[C++] Class (part 2)

Members that are const or reference must be initialized. Similary, members that are of a class type that does not define a default constructor also must be initialized.

class ConstRef{

public:

    ConstRef(int ii);

private:

    int i;

    const int ci;

    int & ri;

};

ConstRef::ConstRef(int ii){

    i = ii;    // ok

    ci = ii;    // error: cannot assign to a const

    ri = i;     // error: ri was never initialized

}

By the time the constructor begin executing, initialization is completed. The correct way to wirtie this constructor is:

// explictly initialize reference and const members

ConstRef::ConstRef(int ii): i(ii), ci(ii), ri(i) { }

Members are initiazlied in the order in which they appear in the class definition, not the order in which they appear in the constructor initializer.

class X{

    int i;

    int j;

public:

    // undefined: i is initialized before j

    X(int val): j(val), i(j) { }

}

To avoid using members to initiazlie other members.

    X(int val): j(val), i(val) { }

class Sales_data{

public:

    // non-delegating constructor initialize member before corresponding argument

    Sales_data(string s, unsigned cnt, double price): bookNo(s), units_sold(cnt), revenue(cnt * price) { }

    // remaining constructors all delegate to another constructor

    Sales_data(): Sales_data('', , ) { }

    Sales_data(string s): Sales_data(s, , ){ }

    Sales_data(istream & is): Sales_data() { read(is, *this); }

};

Had the function bodies contain code, that code would be run before control return to the function body of the delegating constructor.

In practice, it is almost right to provide a default constructor if other constructor is defined.

class NoDefault{

public:

    NoDefault( const string &);

    // additional members follow, but no other constructor

};

Struct A{

    NoDefault my_mem;

};

A a;    // error: cannot synthesize a constructor of A

Struct B{

    B() { }    // error: no initialization for b_member

    Default b_member;

};

It is common mistake among programmers new to C++ to try to declare an object initialized with the default constructor as function

    // opps! defines a function taking no parameter and returning an object of type Sales_data

    Sales_data obj();

    // define an object that use the default constructor

    Sales_data obj;

Reference:

C++ Primer, Fifth Edition, chapter 7 Classes

巴特西

[C++] Class (part 2)

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