Please check my C++

1) The ":" symbolizes the beginning of the initializer list. Everything in the initializer list calls the constructor of what you put there. There it is mandatory, for example, if you have a base class with a constructor that takes one argument. You'll have to use the initializer list to call the correct constructor with the correct argument.

2) That's why I said probably make the constructor get the default time.

3) There are two issues:
3a) It's invalid syntax - you can't call a constructor after the object is constructed. You can only assign. Again, this leads to unnecessary waste of resources.
3b) It's typically undefined behavior to call a member function of a class before it's constructed, so it's very bad.

4) Since getCurrentTime does not rely on the object it's a member of (it only gets the current system time and not the current time embedded in the object), it's better to make it static as mats suggests. That way you can call it without having to worry about constructing the object first.

Quote:

Originally Posted by Elysia

2) That's why I said probably make the constructor get the default time.

Elysia, i would also like to initialize a time object with zeros, not current time...

Quote:

4) Since getCurrentTime does not rely on the object it's a member of (it only gets the current system time and not the current time embedded in the object), it's better to make it static as mats suggests. That way you can call it without having to worry about constructing the object first.

I don't know if i understand you, where did mats say this [EDIT: Sorry i missed his post]? anyway, here's my date class with now 'static' keyword...

Code:

class Date { public: Date(); Date(tm dt); void SetDate(tm dt); static tm getCurrentTime(); int getDay(); int getMonth(); int getYear(); int getHour(); int getMinute(); tm getLocalDT(); private: int day; int month; int year; int hour; int minute; tm local_dt; }; // Get the local date and time of system static tm Date::getCurrentTime() { time_t rawtime; tm* timeinfo; time( &rawtime ); timeinfo = localtime ( &rawtime ); return *timeinfo; }

What difference that this 'static' function make?

Quote:

Originally Posted by csonx_p

Elysia, i would also like to initialize a time object with zeros, not current time...

But initializing them with 0s doesn't make much sense, does it? It's an invalid date.

Quote:

I don't know if i understand you, where did mats say this?

Just above my previous reply.

Quote:

anyway, here's my date class with now 'static' keyword...

You should put static only in front on the definition, not the implementation and it would be correct.

Quote:

What difference that this 'static' function make?

Basically, it does so that all of the classes share only one instance of the function. The reason is that getCurrentTime is not bound to the object - your class. It does nothing of the sort - it doesn't return the time of the object, it doesn't set the time in the object.
There it should better be static or you can make it a free function.

A static function is not passed a "this" pointer of the current class object - which means that it's basically a free function, but it's described within a class. This means that you can call it without having an existing object.

You can't call a constructor with the object that it is constructing. That's like using the car you are building to transport the bits you need to build the car - doesn't work, right? [yes, you could have a "half-finished car" - but computers don't work well with "half-done" things].

You also can't call one constructor from another constructor, e.g.

Code:

class X { X() { X(0); // calling X(int x) with zero - not valid. }; X(int x) { ... }; }

--
Mats

Quote:

Originally Posted by Elysia

But initializing them with 0s doesn't make much sense, does it? It's an invalid date.

Well, by the time i initialize start time, i don't know about the end time yet. Customer can return a car beyond the requested/expected time. So i make them zeros... But i doubt if this is an issue i should be stressing about because my program won't use 'returned' date unless until the car is returned... But then yeah, i can also make start time equals returned time.. The point of this is learning C++ also. I could be writing a program which resetting to zeros is essential, by then i wouldn't like to pose this question....

Quote:

Originally Posted by matsp

A static function is not passed a "this" pointer of the current class object

Sorry about this dumb question, what is "this" pointer... i've read through it but can you put it in the context of my classes cause it's not 100% clear to me

Code:

Contract::Contract(): issued( Date::getCurrentTime() );// is this semicolon necessary? // Display the customer's contract, or save it to text file void Contract::Write(ostream& out, bool isScreen) { } // : error C2062: type 'void' unexpected // : error C2630: ';' found in what should be a comma-separated list // : error C2065: 'isScreen' : undeclared identifier // : error C2065: 'out' : undeclared identifier // : warning C4244: '=' : conversion from 'double' to 'int', possible loss of data

Nothing wrong with my write function, think error is related to code above it!

