What are templates in C++ 2

Question

Hello everybody could anyone explain to me the difference between class temlplate member functio return type of type T (parameterized type) and an object of the claas return type

template<class T>
class array
{
public:
     array(T tx): tObj(tx){}
     T getObj()const{return tObj;}
     array GETTOBJ()const;
private:
     T tObj;
}

I have confusion: what is the difference between the return value of the functions getObj() and the GETTOBJ()?????

`T` is just a substitute for a type. If `T` happens to be anything but `A`, then the difference is clear. If it's `A`, they have the same return type. — chris, Feb 08 '14 at 21:58
I'm sorry I just wanted to write array instead of A. so do you mean that both the functions will return the same return type when specialisation? — Raindrop7, Feb 08 '14 at 22:01
If so needed at all, you would need to write `array` for any references to your template type! — πάντα ῥεῖ, Feb 08 '14 at 22:03

score 4 · Accepted Answer · answered Feb 08 '14 at 22:03

4

You've got:

an array
which holds tObj

The tObj's real value is given through the arrays' constructor, and the array remembers it.

The getObj method returns the tObj, the thing that array remembered.

The GETOBJ method returns an array. It's code is not shown, but the difference is already here: it returns some array, not the thing the array remembers.

It's a difference like "returns a candy from a box" and "return the box".

Btw. I think that your sig is missing type parameter. I mean:

not: array GETTOBJ()const;
but: array<T> GETTOBJ()const;

because array is a template, and array without type parameters is meaningless.

answered Feb 08 '14 at 22:03

quetzalcoatl

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for example if we instantiate it like this: array arInt(7); what is then the return value of the two functons getObj() and GETOBJ()???? – Raindrop7 Feb 08 '14 at 22:12
Please ignore my previous (deleted) comment. Too late and not enough sleep. – Angew is no longer proud of SO Feb 08 '14 at 22:13
2

@raindrop7 It's simple substitution. `getObj()` returns and `int`, and `GETOBJ` returns an `array`. – Angew is no longer proud of SO Feb 08 '14 at 22:14
ok I got it thnx 2 u. but could u be a bit accurate in returns an "array"? and what is the point in doing so return array??? – Raindrop7 Feb 08 '14 at 22:27
uh.. but we have just explained it..? what exactly do you not understand? do you understand the difference between returning "the box" and "the contents of the box"? – quetzalcoatl Feb 08 '14 at 22:29
@raindrop7 Just grab a [good book](http://stackoverflow.com/q/388242/1782465). – Angew is no longer proud of SO Feb 08 '14 at 22:32
ok guys thnk you I got it. the examp0le of the box is so useful many thnx – Raindrop7 Feb 08 '14 at 22:35

score 0 · Answer 2 · answered Feb 08 '14 at 22:12

In the folowing example "class T" is not defined so, then when initializing this class, you can add as parameters what ever you what. For example:

array <int> _array_var(); //OR
array <char> _array_var(); // OR WHAT TYPE YOU WHANT, EVEN SOME STRUCTURES OR CLASSES

in your example "T tOBJ()" will return something of the type you initialized. For example:

if you

array <int> _array_var(); // THAN T=int.... SO T tOBJ will return int type, equivalent to

int tOBJ();

and so on... class T = typename T

for more info check:

http://en.wikipedia.org/wiki/Template_(C%2B%2B)

ow yeah... and GETTOBJ returnes type of the class... always... the can be equal GETTOBJ with tOBJ in case you define:

array <array> _array_var();

What are templates in C++ 2

2 Answers2