Initialization of C++ Struct and Class Instances

I. If all data members of a class or struct are public, initialization can be performed using the brace-enclosed form as follows.

Note:

① Regardless of the number of values, curly braces must be used for delimitation.

② Data members for which no value is specified will be automatically initialized to their default values by the compiler.

③ This method of object initialization requires all data members to be public.

④ This method of object initialization requires that no constructors be written in the class.

struct S {  // class S has the same effect
    int           x;
    unsigned short y;
};

S testS1={100,123};
S testS2={200};// Data members for which no value is specified are initialized to default values; here, os2.y=0;
S TestS[4]={ {100,10},
             {200,20},
             {300} };// Unspecified values are initialized to default values, e.g., os[2].y, os[3].x, os[3].y.

II. If data members are private or protected, or if constructors are provided, initialization must be performed using constructors.

struct S {  // You can experiment with class S; the result will be the same
    private:
        int x;
    public:
        double y;
        S(void){}
        S(int idemo,double ddemo) {x=idemo;y=ddemo;}
        void show(void) {cout<<x<<'\t'<<y<<endl;}
};

S os1;// The default constructor (no-argument constructor) will be called.
S os2(1000,2.345);
S os3=S(2000,4.567);
S os[4]={S(10,1.234),S(20,2.234)};// Uninitialized elements will call the default constructor. If no default constructor exists at this point, an error will occur.

Important Notes:

① In the statement S os3=S(2000,4.567);, because os3 is declared and initialized, the S(int,double) constructor will be called to initialize os3.

② S os3(2000,4.567); is equivalent to S os3=S(2000,4.567);

③ However, if os3 already exists, e.g., S os3(100,1.234); os3=S(2000,4.567);, this means a temporary object is assigned to os3, which will call operator=. The system then releases this temporary object. The system's default = operation copies the values of the source object's data members to the target object's data members.

III. Constructors that accept a single argument allow objects to be initialized using assignment syntax.

The illustrative code is as follows:

#include <iostream>

using namespace std;

class C {
    private:
        int x;
    public:
        C(int idemo) {x=idemo;}
        void show(void) {cout<<x<<endl;}
};

struct S {
    private:
        int x;
    public:
        S(int idemo) {x=idemo;}
        void show(void) {cout<<x<<endl;}
};

int main(int argc, char *argv[]){
    C oc=1000;// Do not attempt to add curly braces here.
    oc.show();
    S os=2000;// Do not attempt to add curly braces here.
    os.show();
    return EXIT_SUCCESS;
}