undefined behaviour

*/

f = (foo) {.m_data1=42, .m_data2=1.99};

/* f is now initialised */

In C++, a struct is exactly the same as a class except it has public access by default, rather than private access by default. As such, you can use the class constructor initialisation list to initialise the data members to default values:

struct foo

{

foo():m_data1(42),m_data2(1.99){}

private:

int m_data1;

double m_data2;

};

Typically, you will also provide an overloaded constructor to initialise the members to more specific values:

foo(const int data1, const double data2): m_data1(data1), m_data2(data2){}

However, you can also combine the default constructor and the overloaded constructor to produce an overloaded default constructor:

foo(const int data1=42, const double data2=1.99): m_data1(data1), m_data2(data2){}

You can also implement a copy constructor with an initialisation list:

foo(const foo& f):m_data1(f.m_data1),m_data2(f.m_data2){}

Note that initialisation lists are more efficient than using the constructor body to perform initialisation. Therefore you should always perform as much initialisation as possible in the initialisation list and only use the constructor body for any additional initialisation processes that cannot be handled by the initialisation list. Ideally, members should be initialised in the same order they were declared.

Note also that the initialisation list employs construction syntax rather than assignment syntax, even for primitive data types. If the class or structure is a derived class, you can also invoke specific base class constructors through the initialisation list. Again, these base classes should be listed in the same order specified by the inheritance list, and should be listed BEFORE the data members since the base classes must be constructed first.