Constructor Defaults
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The default constructor for class X is one that takes no arguments; it usually has the form X::X(). If no user-defined constructors exist for a class, the compiler generates a default constructor. On a declaration such as X x, the default constructor creates the object x.
Like all functions, constructors can have default arguments. For example, the constructor
X::X(int, int = 0)
can take one or two arguments. When presented with one argument, the missing second argument is assumed to be a zero int. Similarly, the constructor
X::X(int = 5, int = 6)
could take two, one, or no arguments, with appropriate defaults. However, the default constructor X::X() takes no arguments and must not be confused with, say, X::X(int = 0), which can be called with no arguments as a default constructor, or can take an argument.
You should avoid ambiguity in defining constructors. In the following case, the two default constructors are ambiguous:
class X
{
public:
X();
X(int i = 0);
};
int main()
{
X one(10); // OK; uses X::X(int)
X two; // Error;ambiguous whether to call X::X() or
// X::X(int = 0)
return 0;
}