C++ for Scientists - Technische Universität Dresden

3.3. CONSTRUCTORS 69
class complex
{
public:
complex(double rnew, double inew)
{
r= rnew; i= inew;
}
// ...
};
Thus, a constructor is a member function with the same name as the class itself. It can have
an arbitrary number of arguments. In our case, two arguments are most suitable because we
want to set two member variables. This constructor allows us to set c1’s values directly in the
definition:
complex c1(2.0, 3.0);
There is a special syntax for setting member variables in constructors
class complex
{
public:
complex(double rnew, double inew) : r(rnew), i(inew) {}
// ...
};
This not only shorter but has also another advantage. It calls the constructors of the variables in
class’s constructor. For plain old data types (POD) this does not make a significant difference.
The situation is another one if the members are themselves classes.
Imagine you have a class that solves linear systems with the same matrix and you store the
matrix in your class
class solver
{
public:
solver(int nrows, int ncols) // : A() #1 → error
{
A(nrows, ncols); // this is not a constructor here #2 → error
}
// ...
private:
matrix type A;
};
Suppose our matrix class has a constructor setting the dimensions. This constructor cannot
be called in the function body of the constructor (#2). The call in #2 is interpreted as
A.operator()(nrows, ncols), see § 4.8.
All member variables of the class are constructed before the class constructor reaches the opening
{. Those members — like A — that do not appear in the list after the colon are built by a constructor
without arguments, called the default constructor. Correspondingly, classes that have
such a constructor are called default-constructible. Our matrix class is not default-constructible
and the compiler will tell us something like “Operator matrix type::matrix type() not
found”. Thus, we need