You should have

Code:

{}

in your Contract constructor, instead of the semicolon. Even if you don't do anything in the constructor, it does need a (empty) function body

--
Mats

Quote:

Originally Posted by csonx_p

Sorry about this dumb question, what is "this" pointer... i've read through it but can you put it in the context of my classes cause it's not 100% clear to me

It is "this" object. e.g.

Code:

class X { public: X(); void foo(); private: int x; } void X::foo() { x = 7; // the above is short for this->x = 7; }

Most of the time, you don't need the this pointer, but it is passed to all non-static member functions, including constructor.

--
Mats

Quote:

Originally Posted by csonx_p

Well, by the time i initialize start time, i don't know about the end time yet. Customer can return a car beyond the requested/expected time. So i make them zeros... But i doubt if this is an issue i should be stressing about because my program won't use 'returned' date unless until the car is returned... But then yeah, i can also make start time equals returned time.. The point of this is learning C++ also. I could be writing a program which resetting to zeros is essential, by then i wouldn't like to pose this question....

Then perhaps you should consider not having a constructor that takes no arguments instead. There's no sense in creating an "invalid" object and it saves you from design woes when an object has been initialized with a faulty time.

Quote:

Originally Posted by Elysia

Then perhaps you should consider not having a constructor that takes no arguments instead. There's no sense in creating an "invalid" object and it saves you from design woes when an object has been initialized with a faulty time.

I agree with this. An object that has "nonsense" in it because the constructor placed it there is pretty meaningless.

On a different not, more towards the contract of rentals: You may want to actually fill in the date of expected return, even if that's not the actual return date. You can then correct it when the car is actually returned.

--
Mats

overloading '='

Anything wrong with this overloading

Code:

Car::TankStatus operator=(int number, Car::TankStatus ts) { return ts = number; } //: error C2801: 'operator =' must be a non-static member

ts is enum type....

I believe the left-hand side of operator= must be a user-defined type (you have int) because it can only be overloaded as a non-static member.

Quote:

Originally Posted by anon

I believe the left-hand side of operator= must be a user-defined type (you have int) because it can only be overloaded as a non-static member.

Lost you, LHS as in ts? that's not int, that's enum...

Quote:

Originally Posted by csonx_p

Lost you, LHS as in ts? that's not int, that's enum...

If you have TWO arguments for operator=, then it must be a static (or free) function, because the class operator= would operate between "this" and the right-handside value.

--
Mats

Code:

Car::TankStatus operator=(int number, Car::TankStatus ts)

The bold part is what would be on the left of the assignment operator.

But anyway, it seems that you can't overload operator= for enum types anyway since it can only be overloaded as non-static member function.

Come to think of it, how would you implement the assignment of the actual enum value, if operator= is not available (since you are in the process of defining it)?

You can already assign enum to int and to enum, and you can assign int to enum with the help of a cast (but be careful with that since the int may not contain one of the enumerated values).

Operator = must be a member of the class to which it is defined to work with.
As thus, operator = does not work for enums, only classes.
Either way, you should not assign an int to an enum; you're essentially circumventing the whole C++ type safety.

Quote:

Originally Posted by Elysia

Operator = must be a member of the class to which it is defined to work with.
As thus, operator = does not work for enums, only classes.
Either way, you should not assign an int to an enum; you're essentially circumventing the whole C++ type safety.

You can certainly make "global" operator=() operator, that takes two arbitrary classes [that you have some idea of how you would convert to the left-hand side type].

The reason, I believe, for this operator is the need to serialize the data to file. If it was stored from the enum on the file, then it should be safe to read back. We could add checks in the operator to ensure that's the case.

--
Mats

Quote:

You can certainly make "global" operator=() operator, that takes two arbitrary classes [that you have some idea of how you would convert to the left-hand side type].

But how?

Code:

struct X {}; struct Y {}; X& operator= (X& x, const Y&) //X& operator=(X&, const Y&)' must be a nonstatic member function { return x; }

Searching the web, I can't actually find it, nor in Stroustrup (but the index is a bit hard to use in Stroustrup - doesn't always list what you want).

--
Mats

Well, the standard says:

Quote:

13.5.3 Assignment
An assignment operator shall be implemented by a non-static member function with exactly one parameter.

