Programming Forums > Application Development > C++

omdown · Oct 3rd, 2005, 7:54 PM

So I'm trying to teach myself C++, but I've stumbled across a programming exercise that I can't figure out. The general idea is this: create a class called Dog which has a string field for the dog's name. In main(), create a dog and assign a name to it. The problem is, it doesn't seem to work the same way as assigning a string in a normal fashion. The way I've been going about it is:

(Toggle Plain Text)

       // this is the class
       class Dog
             {
             public:
	          char dogName[5];
              };

      // then skipping down to the main () section where I assign a name

	Dog oneDog;
	oneDog.dogName = {"Spot"};

Which produces errors indicating that the { } should not be there. When I remove the braces:

(Toggle Plain Text)

	Dog oneDog;
	oneDog.dogName = "Spot";

It gives me the error that "left operand must be l-value." How can I assign a string value to this? I'm also a little confused as to how you would pass an object's value to a function as well. The book I bought has a bad habit of going over how you would do something in an ideal situation but not if it's anything more complicated. Would it be the same as a regular variable like this:

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function_name(oneDog.dogName)

Thanks in advance.

Animatronic · Oct 3rd, 2005, 8:25 PM

If you want the dog just to have a reference to the name and not take ownership of it ( not be able to change it ) do this:

(Toggle Plain Text)

class Dog
{
public:
	const char * Name;
};

Dog oneDog;
oneDog.Name = "Spot";
function( oneDog.Name );

void function( const char * name )
{
}

If you want the dog to own the name, so it can change it's name later do this:

(Toggle Plain Text)

#include <cstring>

class Dog
{
public:
	char Name[5];
};

Dog oneDog;
std::strncpy( oneDog.Name, "Dog", 5 );
function( oneDog.Name );

void function( char * name )		//if you want to change it
{
}
void function( const char * name )	//if you dont want to change it
{
}

There is an easier route however, as you are using C++ you can take advantage of the string class defined in the standard library. This is, by far, the prefered option:

(Toggle Plain Text)

#include <string>

class Dog
{
public:
	std::string Name;
};

Dog oneDog;
oneDog.Name = "Spot";
function( oneDog.Name );

void function( string & name )		//take a reference you can change
{
}
void function( const string & name )	//take a reference you can't change
{
}
void function( string name )			//create a copy of the name (slowest method)
{
}

The string class makes string operations much easier e.g. defining the == opertor to test if two strings are equal. See here for more examples.

edit: added in functions.

omdown · Oct 3rd, 2005, 9:10 PM

Wow, thanks man, it would be nice if this book listed that you had to strcpy to move the info in.

Sometimes I think I should write to the authors and bring up such things.

bl00dninja · Oct 4th, 2005, 12:48 AM

minimal working code:

(Toggle Plain Text)

#include <string>
    class Dog
    {
	  public:
		  std::string dogName;
    };

	int main()
	{
	    Dog oneDog;
	    oneDog.dogName = "pascal";
	    return 0;
	}

Oct 3rd, 2005, 7:54 PM	#1
omdown Newbie Join Date: Oct 2005 Posts: 3 Rep Power: 0	Problem assigning a string to a class object. So I'm trying to teach myself C++, but I've stumbled across a programming exercise that I can't figure out. The general idea is this: create a class called Dog which has a string field for the dog's name. In main(), create a dog and assign a name to it. The problem is, it doesn't seem to work the same way as assigning a string in a normal fashion. The way I've been going about it is: (Toggle Plain Text) // this is the class class Dog { public: char dogName[5]; }; // then skipping down to the main () section where I assign a name Dog oneDog; oneDog.dogName = {"Spot"}; // this is the class class Dog { public: char dogName[5]; }; // then skipping down to the main () section where I assign a name Dog oneDog; oneDog.dogName = {"Spot"}; Which produces errors indicating that the { } should not be there. When I remove the braces: (Toggle Plain Text) Dog oneDog; oneDog.dogName = "Spot"; Dog oneDog; oneDog.dogName = "Spot"; It gives me the error that "left operand must be l-value." How can I assign a string value to this? I'm also a little confused as to how you would pass an object's value to a function as well. The book I bought has a bad habit of going over how you would do something in an ideal situation but not if it's anything more complicated. Would it be the same as a regular variable like this: (Toggle Plain Text) function_name(oneDog.dogName) function_name(oneDog.dogName) Thanks in advance.

Oct 3rd, 2005, 8:25 PM	#2
Animatronic Programmer Join Date: Jun 2005 Posts: 99 Rep Power: 4	If you want the dog just to have a reference to the name and not take ownership of it ( not be able to change it ) do this: (Toggle Plain Text) class Dog { public: const char * Name; }; Dog oneDog; oneDog.Name = "Spot"; function( oneDog.Name ); void function( const char * name ) { } class Dog { public: const char * Name; }; Dog oneDog; oneDog.Name = "Spot"; function( oneDog.Name ); void function( const char * name ) { } If you want the dog to own the name, so it can change it's name later do this: (Toggle Plain Text) #include <cstring> class Dog { public: char Name[5]; }; Dog oneDog; std::strncpy( oneDog.Name, "Dog", 5 ); function( oneDog.Name ); void function( char * name ) //if you want to change it { } void function( const char * name ) //if you dont want to change it { } #include <cstring> class Dog { public: char Name[5]; }; Dog oneDog; std::strncpy( oneDog.Name, "Dog", 5 ); function( oneDog.Name ); void function( char * name ) //if you want to change it { } void function( const char * name ) //if you dont want to change it { } There is an easier route however, as you are using C++ you can take advantage of the string class defined in the standard library. This is, by far, the prefered option: (Toggle Plain Text) #include <string> class Dog { public: std::string Name; }; Dog oneDog; oneDog.Name = "Spot"; function( oneDog.Name ); void function( string & name ) //take a reference you can change { } void function( const string & name ) //take a reference you can't change { } void function( string name ) //create a copy of the name (slowest method) { } #include <string> class Dog { public: std::string Name; }; Dog oneDog; oneDog.Name = "Spot"; function( oneDog.Name ); void function( string & name ) //take a reference you can change { } void function( const string & name ) //take a reference you can't change { } void function( string name ) //create a copy of the name (slowest method) { } The string class makes string operations much easier e.g. defining the == opertor to test if two strings are equal. See here for more examples. edit: added in functions. Last edited by Animatronic; Oct 3rd, 2005 at 8:36 PM.

Oct 4th, 2005, 12:48 AM	#4
bl00dninja Programming Guru Join Date: Oct 2004 Location: namespace std Posts: 1,246 Rep Power: 5	minimal working code: (Toggle Plain Text) #include <string> class Dog { public: std::string dogName; }; int main() { Dog oneDog; oneDog.dogName = "pascal"; return 0; } #include <string> class Dog { public: std::string dogName; }; int main() { Dog oneDog; oneDog.dogName = "pascal"; return 0; } __________________ i put on my robe and wizard hat... Have you ever heard of Plato, Aristotle, Socrates?...Morons.

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Oct 3rd, 2005, 9:10 PM	#3
omdown Newbie Join Date: Oct 2005 Posts: 3 Rep Power: 0	Wow, thanks man, it would be nice if this book listed that you had to strcpy to move the info in. Sometimes I think I should write to the authors and bring up such things.

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Programming Forums > Application Development > C++

Problem assigning a string to a class object.