CODEWITHSUNDEEP
c++templatesstruct
Kalec
Kalec

Reputation: 2881

Having problems using a template on a struct

I am a beginner both in programming and in C++. I have used templates before but in a very limited way and I have no idea what I am doing wrong:

template <typename TElement>
struct list{    // if I try list<TElement> => list is not a template
    TElement data;
    struct list *next;
} node_type;    // also tried node_type<TElement>, other incomprehensible errors
node_type *ptr[max], *root[max], *temp[max];

I find the error a bit incomprehensible: "node_type does not name a type"
What am I doing wrong ?

What I intend to do:
I want to have a list of type (so I can use this on several completely different Abstract Data Types - ADTs), so I'd like to end up with something like this:

Activities list *ptr[max], *root[max], *temp[max]

if that makes any sense (Where Activities is a class but can be any other class).

Upvotes: 0

Views: 133

Answers (2)

Bojan Komazec
Bojan Komazec

Reputation: 9526

Try this:

template <typename TElement>
struct node{   
    TElement data;
    node* next;
};    

node<int>* ptr[max], *root[max], *temp[max];

One additional suggestion: avoid naming your custom types after types from standard C++ library (like list, vector, queue...which are all in namespace std). It is confusing and can result with name clashes (unless you place it in your own namespace which you'll need explicitly to use wherever you put using namespace std;).

Upvotes: 5

Jonathan Wakely
Jonathan Wakely

Reputation: 171263

Don't try to learn C++ by trial and error and guessing at the syntax, read a good book.

template <typename TElement>
  struct list{

This declares a struct template called list which has a single template parameter, TElement, which serves as an alias (only within the body of the template) for the type you instantiate the template with.

If you instantiate list<int> then TElement will refer to int. If you instantiate list<char> then TElement will refer to char. etc.

So the type you instantiate the template with will be substituted for TElement in your template definition.

The error you got when trying:

// if I try list<TElement> => list is not a template
template <typename TElement>
  struct list<TElement> {

is because this is not valid syntax, the error is telling you list isn't a template because at the point where you write list<TElement> you haven't finished declaring list yet, so the compiler has no idea what it is and you can't have a list of something if the list template isn't defined.

template <typename TElement>
  struct list{
    TElement data;
    struct list *next;
  }node_type;

This attempts to declares an object of type list, similar to this syntax:

struct A {
  int i;
} a;        // the variable 'a' is an object of type 'A'

But in your case list is not a type, it's a template. list<int> is a type, but list on its own is not a valid type, it's just a template that a type can be made from when you "fill in the blanks" i.e. provide a type to substitute for the parameter TElement

It looks like you weren't even trying to declare a variable anyway, just blindly guessing at syntax.

// also tried node_type<TElement>, other incomprehensible errors

That won't help either, node_type<TElement> is nonsense, if you want to declare a variable it needs a type, e.g. list<int>. The parameter TElement needs to be replaced with a type, it isn't a type itself. Stop trying to string together random bits of syntax hoping it will work. You could start by reading http://www.parashift.com/c++-faq/templates.html

On the last line:

node_type *ptr[max], *root[max], *temp[max];

node_type isn't a type so that won't work. Also, you should avoid getting into the bad habit of declaring multiple variables on a single line. It's much clearer to write:

int* p1;
int* p2;

Instead of 

int *p1, *p2;

Also, are you sure youu want arrays of pointers? Since you clearly don't know what you're doing it would be more sensible to use standard container types that do the work for you.

Upvotes: 2

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