What does "missing template argument" mean? - c++

Question

I'm pretty new to C++ and this site so there are bound to be errors. When I try to compile my code I get errors like error: missing template argument before 'b'. I've been searching the world for answers for hours and it has led me here.
My assignment is to implement a templated class Collection that stores a collection of
Objects using an array, along
with the current size of the collection.
#include <iostream>
#include "collection.h"
using namespace std; v
int main(int argc, char* argv[])
{
collection b; //<----error missing template argument before 'b'
return 0;
}
#ifndef COLLECTION_H
#define COLLECTION_H
#include <iostream>
template <typename obj>
class collection
{
public:
collection();
bool isEmpty() const;
void makeEmpty();
void insert(obj val);
void remove(obj val);
bool contains(obj val) const;
private:
size_t size;
obj* col[];
};
#endif
#include "collection.h"
template <typename obj>
collection<obj>::collection() :size(10)
{
col = new obj*[size];
}
template <typename obj>
bool collection<obj>::isEmpty() const
{
for(size_t k = 0; k < size; k++)
{
if(col[k] != NULL)
return false;
}
return true;
}
template <typename obj>
void collection<obj>::makeEmpty()
{
for(size_t k = 0; k < size; k++)
{
col[k] = NULL;
}
}
template <typename obj>
void collection<obj>::insert(obj val)
{
int temp = 0;
for(size_t s = 0; s < size; s++)
{
if(col[s] != NULL)
temp++;
}
if(temp >= size)
{
obj* temp = new obj*[size*2];
for(size_t c = 0; c < size; c++)
temp[c] = col[c];
delete col;
col = temp;
}
else
col[temp] = val;
}
template <typename obj>
void collection<obj>::remove(obj val)
{
for(size_t x = 0; x < size; x++)
{
if (col[x] == val)
{
for(size_t y = x; y < size-1; y++)
{
col[y] = col[y+1];
}
col[size-1] = NULL;
return;
}
}
}
template <typename obj>
bool collection<obj>::contains(obj val) const
{
for(size_t z = 0; z < size; z++)
{
if(col[z] == val)
return true;
}
return false;
}

Answer 1

You have to say what it's a collection of.
template <class A> class collection {}
requires that you use it as
collection<int> b;
or some appropriate type. That then makes a collection of ints. You haven't told it what you want a collection of.

Answer 2

First : Instantiate template by type. So if you have template <typename obj> class T {...}; you should use it like
void main {
T<int> t;
T<bool> t1; // .. etc
}
You can use a template with default value for the typename parameter defined in the class template declaration
template <typename obj = int> class T {/*...*/};
void main {
T<> t;
}
but anyway you should put empty angle brackets when use it without parameter.
Second: While declaring template, place it whole in the header file. Each definition of his methods should be in the file "*.h", don't ask me why, just don't split it to the header and "cpp" file.
Hope it helps.

Answer 3

Well, you're missing a template argument. You can't create a collection object, that's just a template.
You can only create e.g. a collection<int> or collection<std::string>.

Answer 4

You need to specify the type for template, like int or some other type:
collection<int> b;

Answer 5

There are a large number of errors in your code. First define your template in a header file:
In collection.h put the following:
#ifndef COLLECTION_H
#define COLLECTION_H
template <typename obj>
class collection
{
public:
collection() {}
bool isEmpty() const;
void makeEmpty();
void insert(obj val);
void remove(obj val);
bool contains(obj val) const;
private:
size_t size;
obj* col[];
};
#endif
Then in a .cpp file inside a main function do the following:
collection<int> b;
Instead of int you can use a different type but the main point is that YOU NEED A TYPE to instantiate a template.