It is possible to add a contructor to make Y assignable to X (in which case the constructor would do the hard work).

Code:

struct Y {}; struct X { X() {} X(const Y&) {} }; int main() { X x; Y y; x = y; }

It should be equally possible to create an assignment operator in X that takes Y as argument.
It just can't be global.

Hello, read your posts...

Quote:

But anyway, it seems that you can't overload operator= for enum types anyway since it can only be overloaded as non-static member function.

Quote:

Operator = must be a member of the class to which it is defined to work with.
As thus, operator = does not work for enums, only classes.
Either way, you should not assign an int to an enum; you're essentially circumventing the whole C++ type safety.

Quote:

You can certainly make "global" operator=() operator, that takes two arbitrary classes [that you have some idea of how you would convert to the left-hand side type].

The reason, I believe, for this operator is the need to serialize the data to file. If it was stored from the enum on the file, then it should be safe to read back. We could add checks in the operator to ensure that's the case.

Am not sure if i have a solution to my problem.... :(

I would say you have to cast the enum to int.
But what's the code that uses the enum that relies on that assignment operator?

Quote:

Originally Posted by Elysia

I would say you have to cast the enum to int.
But what's the code that uses the enum that relies on that assignment operator?

i figured there's not even a need to assign the int, cause i have the enums to assign anyway... No need for overloading. Thanx

Quote:

Car::TankStatus ts;

// Ensure correct tank details
if ( (ts>3) || (ts<0) ) {
ts=Car::unknown;
}

sorting

My sort function works but not according to my desires. I have two fields that it needs to consider when sorting [car make & car model]... i.e If i have Mercedes Vito before Mercedes Benz in the list, then the two wont be sorted since both have the same make...

this is how i use the sort function...

Code:

vector <Car> vehicle; sort(vehicle.begin(), vehicle.end());

How do i modify this to achieve my desires?

You need a operator> (or is it operator<) implemented for your Car class. You would also, most likely, want to combine the effect of make and model, so for exampel Ford Escort is before Ford Focus. This would mean comparing the make first, and if they are equal, then compare the make component.

--
Mats

Quote:

Originally Posted by matsp

You need a operator> (or is it operator<) implemented for your Car class. You would also, most likely, want to combine the effect of make and model, so for exampel Ford Escort is before Ford Focus. This would mean comparing the make first, and if they are equal, then compare the make component.

--
Mats

Cool, i already have that operator, just need to expand on it!

Code:

bool Car::operator<(const Car& car) const { return make < car.make; }

thnx... Sorry, how do you clear unwanted characters on stin again? is it cin.clear()?

Quote:

Originally Posted by csonx_p

Cool, i already have that operator, just need to expand on it!

Code:

bool Car::operator<(const Car& car) const { return make < car.make; }

thnx... Sorry, how do you clear unwanted characters on stin again? is it cin.clear()?

cin.ignore() perhaps? cin.clear() sets (or rather clears) the error flags, but doesn't affect input itself. So cin.clear() may be useful if you reach EOF in a file and then want to read it again.

--
Mats

Quote:

Originally Posted by csonx_p

thnx... Sorry, how do you clear unwanted characters on stin again? is it cin.clear()?

http://cboard.cprogramming.com/showp...0&postcount=46

Quote:

Originally Posted by Elysia

http://cboard.cprogramming.com/showp...0&postcount=46

Oh! that boost thing again... Lol! well i said am gonna return to you to explain it more, but didn't as yet... Let me try't then will flood you with questions :)

Code:

boost::integer_traits<int>::const_max

Please explain, what does this have to do with STDIN, what's boost anyway?

Quote:

Originally Posted by csonx_p

Code:

boost::integer_traits<int>::const_max

Please explain, what does this have to do with STDIN, what's boost anyway?

It is simply a constant describing the largest positive value that an int can have, and nothing else. So it's really not necessary to use boost for this purpose - it's just a convenient place to get such a constant. It is also unlikely that you really need to ignore 2GB of input - most likely any number between 100 and 10000 would be perfectly find for NEARLY all purposes.

--
Mats

Thanx mats for explaining boost..

