Programming Forums > Application Development > C++

Arevos · Nov 30th, 2006, 3:14 PM

In C++ you can assign custom operators to classes. For instance, lets say I have a class Pet, and I consider two Pets as being equal if they have the same name and the same species. I can go about this one of two ways. I could have a function on the class like so:

 C++ Syntax (Toggle Plain Text) 
bool Pet::equals(const Pet& pet) {
     return this->name == pet.name && this->species == pet.species;
}
bool Pet::equals(const Pet& pet) {
     return this->name == pet.name && this->species == pet.species;
}

Then later on, I can compare two Pet instances like so:

 C++ Syntax (Toggle Plain Text) 
if (fluffy.equals(unknownCat)) { ... }
if (fluffy.equals(unknownCat)) { ... }

However, a better solution would be if we put a custom "==" operator on the Pet class:

 C++ Syntax (Toggle Plain Text) 
bool Pet::operator==(const Pet& pet) {
     return this->name == pet.name && this->species == pet.species;
}
bool Pet::operator==(const Pet& pet) {
     return this->name == pet.name && this->species == pet.species;
}

Then we could do this:

 C++ Syntax (Toggle Plain Text) 
if (fluffy == unknownCat) { ... }
if (fluffy == unknownCat) { ... }

As well as making things look nicer, operator overloading also gives one a standard ways of accessing objects. For instance, in order to sort a set of objects, we need to have some information about how the objects should be ordered. The < and > operators give this capability; in strings, "A" < "B", and in ints, 1 < 2. In fact, we don't need both operators; only one is needed, and this happens to be the less than operator.

So all you need do is override the less than operator, and sort your container class with std::sort. For instance, if you have a vector<Customer> object called customers:

 C++ Syntax (Toggle Plain Text) 
std::sort(customers.begin(), customers.end());
std::sort(customers.begin(), customers.end());

The less than operator on the Customer class gives std::sort the information to achieve the right ordering.

codylee270 · Nov 30th, 2006, 6:35 PM

I'm pretty familiar with operator overloading, my question is where in the sort function syntax is there a greater than or lesser than operator. Could you explain the use of the sort function with these operators inside the function call. Or could you give me a link to a site that explains the use of the sort function with respect to these operators.

Quote:

The less than operator on the Customer class gives std::sort the information to achieve the right ordering.

(Toggle Plain Text)

C++ Code:
std::sort(customers.begin(), customers.end());
The less than operator on the Customer class gives std::sort the information to achieve the right ordering.

Where is the operator?

Arevos · Dec 1st, 2006, 2:59 AM

The operator is called within the sort function itself.

codylee270 · Dec 1st, 2006, 12:00 PM

So you just overload the < operator as member function for the Customer class? And due to the implementation of the sort function, where ever it is located, will recognize to use the overloaded operator since it is comparing 2 customer objects. Correct?

Arevos · Dec 1st, 2006, 2:59 PM

You got it.

codylee270 · Dec 1st, 2006, 3:41 PM

Using your implementation a few posts above, why did you pass a reference to a customer object. I see that you are using this, so how does the sort function know the invoking object? Does the object on the left side invoke the overloaded function, thus requires accepting the other one as an argument? Does that make sense? Also, to take into account sorting by first name, if the last names are equal, would it make sense to use some conditionals to choose wha treturn to make so if the last names are equal, you would return one as less than the other.

e.g. -

(Toggle Plain Text)

if( (customer.lastName == this->lastName) && customer.firstName < this->firstName)
     return (customer.lastName < this->lastName);
else if( (customer.lastName == this->lastName) && customer.firstName) > this->firstName)
     return (this->lastName < customer.lastName);
else
     return ( customer.lastName < this->lastName);
}

Would something like this work in this situation?

Arevos · Dec 1st, 2006, 4:02 PM

Quote:

Originally Posted by codylee270

I see that you are using this, so how does the sort function know the invoking object? Does the object on the left side invoke the overloaded function, thus requires accepting the other one as an argument?

If I'm understanding you right, yes, that's what happens. The object on the left hand side is the object that calls the operator and the object on the right is passed in as an argument.

Quote:

Originally Posted by codylee270

Also, to take into account sorting by first name, if the last names are equal, would it make sense to use some conditionals to choose wha treturn to make so if the last names are equal, you would return one as less than the other.

e.g. -

(Toggle Plain Text)

if( (customer.lastName == this->lastName) && customer.firstName < this->firstName)
     return (customer.lastName < this->lastName);
else if( (customer.lastName == this->lastName) && customer.firstName) > this->firstName)
     return (this->lastName < customer.lastName);
else
     return ( customer.lastName < this->lastName);
}

Erm. The above code doesn't make logical sense. Your code is logically equivalent to this:

 C++ Syntax (Toggle Plain Text) 
if (customer.lastName == this->lastName && customer.firstName != this->firstName)
     return false;
else
     return ( customer.lastName < this->lastName);
}
if (customer.lastName == this->lastName && customer.firstName != this->firstName)
     return false;
else
     return ( customer.lastName < this->lastName);
}

Which is clearly incorrect. That said, my previous function was also incorrect, so I'm not really one to talk :p

