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C++ In Action: Language

Initialization of Aggregates

Explicit initialization of classes and arrays.


Just as you can explicitly initialize an array of characters using a literal string

char string [] = "Literal String";
you can initialize other aggregate data structures--classes and arrays. An object of a given class can be explicitly initialized if and only if all its non-static data members are public and there is no base class, no virtual functions and no user-defined constructor. All public data members must be explicitly initializable as well (a little recursion here). For instance, if you have
class Initializable
{
public:
    // no constructor
    int          _val;
    char const * _string;
    Foo        * _pFoo;
};
you can define an instance of such class and initialize all its members at once
Foo foo;
Initializable init = { 1, "Literal String", &foo };

Since Initializable is initializable, you can use it as a data member of another initializable class.

class BigInitializable
{
public:
    Initializable   _init;
    double          _pi;
};

BigInitializable big = { { 1, "Literal String", &foo }, 3.14 };

As you see, you can nest initializations.

You can also explicitly initialize an array of objects. They may be of a simple or aggregate type. They may even be arrays of arrays of objects. Here are a few examples.

char string [] = { 'A', 'B', 'C', '\0' };
is equivalent to its shorthand
char string [] = "ABC";

Here's another example

Initializable init [2] = { 
        { 1, "Literal String", &foo1 }, 
        { 2, "Another String", &foo2 } };

We used this method in the initialization of our array of FuncitionEntry objects.

If objects in the array have single-argument constructors, you can specify these arguments in the initializer list. For instance,

CelestialBody solarSystem [] = { 0.33, 4.87, 5.98, 0.64, 1900, 569, 87, 
                                                             103, 0.66 };

where masses of planets are given in units of 1024kg.


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