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# Help a beginner

 P: n/a Hello experts, I am a complete beginner in C++ (although I know C). I am trying to compile the code below, and I get the following error. Can anyone explain to me my mistake? Thanks! PhilB myprog2.cpp: In method `Line::Line (Point, Point)': myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) //------------------------ class Point { protected: int x; int y; public: Point::Point(int, int); }; Point::Point(int initx, int inity) { x=initx; y=inity; return; } class Line:public Point { public: Point start_point; Point end_point; public: Line::Line(Point, Point); }; Line::Line(Point initp1, Point initp2) { start_point=initp1; end_point=initp2; return; } main() { Point p1(10,20); Point p2(20,40); Line l1(p1,p2); } //------------------------ Jul 22 '05 #1
15 Replies

 P: n/a > //------------------------ class Point { protected: int x; int y; public: Point::Point(int, int); }; not Point::Point(int, int); but just Point(int, int); same in Line class. Jul 22 '05 #2

 P: n/a "PhilB" wrote in message news:10**************************@posting.google.c om... Hello experts, I am a complete beginner in C++ (although I know C). I am trying to compile the code below, and I get the following error. Can anyone explain to me my mistake? Thanks! PhilB myprog2.cpp: In method `Line::Line (Point, Point)': myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) //------------------------ class Point { protected: int x; int y; public: Point::Point(int, int); }; Point::Point(int initx, int inity) { x=initx; y=inity; return; } class Line:public Point { public: Point start_point; Point end_point; public: Line::Line(Point, Point); }; Line::Line(Point initp1, Point initp2) { start_point=initp1; end_point=initp2; return; } main() { Point p1(10,20); Point p2(20,40); Line l1(p1,p2); } //------------------------ Hello, In the class as: class Point { protected: int x; int y; public: Point::Point(int, int); ^^^^remove the "Point::" }; Same for the other class. -- Elias Jul 22 '05 #3

 P: n/a PhilB wrote: Hello experts, I am a complete beginner in C++ (although I know C). I am trying to compile the code below, and I get the following error. Can anyone explain to me my mistake? Isn't it obvious from the error messages? Thanks! PhilB myprog2.cpp: In method `Line::Line (Point, Point)': myprog2.cpp:32: no matching function for call to `Point::Point ()' You try to use the default constructor (i.e. `Point::Point ()'), but your Point class doesn't have one. myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) //------------------------ class Point { protected: int x; int y; public: Point::Point(int, int); }; Point::Point(int initx, int inity) { x=initx; y=inity; return; } class Line:public Point { public: Point start_point; Point end_point; public: Line::Line(Point, Point); }; Line::Line(Point initp1, Point initp2) { start_point=initp1; end_point=initp2; return; } Since you have not specified an initializer for your Point objects, the default constructor will be used to create them, but you don't have one. Try this instead: Line::Line(Point initp1, Point initp2) : start_point(initp1), end_point(initp2) { } This will use the copy constructor (which is generated by the compiler automatically) to create your two Point members. main() { Point p1(10,20); Point p2(20,40); Line l1(p1,p2); } //------------------------ Jul 22 '05 #4

 P: n/a "PhilB" wrote in message news:10**************************@posting.google.c om... Hello experts, I am a complete beginner in C++ (although I know C). I am trying to compile the code below, and I get the following error. Can anyone explain to me my mistake? Thanks! PhilB myprog2.cpp: In method `Line::Line (Point, Point)': myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) //------------------------ class Point { protected: int x; int y; public: Point::Point(int, int); }; Point::Point(int initx, int inity) { x=initx; y=inity; return; } class Line:public Point { public: Point start_point; Point end_point; public: Line::Line(Point, Point); }; Line::Line(Point initp1, Point initp2) { start_point=initp1; end_point=initp2; return; } main() { Point p1(10,20); Point p2(20,40); Line l1(p1,p2); } //------------------------ Please note it one of my top ten that a number of you objected to having on the list. To poster: Your Point constructor is protected, which means it can only be called by code of the Point class or one of its subclasses (Line). You are calling it from main. When you create a Line object, C++ will automatically construct the Point inherited portion by calling a constructor for a Point using a default constructor. And there isn't one. In addition, I'd say your design is flawed: A Line is not a kind of Point so you should not be using an is-a design. Lines have Points which is a has-a design. This is one way it could look: #include using namespace std; class Linex; class Pointx { friend class Linex; int x, y; //coordinates public: Pointx (int x, int y):x(x), y(y){}; Pointx ( ){}; //This is the missing constructor }; class Linex { Pointx startPoint, endPoint; //points for line public: void show( ); Linex (Pointx start, Pointx end) { startPoint = start; endPoint = end; } }; void Linex::show( ) { cout << "Start=(" << startPoint.x << ", " << startPoint.y << ") : End=(" << endPoint.x << ", " << endPoint.y << ")" << endl; } int main( ) { Pointx point1(10,20); Pointx point2(20,30); Linex myLine(point1, point2); cout << "Line is "; myLine.show( ); cout << endl; return 0; } Note: I made the entire class Linex a friend because my compiler is objecting to making single function a friend. Don't know why. MingW32: Is this a known bug? -- Gary Jul 22 '05 #5