Here's the correct boolean logic:

 C++ Syntax (Toggle Plain Text) 
return this->lastName < customer.lastName ||
    (this->lastName == customer.lastName &&
     this->firstName < customer.firstName);
return this->lastName < customer.lastName ||
    (this->lastName == customer.lastName &&
     this->firstName < customer.firstName);

codylee270 · Dec 1st, 2006, 7:29 PM

Haha, well at least you understood me! I'm probably going to start coding this tonight or tomorrow, so be on the lookout for actual source. I'm thinking of using a large MegaStore class, as you stated before to handle all of the queues inside the store, and manage statistics for each line. Sound good? Looks like that will be the easiest way to make function calls and what not and not have to worry about what object is invoking what and where/if I need to pass this object so it can be used to process this, etc...

Nov 30th, 2006, 3:14 PM	#21
Arevos Programming Guru Join Date: Aug 2005 Location: England Posts: 1,499 Rep Power: 5	In C++ you can assign custom operators to classes. For instance, lets say I have a class Pet, and I consider two Pets as being equal if they have the same name and the same species. I can go about this one of two ways. I could have a function on the class like so: C++ Syntax (Toggle Plain Text) bool Pet::equals(const Pet& pet) { return this->name == pet.name && this->species == pet.species; } bool Pet::equals(const Pet& pet) { return this->name == pet.name && this->species == pet.species; } Then later on, I can compare two Pet instances like so: C++ Syntax (Toggle Plain Text) if (fluffy.equals(unknownCat)) { ... } if (fluffy.equals(unknownCat)) { ... } However, a better solution would be if we put a custom "==" operator on the Pet class: C++ Syntax (Toggle Plain Text) bool Pet::operator==(const Pet& pet) { return this->name == pet.name && this->species == pet.species; } bool Pet::operator==(const Pet& pet) { return this->name == pet.name && this->species == pet.species; } Then we could do this: C++ Syntax (Toggle Plain Text) if (fluffy == unknownCat) { ... } if (fluffy == unknownCat) { ... } As well as making things look nicer, operator overloading also gives one a standard ways of accessing objects. For instance, in order to sort a set of objects, we need to have some information about how the objects should be ordered. The < and > operators give this capability; in strings, "A" < "B", and in ints, 1 < 2. In fact, we don't need both operators; only one is needed, and this happens to be the less than operator. So all you need do is override the less than operator, and sort your container class with std::sort. For instance, if you have a vector<Customer> object called customers: C++ Syntax (Toggle Plain Text) std::sort(customers.begin(), customers.end()); std::sort(customers.begin(), customers.end()); The less than operator on the Customer class gives std::sort the information to achieve the right ordering.

Dec 1st, 2006, 3:41 PM	#26
codylee270 Unverified User Join Date: Sep 2005 Posts: 209 Rep Power: 0	Using your implementation a few posts above, why did you pass a reference to a customer object. I see that you are using this, so how does the sort function know the invoking object? Does the object on the left side invoke the overloaded function, thus requires accepting the other one as an argument? Does that make sense? Also, to take into account sorting by first name, if the last names are equal, would it make sense to use some conditionals to choose wha treturn to make so if the last names are equal, you would return one as less than the other. e.g. - (Toggle Plain Text) if( (customer.lastName == this->lastName) && customer.firstName < this->firstName) return (customer.lastName < this->lastName); else if( (customer.lastName == this->lastName) && customer.firstName) > this->firstName) return (this->lastName < customer.lastName); else return ( customer.lastName < this->lastName); } if( (customer.lastName == this->lastName) && customer.firstName < this->firstName) return (customer.lastName < this->lastName); else if( (customer.lastName == this->lastName) && customer.firstName) > this->firstName) return (this->lastName < customer.lastName); else return ( customer.lastName < this->lastName); } Would something like this work in this situation?

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Dec 1st, 2006, 2:59 AM	#23
Arevos Programming Guru Join Date: Aug 2005 Location: England Posts: 1,499 Rep Power: 5	The operator is called within the sort function itself.

Dec 1st, 2006, 12:00 PM	#24
codylee270 Unverified User Join Date: Sep 2005 Posts: 209 Rep Power: 0	So you just overload the < operator as member function for the Customer class? And due to the implementation of the sort function, where ever it is located, will recognize to use the overloaded operator since it is comparing 2 customer objects. Correct?

Dec 1st, 2006, 2:59 PM	#25
Arevos Programming Guru Join Date: Aug 2005 Location: England Posts: 1,499 Rep Power: 5	You got it.

Dec 1st, 2006, 7:29 PM	#28
codylee270 Unverified User Join Date: Sep 2005 Posts: 209 Rep Power: 0	Haha, well at least you understood me! I'm probably going to start coding this tonight or tomorrow, so be on the lookout for actual source. I'm thinking of using a large MegaStore class, as you stated before to handle all of the queues inside the store, and manage statistics for each line. Sound good? Looks like that will be the easiest way to make function calls and what not and not have to worry about what object is invoking what and where/if I need to pass this object so it can be used to process this, etc...

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Programming Forums > Application Development > C++

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