Code:

bool Car::operator<(const Car& car) const { return make < car.make; }

Q.

1. i know i wrote this but have a question... In the above, what is the different between make & car.make, is it a this pointer VS current object...?

2. I tried doing this but doesn't work...

Code:

bool Car::operator<(const Car& car) const { return (make < car.make) && (model < car.model); }

Maybe because i don't understand how does find() use the operator '<'...

What you need is something like this:

Code:

return (a.x == b.x)? a.y < b.y : a.x < b.x;

Or, if we use if-statement instead of the ternary operator:

Code:

if (a.x == b.x) return a.y < b.y; return a.x < b.x

--
Mats

Quote:

Originally Posted by matsp

What you need is something like this:

Code:

return (a.x == b.x)? a.y < b.y : a.x < b.x;

Or, if we use if-statement instead of the ternary operator:

Code:

if (a.x == b.x) return a.y < b.y; return a.x < b.x

--
Mats

This function is non-static meaning it takes one argument... I think your example is based on static function which actually receives the two objects (a & b)...

Quote:

Originally Posted by csonx_p

This function is non-static meaning it takes one argument... I think your example is based on static function which actually receives the two objects (a & b)...

Yes, I was just demonstrating "how you compare two-level objects", not concerned with any particular implementation you may choose to use.

--
Mats

Quote:

Originally Posted by csonx_p

Thanx mats for explaining boost..

Actually, boost is a collection of libraries for C++. Some of which comes in handy very much.
What I showed in my example is called integer traits. Basically, it defines various properties of certain types.
It provides constant numbers for maximum and minimum values the types can hold along with something else and gets the rest of the information by deriving from STL's integer traits.

Quote:

Originally Posted by Elysia

Actually, boost is a collection of libraries for C++. Some of which comes in handy very much.
What I showed in my example is called integer traits. Basically, it defines various properties of certain types.
It provides constant numbers for maximum and minimum values the types can hold along with something else and gets the rest of the information by deriving from STL's integer traits.

My question was why use this with stdin.... Is it to allow all types and size of data to be ignored/cleared? Where else are boosts commonly used? a simple example maybe!

new line at eof

When i write records on a text file i use '\n' at the end of each record so that all records can be on separate lines... Now the challenge is there's always going to be an empty line a at the last record... Been thinking of a way to avoid cause when reading from same file, this record reads as junk... any ideas??

If you read using >> x, then it will "fail" if it can't read valid data after the newline, so all you need to do is to check the result of the read operation. Fortunately, that's easy, because the bool operator of the stream class returns false if the stream encountered an error (such as end of file).

So just do

Code:

if (!fin >> car) // Reached end of file, stop reading else // process car (e.g. insert into vector).

Note that it doesn't matter that we tried to read the ENTIRE car record - it will fail when it reaches end of file, and assuming your data file is correctly built, that would happen just after your last end-of-line marker.

--
Mats

Quote:

Originally Posted by matsp

If you read using >> x, then it will "fail" if it can't read valid data after the newline, so all you need to do is to check the result of the read operation. Fortunately, that's easy, because the bool operator of the stream class returns false if the stream encountered an error (such as end of file).

So just do

Code:

if (!fin >> car) // Reached end of file, stop reading else // process car (e.g. insert into vector).

Note that it doesn't matter that we tried to read the ENTIRE car record - it will fail when it reaches end of file, and assuming your data file is correctly built, that would happen just after your last end-of-line marker.

--
Mats

as this is my record

Code:

in >> make >> model; in >> regNo; in >> ymodel; in >> engCap; in >> ts;

i'm guessing then it would be more appropriate to check the first field

i.e

Code:

if (!in >> make ) return -1; // Can i return -1 to istream? else { in >> model; in >> regNo; in >> ymodel; in >> engCap; in >> ts; }

Quote:

Originally Posted by csonx_p

as this is my record

Code:

in >> make >> model; in >> regNo; in >> ymodel; in >> engCap; in >> ts;

i'm guessing then it would be more appropriate to check the first field

i.e

Code:

if (!in >> make ) return -1; // Can i return -1 to istream? else { in >> model; in >> regNo; in >> ymodel; in >> engCap; in >> ts; }

Well, in that case, you should check ALL of them - what if you have make, model, regno <end of file> because of some system error (full disk, power failure/system crash)?

