Question

I have a set of code, which mimics a basic library cataloging system. There is a base class named items, in which the the general id,title and year variables are defined and 3 other derived classes (DVD,Book and CD).

Base [Items]

Derived [DVD,Book,CD].

The programs runs, however I get the following warnings, I’m not sure how to fix these.

>"C:\Program Files\gcc\bin/g++"  -Os -mconsole -g -Wall -Wshadow -fno-common mainA4.cpp -o mainA4.exe
In file included from mainA4.cpp:5:
a4.h: In constructor `DVD::DVD(int, std::string, int, std::string)':
a4.h:28: warning: `DVD::director' will be initialized after
a4.h:32: warning:   base `Items'
a4.h:32: warning:   when initialized here
a4.h: In constructor `Book::Book(int, std::string, int, std::string, int)':
a4.h:48: warning: `Book::numPages' will be initialized after
a4.h:52: warning:   base `Items'
a4.h:52: warning:   when initialized here
a4.h: In constructor `CD::CD(int, std::string, int, std::string, int)':
a4.h:66: warning: `CD::numSongs' will be initialized after
a4.h:70: warning:   base `Items'
a4.h:70: warning:   when initialized here
>Exit code: 0

Answer 1

When you declare member variables in a class, they’re initialized in the order you declare them. But you may write them in any order in your constructor’s initializer list. For example,

struct foo {
   int a;
   int b;

   foo(): b(5), a(3) {}
};

will construct a and then b, even though it appears that you’re initializing them in the other order.

The compiler issues a warning because you can trick yourself into writing incorrect code. For example,

struct foo {
    int a;
    int b;

    foo(): b(5), a(b) {}
};

the value of a will be undefined.