 P: n/a "PhilB" wrote in message news:10**************************@posting.google.c om... Hello experts, I am a complete beginner in C++ (although I know C). I am trying to compile the code below, and I get the following error. Can anyone explain to me my mistake? Thanks! PhilB myprog2.cpp: In method `Line::Line (Point, Point)': myprog2.cpp:32: no matching function for call to `Point::Point ()' The compiler is trying to tell you there is no default constructor for Point. myprog2.cpp:12: candidates are: Point::Point (int, int) The compiler says it only sees this constructor, to create a point from two ints. [snip] class Point { protected: int x; int y; Protected data is usually a mistake; it tends to introduce confusing dependencies between a base and its derived classes. Also, if you intend to use Point as a base class, you should declare a virtual destructor, (virtual ~Point()) or else derived objects may not be properly destroyed. public: Point::Point(int, int); (Note: the "Point::" is unnecessary inside the class definition, it is only needed when you define the function outside of the class definition.) This declares a constructor from two ints, which hides the compiler-generated default constructor. If you want the default constructor as well, you would need to define one: Point () {} This is not really necessary though, there's a better solution to your immediate problem. }; Point::Point(int initx, int inity) { x=initx; y=inity; return; } You don't need to explicitly return here. This function would usually be written (for reasons explained later): Point (int initx, int inity) : x (initx), y (inity) {} This uses initialization syntax to assign the values of x and y. Your syntax instead, will first create the two ints and then assign new values for them. This isn't too big a deal for ints, but see below. (Also, many C++ coders prefer to mark member data in some way to differentiate between member data and function arguments. Popular ways include m_x ("m_" for member) and x_.) class Line:public Point { public: Point start_point; Point end_point; As someone trying to break old C habits it will probably be better for you to only use private data until you learn the exceptions to this rule. Instead, make functions that act on Lines and Points members of those classes. public: Line::Line(Point, Point); }; Line::Line(Point initp1, Point initp2) { start_point=initp1; end_point=initp2; return; } OK, here's the compiler's problem; not just a style one :). This tells the compiler to first default construct two points, and then assign the new point values to them. However, the Point's constructor has hidden the default constructor, so this won't work. Instead, you should write: Line (Point initp1, Point initp2) : start_point (initp1), end_point (initp2) {} This will use the compiler-generated Point copy constructor (which isn't hidden by the user-declared constructor), instead. In general it is good practice to use initialization syntax where possible, since 1) The compiler may be able to optimize it better. 2) It's a familiar idiom to C++ coders. 3) It removes the necessity of defining default constructors for objects. As you get more used to OO programming, you will find that default objects do not make sense for all kinds of classes, and often require special-purpose code to handle the default-constructed case. I think "Point" is a class where a default constructor may or may not be appropriate, depending on your other requirements. HTH -- KCS Jul 22 '05 #6

 P: n/a PhilB wrote: Hello experts, I am a complete beginner in C++ (although I know C). I am trying to compile the code below, and I get the following error. Can anyone explain to me my mistake? Thanks! PhilB myprog2.cpp: In method `Line::Line (Point, Point)': myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) You dont have a default constructor, so convert the existing one to one. //------------------------ class Point { protected: int x; int y; public: Point::Point(int, int); }; Point::Point(int initx, int inity) make this Point::Point(int initx=0, int inity=0) { x=initx; y=inity; return; } class Line:public Point { public: Point start_point; Point end_point; public: Line::Line(Point, Point); }; Line::Line(Point initp1, Point initp2) { start_point=initp1; end_point=initp2; return; } main() { Point p1(10,20); Point p2(20,40); Line l1(p1,p2); } //------------------------ -- regards, Vardhan -- Jul 22 '05 #7

 P: n/a "Rolf Magnus" wrote in message news:br*************@news.t-online.com... PhilB wrote: Hello experts, I am a complete beginner in C++ (although I know C). I am trying to compile the code below, and I get the following error. Can anyone explain to me my mistake? Isn't it obvious from the error messages? The only thing that's obvious is that it isn't. Jul 22 '05 #8

 P: n/a "lallous" wrote in message news:br************@ID-161723.news.uni-berlin.de... In the class as: class Point { protected: int x; int y; public: Point::Point(int, int); ^^^^remove the "Point::" }; Same for the other class. Actually, I believe that's fine. Anyway, that's not the main problem. The main problem is that he's trying to create Points with the default constructor, and there's no default constructor. Jul 22 '05 #9

