Let''s work an instance. Imagine you need your constructor Foo::Foo(char) to call another constructor of the similar class, namely Foo::Foo(char,int), in order that Foo::Foo(char,int) would help initialize this object. Unluckily there''s no way to do this in C++.
Some of people do it anyway. Unluckily it doesn''t do what they want. For instance, the line Foo(x, 0); does not call Foo::Foo(char,int) on the this object. Rather then it calls Foo::Foo(char,int) to initialize temporary, local object (not this), then it instantly destructs that temporary while control flows over the ;.
class Foo { public: Foo(char x);
Foo(char x, int y);
...
};
Foo::Foo(char x)
{
...
Foo(x, 0); // this line does NOT help initialize the this object!!
...
}
You can combine sometimes two constructors through a default parameter:
class Foo {
public:
Foo(char x, int y=0); // this line combines the two constructors
...
};
If that doesn''t work, for example if there isn''t an suitable default parameter which combines the two constructors, every so often you can share their common code in a private init() member function:
class Foo { public: Foo(char x);
Foo(char x, int y);
... private:
void init(char x, int y);
};
Foo::Foo(char x)
{
init(x, int(x) + 7);
...
}
Foo::Foo(char x, int y)
{
init(x, y);
...
}
void Foo::init(char x, int y)
{
...
}
BTW do NOT attempt to get this via placement new. Some of the people think they can say new(this) Foo(x, int(x)+7) in the body of Foo::Foo(char). Constructors do a bunch of little magical things behind the scenes, but that bad technique steps on those partly constructed bits.