Edit: Just checking if(!in) after you have read all the fields [any or all of which may have failed] will tell you if it failed, or on the else-side, succeeded.
Edit2: If it's a operator>> then you will return in itself, and that will be sufficient to check at the calling point. No need to return a bool/integer value from this function in that case.

--
Mats

Quote:

Originally Posted by matsp

Edit2: If it's a operator>> then you will return in itself, and that will be sufficient to check at the calling point. No need to return a bool/integer value from this function in that case.

calling point ...?

Code:

void Car::Read(istream& indata, Car& car, bool isKeyboard) { indata >> car; }

If i check here. what do i check for, indata.fail() ?

OR

Code:

ifstream fleetin; car.Read(fleetin, car, false);

If i check here, fleetin.fail()?

... just checking .. :)

You could just do:

Code:

istream& Car::Read(istream& indata, Car& car, bool isKeyword) { return indata >> car; }

and use it like:

Code:

if (!car.Read(fleetin, car, false) { //input failed }

Then, in case of failed input you could use more specialized functions to find out what went wrong (if that is important to you).

Also, in the above usage, what is the point to pass car as an argument if this function is already called on the car instance and car is already passed as the this pointer. Simpler:

Code:

car.Read(fleetin, false);

Quote:

Originally Posted by anon

Also, in the above usage, what is the point to pass car as an argument if this function is already called on the car instance and car is already passed as the this pointer. Simpler:

Code:

car.Read(fleetin, false);

how do i read with "this" pointer?

Code:

void Car::Read(istream& indata, bool isKeyboard) { indata >> this; }

You dereference it like other pointers:

Code:

indata >> *this;

You may want *this, as this is a pointer to car, which won't be available as a "read into this type" overloaded operator>> .

--
Mats

vector vs dequeue vs maps

For this program, which would you recommend and why (advantages)?

Seeing as you need to have a car to read in the value anyways, you may just as well use the "this" pointer that is already there. Is Car::Read a static function? If not, it is being passed "this" as well as car, so if you do car.Read(... car ...) then you are effectively passing car along twice - once as this, and second as a parameter. Seems unnecessary, and there's no purpose (or advantage) to having a Car reference parameter to a function that requires to have a Car object to be called. Since "simpler is better", this gives the advantage to the model that anon is suggesting.

--
Mats

Quote:

Originally Posted by matsp

Seeing as you need to have a car to read in the value anyways, you may just as well use the "this" pointer that is already there. Is Car::Read a static function? If not, it is being passed "this" as well as car, so if you do car.Read(... car ...) then you are effectively passing car along twice - once as this, and second as a parameter. Seems unnecessary, and there's no purpose (or advantage) to having a Car reference parameter to a function that requires to have a Car object to be called. Since "simpler is better", this gives the advantage to the model that anon is suggesting.

--
Mats

Oops, i took it for granted that you'll see the title of my post...

Quote:

For this program, which would you recommend and why (advantages)?
[vector vs dequeue vs maps]

Ok, I see now. I think the general rule is "use vector unless you are sure you need one of the others".

--
Mats

Friend Classes

Here's my scenario concerning my friend class..

in FleetMenu.cpp

Code:

class FleetMenu { public: friend class ContractMenu; FleetMenu(); private: void Pause(); void Clrscr();

in ContractMenu.h

Code:

class ContractMenu { public: ContractMenu(); void Run(); private: .... private: .... };

in ContractMenu.cpp

Code:

void ContractMenu::Run() { Clrscr(); }

Problem

Code:

: error C3861: 'Clrscr': identifier not found

am new to friends... :(

You need to:
1. make Clrscr static [I presume it doesn't actually use any member content]
2. call FleetMenu::Clrscr().

By making to classes friends, you are not inheriting the behaviour, you are just letting the other class use the private member functions of the class.

However, I'm not convinced that is the best way to form a menu - perhaps something based on tables is a better suggestion.

--
Mats

Quote:

Originally Posted by matsp

You need to:
1. make Clrscr static [I presume it doesn't actually use any member content]
2. call FleetMenu::Clrscr().