 P: n/a "Vardhan Prabhakar N [C]" wrote in message news:3F**************@motorola.com... //------------------------ class Point { protected: int x; int y; public: Point::Point(int, int); }; Point::Point(int initx, int inity) make this Point::Point(int initx=0, int inity=0) Not really a good explanation. That very well might not be what he wants. Jul 22 '05 #10

 P: n/a "Gary Labowitz" wrote in message news:K9********************@comcast.com... Please note it one of my top ten that a number of you objected to having on the list. To poster: Your Point constructor is protected, which means it can only be called by code of the Point class or one of its subclasses (Line). No, you missed the public. Jul 22 '05 #11

 P: n/a "Marko Becirevic" wrote in message news:br**********@ls219.htnet.hr... //------------------------ class Point { protected: int x; int y; public: Point::Point(int, int); }; not Point::Point(int, int); but just Point(int, int); same in Line class. That is not the problem. The problem is that he has Points in his Line. There is no way to construct those Points when he goes to create a Line in his main program. Jul 22 '05 #12

 P: n/a "Gary Labowitz" wrote in message news:... Please note it one of my top ten that a number of you objected to having on the list. To poster: Your Point constructor is protected, which means it can only be called by code of the Point class or one of its subclasses (Line). You are calling it from main. When you create a Line object, C++ will automatically construct the Point inherited portion by calling a constructor for a Point using a default constructor. And there isn't one. In addition, I'd say your design is flawed: A Line is not a kind of Point so you should not be using an is-a design. Lines have Points which is a has-a design. This is one way it could look: #include using namespace std; class Linex; class Pointx { friend class Linex; int x, y; //coordinates public: Pointx (int x, int y):x(x), y(y){}; Pointx ( ){}; //This is the missing constructor Hello Gary Thank you for your reply; I just added the line Pointx ( ){}; and it compiles with no error! Thanks to all for sharing your expertise. PhilB Jul 22 '05 #13

 P: n/a On Tue, 16 Dec 2003 05:51:02 -0800, PhilB wrote: Hello experts, I am a complete beginner in C++ (although I know C). I am trying to compile the code below, and I get the following error. Can anyone explain to me my mistake? Thanks! PhilB myprog2.cpp: In method `Line::Line (Point, Point)': myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) myprog2.cpp:32: no matching function for call to `Point::Point ()' myprog2.cpp:12: candidates are: Point::Point (int, int) myprog2.cpp:9: Point::Point (const Point &) //------------------------ class Point { protected: int x; int y; public: Point::Point(int, int); }; Point::Point(int initx, int inity) { x=initx; y=inity; return; } class Line:public Point { public: Point start_point; Point end_point; public: Line::Line(Point, Point); }; Line::Line(Point initp1, Point initp2) { start_point=initp1; end_point=initp2; return; } main() { Point p1(10,20); Point p2(20,40); Line l1(p1,p2); } //------------------------ The answer has been covered over and over, here so I will only include my alternate implementation that doesn't require definition of the default constructor class Point { protected: int x; int y; public: Point(int, int); }; Point::Point(int initx, int inity) { x=initx; y=inity; return; } class Line { public: Point* start_point; Point* end_point; public: Line(Point*, Point*); }; Line::Line(Point* initp1, Point* initp2) { start_point=initp1; end_point=initp2; return; } main() { Point p1(10,20); Point p2(20,40); Line l1(&p1,&p2); } I removed the line is-a point relationship which makes little sence. Additionally, I used pointers to avoid the calling of the default constructor. --Steve Jul 22 '05 #14

 P: n/a Steven Green wrote in message news:. .. The answer has been covered over and over, here so I will only include my alternate implementation that doesn't require definition of the default constructor class Point { protected: int x; int y; public: Point(int, int); }; Point::Point(int initx, int inity) { x=initx; y=inity; return; } class Line { public: Point* start_point; Point* end_point; public: Line(Point*, Point*); }; Line::Line(Point* initp1, Point* initp2) { start_point=initp1; end_point=initp2; return; } main() { Point p1(10,20); Point p2(20,40); Line l1(&p1,&p2); } I removed the line is-a point relationship which makes little sence. Additionally, I used pointers to avoid the calling of the default constructor. --Steve Hi Steve, Thanks a lot for your crystal-clear example, and thanks for sharing your expertise. PhilB Jul 22 '05 #15

 P: n/a Steven Green wrote in message news:. .. class Line { public: Point* start_point; Point* end_point; public: Line(Point*, Point*); }; Line::Line(Point* initp1, Point* initp2) { start_point=initp1; end_point=initp2; return; } main() { Point p1(10,20); Point p2(20,40); Line l1(&p1,&p2); } I removed the line is-a point relationship which makes little sence. Additionally, I used pointers to avoid the calling of the default constructor. --Steve Now, though, Line doesn't own its Points anymore ... in this example, if you pass l1 as a return value or do anything that extends its lifetime beyond p1 and p2, you'll have problems. Unless Points are really expensive to create and/or copy, this may not be the best way to do it. My two cents at least, Michael Jul 22 '05 #16

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