Help with dynamically allocated array of pointer to a structure.

Code:

bool AddRecord(StudentRecord**& Records, int& numRecords){	
	struct StudentRecord
{
	char* firstName;	// student key
	char* lastName;	// student name
	int id;		//student id
	float mark;	// course mark
};

                StudentRecord**Temp; /*Declare array of pointers to a struct*/
	Temp=new StudentRecord*[1000]; /*dynamically allocate memory*/
	Temp[0]->firstName=new char[1000]; /*crashes here, why?*/

}

Code:

        StudentRecord**Temp; /*Declare array of pointers to a struct*/
	Temp=new StudentRecord*[1000]; /*dynamically allocate memory*/

at this point in your code you've allocated an array of pointers. Temp[0] is a pointer, but it's not pointing to anything yet so this line

Code:

Temp[0]->firstName=new char[1000];

crashes because you are trying to access uninitialised memory.

you need to do this first.

Code:

        StudentRecord**Temp; /*Declare array of pointers to a struct*/
	Temp=new StudentRecord*[1000]; /*dynamically allocate memory*/
	Temp[0] = new StudentRecord; // allocate space for student record
	Temp[0]->firstName=new char[1000]; // should work fine now

I'm not sure why you're using an array of pointers anyway. A vector would be easier and safer or if you really need a container of pointers use a boost::ptr_vector (see my sig)

The pointers in the array don't point to any StudentRecord structs yet. You have to create some StudentRecord structs and assign their addresses to the pointers in the array--before you can acesss the structs.

Originally Posted by ChaosEngine

Code:

        StudentRecord**Temp; /*Declare array of pointers to a struct*/
	Temp=new StudentRecord*[1000]; /*dynamically allocate memory*/

at this point in your code you've allocated an array of pointers. Temp[0] is a pointer, but it's not pointing to anything yet so this line

Code:

Temp[0]->firstName=new char[1000];

crashes because you are trying to access uninitialised memory.

you need to do this first.

Code:

        StudentRecord**Temp; /*Declare array of pointers to a struct*/
	Temp=new StudentRecord*[1000]; /*dynamically allocate memory*/
	Temp[0] = new StudentRecord; // allocate space for student record
	Temp[0]->firstName=new char[1000]; // should work fine now

I'm not sure why you're using an array of pointers anyway. A vector would be easier and safer or if you really need a container of pointers use a boost:tr_vector (see my sig)

Thank you it works!!

Glad it works, but for the record this is a much more C++ way of coding

Code:

struct StudentRecord
{
	std::string firstName;	// student key
	std::string lastName;	// student name
	int id;		//student id
	float mark;	// course mark
};

std::vector<StudentRecord> Temp(1000);
Temp[0].firstName = "Chaos"

shorter and safer, and uses a lot less memory (unless you know many people with 1000 character first names! )

Originally Posted by ChaosEngine

Glad it works, but for the record this is a much more C++ way of coding

Code:

struct StudentRecord
{
	std::string firstName;	// student key
	std::string lastName;	// student name
	int id;		//student id
	float mark;	// course mark
};

std::vector<StudentRecord> Temp(1000);
Temp[0].firstName = "Chaos"

shorter and safer, and uses a lot less memory (unless you know many people with 1000 character first names! )

I don't understand this code. It is an assignment requirement to program it with array of pointers to struct. see http://ca.geocities.com/web8146@rogers.com assignment0 for your reference.

Ok if you have to code it like that, fair enough. And I have to say it's refreshing to see someone asking for help with a problem in a decent way rather than asking us to do their assignment for them. Keep it up.