How to initialize object in constructor? - 未名空间MITBBS历史存档

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How to initialize object in constructor?

How to initialize object in constructor?# Programming - 葵花宝典

r*r2005-08-21 07:08

1 楼

One rule for constructor:use initialization instead of assignment. Is this
also true for object?For example,I have a simple class
Point{
public:
...
private:
float x;
float y;
}
Suppose I have another class and want to use a point as a private member,
PointColl{
public:
//default constructor
PointColl();
PointColl(const PointColl & thePoint);
...
private:
int number;
Point* thePointer;
}
For the default constructor, can I do like this
PointColl::PointColl():number(0)
{
thePointer = n

c*t2005-08-21 07:08

2 楼

Use the one declaring the initiation parameters before { }

【在 r*****r 的大作中提到】

: One rule for constructor:use initialization instead of assignment. Is this
: also true for object?For example,I have a simple class
: Point{
: public:
: ...
: private:
: float x;
: float y;
: }
: Suppose I have another class and want to use a point as a private member,

y*i2005-08-21 07:08

3 楼

Yes, the rule applied to object.
Reason

r*r2005-08-21 07:08

4 楼

Thank you.And I understand the rule, just not sure about the syntax.for
example,in default constructor,
PointColl::PointColl():
number(0),
thePointer(NULL)
{
}
thePointer(NULL) is equal to " thePointer = new Point();"?I mean, this will
create a new pointer.
For another constructor,I can use directory
PointColl::PointColl(int num,const Point& p):
number(num)
thePointer(p);
{
}
instead of
PointColl::PointColl(int num,const Point& p):number(num)
{
thePointer = new Point(p);
}
Thanks, again

r*r2005-08-21 07:08

5 楼

I mean, I tried for both case in the constructor
PointColl::PointColl(int num,const Point& p):
number(num)
thePointer(p);
{
}
and get an error "cannot convert 'Point' to 'Point*' in initialization
but if i use
PointColl::PointColl(int num,const Point& p):number(num)
{
thePointer = new Point(p);
}
it works fine.
why?

.

【在 y****i 的大作中提到】

: Yes, the rule applied to object.
: Reason

y*i2005-08-21 07:08

6 楼

I mean, I tried for both case in the constructor
PointColl::PointColl(int num,const Point& p):
number(num)
thePointer(p);
{
}
and get an error "cannot convert 'Point' to 'Point*' in initialization
//---------------------------------
thePointer is a pointer(Point*) , however p is an object(Point)
You cannot assign a object to a pointe. If you really want to, you have to
cast it like thePoint = (Point*) p; // dangerous
Are you clear?
but if i use
PointColl::PointColl(int num,const Point& p):numbe

【在 r*****r 的大作中提到】

: I mean, I tried for both case in the constructor
: PointColl::PointColl(int num,const Point& p):
: number(num)
: thePointer(p);
: {
: }
: and get an error "cannot convert 'Point' to 'Point*' in initialization
: but if i use
: PointColl::PointColl(int num,const Point& p):number(num)
: {

r*r2005-08-21 07:08

7 楼

Yeah, very clear. So basically, for pointers, better use new to create instead
of initialization.Right?Thank you .

【在 y****i 的大作中提到】

:
: I mean, I tried for both case in the constructor
: PointColl::PointColl(int num,const Point& p):
: number(num)
: thePointer(p);
: {
: }
: and get an error "cannot convert 'Point' to 'Point*' in initialization
: //---------------------------------
: thePointer is a pointer(Point*) , however p is an object(Point)

y*i2005-08-21 07:08

8 楼

Yes.
For object, it is better to init in init list
for builtin types or pointer, you can use both ways.
However, let p = new A() int constructor is much clear that in the init-list.

for pointers, better use new to create
instead: of initialization.Right?Thank you .

【在 r*****r 的大作中提到】

: Yeah, very clear. So basically, for pointers, better use new to create instead
: of initialization.Right?Thank you .