A class object with a constructor must be explicitly initialized or have a
default constructor. Except for aggregate initialization, explicit
initialization using a constructor is the only way to initialize nonstatic
constant and reference class members.
A class object that has no constructors, no virtual functions, no private or protected members, and no base classes is called an aggregate. Examples of aggregates are C-style structures and unions.
You explicitly initialize a class object when you create that object. There are two ways to initialize a class object:
The syntax for an initializer that explicitly initializes a class object with a constructor is:
>>-+-(--expression--)-------------------+---------------------->< '-=--+-expression------------------+-' | .-,----------. | | V | | '-{----expression-+--+---+--}-' '-,-'
The following example shows the declaration and use of several constructors that explicitly initialize class objects:
// This example illustrates explicit initialization // by constructor. #include <iostream> using namespace std; class complx { double re, im; public: // default constructor complx() : re(0), im(0) { } // copy constructor complx(const complx& c) { re = c.re; im = c.im; } // constructor with default trailing argument complx( double r, double i = 0.0) { re = r; im = i; } void display() { cout << "re = "<< re << " im = " << im << endl; } }; int main() { // initialize with complx(double, double) complx one(1); // initialize with a copy of one // using complx::complx(const complx&) complx two = one; // construct complx(3,4) // directly into three complx three = complx(3,4); // initialize with default constructor complx four; // complx(double, double) and construct // directly into five complx five = 5; one.display(); two.display(); three.display(); four.display(); five.display(); }
The above example produces the following output:
re = 1 im = 0 re = 1 im = 0 re = 3 im = 4 re = 0 im = 0 re = 5 im = 0
Related References