By making to classes friends, you are not inheriting the behaviour, you are just letting the other class use the private member functions of the class.

However, I'm not convinced that is the best way to form a menu - perhaps something based on tables is a better suggestion.

--
Mats

Code:

class FleetMenu { public: //friend class ContractMenu; FleetMenu(); private: static void Pause(); static void Clrscr();

Code:

void ContractMenu::Run() { FleetMenu::Clrscr(); } // error C2248: 'FleetMenu::Clrscr' : cannot access private member declared in class 'FleetMenu'

You STILL need to make it a friend if you want to use that solution, but you ALSO need to have it static.

--
Mats

By the way, here's a post I made a while back with regards to creating a menu system - perhaps you can use it as a base:
http://cboard.cprogramming.com/showp...6&postcount=18

--
Mats

Quote:

Originally Posted by matsp

perhaps something based on tables is a better suggestion.
Mats

What do you mean by tables?

Quote:

Originally Posted by csonx_p

What do you mean by tables?

See post #217.

--
Mats

Quote:

Originally Posted by matsp

See post #217.

--
Mats

looks interesting, lets take for example the following

Code:

case 1: AddNewCar(); Pause(); break; case 2: UpdateFleetFile(); Pause(); break; case 3: EditFleetContract(); Pause(); break; case 4: DeleteCar(); Pause(); break;

1. how do i use your function // addmenu(std::string s, char c, menufunc f)... Or how do i replace the code above! ?
2. If i have two different menus, how do i use your code?

Quote:

Originally Posted by csonx_p

looks interesting, lets take for example the following

Code:

case 1: AddNewCar(); Pause(); break; case 2: UpdateFleetFile(); Pause(); break; case 3: EditFleetContract(); Pause(); break; case 4: DeleteCar(); Pause(); break;

1. how do i use your function // addmenu(std::string s, char c, menufunc f)... Or how do i replace the code above! ?
2. If i have two different menus, how do i use your code?

1. If you move the Pause() to somewhere inside each of those functions, then you can easily just make those functions the "f" parameter (you'd have to add some extra parameters which you may or may not want to ignore).

If you have two different menus, you just create two menu objects - you can even make hierarchical menus by creating a new menu in one of the functions you call as a result of a menu choice.

--
Mats

Quote:

Originally Posted by matsp

See post #217.

--
Mats

1. Can this be called link list?, or is it strictly table...
2. Problem is with this my menu won't be nicely aligned within borders, it will be basic...

Quote:

Originally Posted by csonx_p

1. Can this be called link list?, or is it strictly table...
2. Problem is with this my menu won't be nicely aligned within borders, it will be basic...

Surely you can generate borders around it if you like - just count the number of items in the linked list and the lenght of the strings. You will want to replace this:

Code:

for(p = head; p; p = p->next) { p->item->print(); }

and put it into a function that counts the items + lengths, draws the frame and prints the items within a box - it shouldn't take that much effort.

Yes, it's a linked list, which I call a table in the sense that it is data rather than code, but it's obviously not an array - you could probably replace the linked list stuff with a vector<> and push_back instead of linked list insertion.

--
Mats

Quote:

Originally Posted by matsp

Surely you can generate borders around it if you like - just count the number of items in the linked list and the lenght of the strings. You will want to replace this:

Code:

for(p = head; p; p = p->next) { p->item->print(); }

Can't i just modify this function

Code:

void menuitem::print() { std::cout << choice << ": " << descr << std::endl; }

Maybe something like..

Code:

std::cout << " \t\t#" << choice << "." << descr << " #\n" << std::endl;

then by looping this somehow the hashes will from a border?

Well, print shows only one menuitem - you need some way to determine how long ALL of the menu items are [or decide beforehand that everything should fit in X characters].

Obviously, if you have a constant width for all menus, then (as discussed elsewhere), you could just modify print() to do the right thing. It is probably STILL a good idea to add a function "drawmenu" or some such.

--
Mats

Quote:

Originally Posted by matsp

By the way, here's a post I made a while back with regards to creating a menu system - perhaps you can use it as a base:
http://cboard.cprogramming.com/showp...6&postcount=18

--
Mats

I'm trying to understand your code, well, i've never worked on linked list but have an idea..

Code:

// Class menucontext class menucontext { public: bool fquit; menucontext() : fquit(false) {} ~menucontext() {}; }; typedef void (*menufunc)(menucontext &ctxt);

Could you explain the purpose of this function pointer menufunc & class menucontext? Expanding to that, how/for what reasons it's used here in"domenu"?

Code:

for(p = head; p; p = p->next) { if (p->item->isChoice(c)) { p->item->domenu(ctxt); choosen = true; } } ............. void menuitem::domenu(menucontext &ctxt) { func(ctxt); }

The context is there so that you can have some data [that is not global] that you can pass between the function that uses the menu class itself, and the functions provided as the menu functions. This class probably would need to be expanded - right now it only contains a flat to indicate if we want to quit or not.

--
Mats

Quote:

Originally Posted by matsp

The context is there so that you can have some data [that is not global] that you can pass between the function that uses the menu class itself, and the functions provided as the menu functions. This class probably would need to be expanded - right now it only contains a flat to indicate if we want to quit or not.

--
Mats

In simple terms, could you explain this for loop

Code:

for(p = head; p; p = p->next) { if (p->item->isChoice(c)) { p->item->domenu(ctxt); choosen = true; } }

thnx

The for loop walks the linked list, from head until it finds a 0 pointer. It checks if the choice is matching the current menuitem, and if so, calls the menu for that and sets the "isChoosen" variable [for optimization, on long menus, you may want to change the ; p ; in the for-loop into ; p && !isChoosen;

--
Mats

Understanding link lists

Quote:

Originally Posted by matsp

The for loop walks the linked list, from head until it finds a 0 pointer. It checks if the choice is matching the current menuitem, and if so, calls the menu for that and sets the "isChoosen" variable [for optimization, on long menus, you may want to change the ; p ; in the for-loop into ; p && !isChoosen;

--
Mats

To help myself understand link lists, i'm gonna try explain this function and stand for corrections

Code:

void menu::addmenu(std::string s, char c, menufunc f) { menunode *node = new menunode; node->item = new menuitem(s, c, f); node->next = 0; if (!head) head = tail = node; else { tail->next = node; tail = node; } };

If called for the first time, head and tail will be NULL, thus item will be contained at the head and tail after the 'if' execution. If called the second time where both head & tail contain same data, tail will now point to the new data (node) hence the 'else' part of the if statement.

Confusions
1. tail->next = node points to node already, why tail=node
2. If called for the third time, you replace tail with new node. What happens to what tail pointed to before (second time)? I know you inserting new item at the end.. can't see the logic of that though!

In a linked list, you often only track the head and/or tail - the other components are held together with the links within the list. So, if we have 3 elements (1, 2, 3) in a linked list:
head->1->2->3 <- tail

So tail points to the currently last element in the list at the end of the function. When the first element is inserted, obviously the first AND the last element are the same one, so tail and head point to the same thing. Once we start adding more element, tail will point to the most recently added [since we don't do any sorting or such - elements are in the order they were inserted].

--
Mats

Quote:

Originally Posted by matsp

In a linked list, you often only track the head and/or tail - the other components are held together with the links within the list. So, if we have 3 elements (1, 2, 3) in a linked list:
head->1->2->3 <- tail

So tail points to the currently last element in the list at the end of the function. When the first element is inserted, obviously the first AND the last element are the same one, so tail and head point to the same thing. Once we start adding more element, tail will point to the most recently added [since we don't do any sorting or such - elements are in the order they were inserted].

--
Mats

mind you the if/else you have is only used to point, not to insert... Maybe that's where my confusion was!

Quote:

Originally Posted by csonx_p

mind you the if/else you have is only used to point, not to insert... Maybe that's where my confusion was!

Well, the "insert" in a linked list is just a "link it in", which is done in the else-part. In the initial link, there is nothing other than the head to "link in" - and the next link is NULL, which is already done above the if-else (since we ALWAYS insert at the end, next is always NULL on a new node). The else part sets the current tail's next pointer to the new node, then moves the tail to the new last element.
Insert element 3 in a 1, 2 list:

Code:

before: head -> 1 | v tail -> 2 | v NULL after tail->next = node: head -> 1 | v tail -> 2 | v 3 | v NULL after tail = node: head -> 1 | v 2 | v tail -> 3 | v NULL

--
Mats

Quote:

Originally Posted by matsp

Well, the "insert" in a linked list is just a "link it in", which is done in the else-part. In the initial link, there is nothing other than the head to "link in" - and the next link is NULL, which is already done above the if-else (since we ALWAYS insert at the end, next is always NULL on a new node). The else part sets the current tail's next pointer to the new node, then moves the tail to the new last element.
Insert element 3 in a 1, 2 list:

Code:

before: head -> 1 | v tail -> 2 | v NULL after tail->next = node: head -> 1 | v tail -> 2 | v 3 | v NULL after tail = node: head -> 1 | v 2 | v tail -> 3 | v NULL

--
Mats

makes sense, thanx...

Lastly on this topic...

Quote:

This class (menucontext) probably would need to be expanded - right now it only contains a flag to indicate if we want to quit or not.

What do you imagine should be added as a member to this class... Say i have 5 Menu Items...

Quote:

Originally Posted by csonx_p

makes sense, thanx...

Lastly on this topic...

What do you imagine should be added as a member to this class... Say i have 5 Menu Items...

It depends very much on how you want to design the user-interface for your system. I can imagine something like this for renting a car out:
* Register new hirer (or find existing).
* Find suitable car
* finalize contract

If we do it this way, when you register new hirer, you need to remember that hirer for when you finalize the contract. Likewise for the car. Once you have issued the contract, you probably want to "forget" what the current hirer and car is.

--
Mats

Quote:

Originally Posted by matsp

It depends very much on how you want to design the user-interface for your system. I can imagine something like this for renting a car out:
* Register new hirer (or find existing).
* Find suitable car
* finalize contract

If we do it this way, when you register new hirer, you need to remember that hirer for when you finalize the contract. Likewise for the car. Once you have issued the contract, you probably want to "forget" what the current hirer and car is.

--
Mats

Am not sure if you understood me, take for example..

Code:

case ADDCAR: AddNewCar(); // Add new car break; case UPDATEF: UpdateFleetFile(); // Save changes to text file break; case EDITF: EditFleetContract(); // Edit contract details break; case DELCAR: DeleteCar(); // Remove a specific car from fleet Pause();

This is what needs to be part of Menu, or currently is... But, the class 'menucontext' has fquit as the only functionality, to help quit the program. My question is how do i relate the above options/functionalities with this class, if at all. My knowledge is i have to pass all these functions above any many others using the addmenu() function, which inserts them to the list or table. So, where does menucontext come in? or of what help except 'Quitting' would it be! Hope am clear....

If all you "need" from one menu to another is to know if you wanted to quit or not, then fQuit is all you need in menucontext. If on the other hand you would like to remember something from one menu selection to another (for example the car you select in "find car"), then you may want to add that to the menucontext.

--
Mats

Quote:

Originally Posted by matsp

If all you "need" from one menu to another is to know if you wanted to quit or not, then fQuit is all you need in menucontext. If on the other hand you would like to remember something from one menu selection to another (for example the car you select in "find car"), then you may want to add that to the menucontext.

--
Mats

mhh... notice menufunc() has menucontext as a ref parameter... How do i pass a function with a different parameter, and a none void return value? Let me guess, this is going back to what you said (modifying menucontext for all functions :rolleyes:) ... Well, let take one function ..

Code:

bool ShowFleetList(vector<car>::iterator i , bool status);

How would i cater for this?

What do you currently pass to ShowFleetList? [that is, what does the code look like in the calling place].

If the vector of cars is a global, then you don't need to pass anything. If it's not a global, then you need some way to pass it along, and menucontext would be one way to achieve that.

--
Mats

Quote:

Originally Posted by matsp

What do you currently pass to ShowFleetList? [that is, what does the code look like in the calling place].
Mats

Code:

FleetIterator iterate; ShowFleetList(iterate=fleet.begin(), found);

only fleet is global...

Quote:

If the vector of cars is a global, then you don't need to pass anything.

They may be other functions which receives parameters, and return values

Quote:

If it's not a global, then you need some way to pass it along, and menucontext would be one way to achieve that.

one example would help, maybe based on the above function...