Hello Experts!
I have the following Array template class see below.
I execute these three statements
statement 1: Array<int> x(5);
statement 2: cin >>x[1];
statement 3: Array<int>::element_type y = x[1];
but I can't understand the last one which is
Array<int>::element_type y = x[1];
We have a typedef T element_type; in the template Array class see below
and in my example I use int as the type parameter for T so
that would mean that it says
Array<int>::int y = x[1];
this is even more confusing for me.
If the Array template class for example had a static variable with name
number then you would
use Array<int>::number; to access this static variable.
template <typename T>
class Array
{
public:
Array(size_t=100); // size_t is predefined
Array(const Array<T>&);
Array<T>& operator=(const Array<T>&);
virtual ~Array();
virtual const T& operator[](int) const;
virtual T& operator[](int);
int size() const;
typedef T element_type;
protected:
size_t size_;
T* array;
};
Many thanks
//Tony 7 2120
Tony Johansson wrote: I have the following Array template class see below. I execute these three statements statement 1: Array<int> x(5); statement 2: cin >>x[1]; statement 3: Array<int>::element_type y = x[1]; but I can't understand the last one which is Array<int>::element_type y = x[1]; We have a typedef T element_type; in the template Array class see below and in my example I use int as the type parameter for T so that would mean that it says Array<int>::int y = x[1]; this is even more confusing for me.
No, it doesn't mean it says "Array<int>::int y = x[1];".
It means what it says,
Array<int>::element_type y = x[1];
or you can rewrite it to say
int y = x[1];
'element_type' does not mean 'int' by itself. It only means 'int' if
used _along_ with the class in which it's defined. For example, in
Array<string>, 'element_type' would mean 'string'. So, if you write
Array<string>::element_type
it would be the same as writing
string
If the Array template class for example had a static variable with name number then you would use Array<int>::number; to access this static variable.
Yes.
V
"Victor Bazarov" <v.********@comAcast.net> skrev i meddelandet
news:aV*******************@newsread1.mlpsca01.us.t o.verio.net... Tony Johansson wrote: I have the following Array template class see below. I execute these three statements statement 1: Array<int> x(5); statement 2: cin >>x[1]; statement 3: Array<int>::element_type y = x[1]; but I can't understand the last one which is Array<int>::element_type y = x[1]; We have a typedef T element_type; in the template Array class see below and in my example I use int as the type parameter for T so that would mean that it says Array<int>::int y = x[1]; this is even more confusing for me.
No, it doesn't mean it says "Array<int>::int y = x[1];". It means what it says,
Array<int>::element_type y = x[1];
or you can rewrite it to say
int y = x[1];
'element_type' does not mean 'int' by itself. It only means 'int' if used _along_ with the class in which it's defined. For example, in Array<string>, 'element_type' would mean 'string'. So, if you write
Array<string>::element_type
it would be the same as writing
string
If the Array template class for example had a static variable with name number then you would use Array<int>::number; to access this static variable.
Yes.
V
I can't understand how Array<int>::element_type
is the same as int in my example.
or that Array<string>::element_type is the same as string
Some question that might help me to understand this.
How does the scope operation be understood here?
What meaning have T in the typedef T element_type;?
I want to understand the scope operatorn in such way that there is some
variable
in the Array template class that has name element_type.
The fully name of the Array template class when you have int as the
parameter is
Array<int>
Can you give me some more explanation about Array<int>::element_type
that might help me to understand this.
//Tony
Tony wrote: "Victor Bazarov" <v.********@comAcast.net> skrev i meddelandet news:aV*******************@newsread1.mlpsca01.us.t o.verio.net...
Tony Johansson wrote:
I have the following Array template class see below. I execute these three statements statement 1: Array<int> x(5); statement 2: cin >>x[1]; statement 3: Array<int>::element_type y = x[1]; but I can't understand the last one which is Array<int>::element_type y = x[1]; We have a typedef T element_type; in the template Array class see below and in my example I use int as the type parameter for T so that would mean that it says Array<int>::int y = x[1]; this is even more confusing for me. No, it doesn't mean it says "Array<int>::int y = x[1];". It means what it says,
Array<int>::element_type y = x[1];
or you can rewrite it to say
int y = x[1];
'element_type' does not mean 'int' by itself. It only means 'int' if used _along_ with the class in which it's defined. For example, in Array<string>, 'element_type' would mean 'string'. So, if you write
Array<string>::element_type
it would be the same as writing
string
If the Array template class for example had a static variable with name number then you would use Array<int>::number; to access this static variable.
Yes.
V
I can't understand how Array<int>::element_type is the same as int in my example.
Try a simple substitution for this struct template:
template<class T> struct A { typedef T type; };
If I instantiate A<int>, what happens? It's as if I declare a type
struct A<int> /* incorrect syntax, but will do */ {
typedef int type;
};
(anywhere 'T' was used inside the 'A' definition, I put 'int'). Now, what
does 'A<int>::type' stand for? It's typedef'ed to be the same as 'int'.
So, anywhere I want to use 'int' I _can_ use 'A<int>::type' because they
are defined to mean the same thing.
or that Array<string>::element_type is the same as string Some question that might help me to understand this. How does the scope operation be understood here?
It's not "the scope operation". It's what you may call a "class-wide
member access operation". A typedef inside a class is a class-wide
member. It has a name which you can access using the '::'. The name
of that member in my example is 'type'. A way to access it is to supply
the name of the _class_ with the scope resolution operator:
A<int> :: type
^^^^^^ ^^ ^^^^
Name of the class Scope resolution Member name
What meaning have T in the typedef T element_type;?
Just like with any other typedef, 'element_type' is now the synonym for
whatever 'T' is. For any particular instantiation of the template, 'T'
means something specific. So, in any particular instantiation of your
'Array' template, 'element_type' will mean something in particular.
I want to understand the scope operatorn in such way that there is some variable in the Array template class that has name element_type.
I don't understand the sentence above.
The fully name of the Array template class when you have int as the parameter is Array<int>
It's called "instantiation", not "fully name of the Array template class".
Can you give me some more explanation about Array<int>::element_type that might help me to understand this.
See above. And, yes, I can give you more if you ask more specific
questions.
V
"Victor Bazarov" <v.********@comAcast.net> skrev i meddelandet
news:bl******************@newsread1.mlpsca01.us.to .verio.net... Tony wrote: "Victor Bazarov" <v.********@comAcast.net> skrev i meddelandet news:aV*******************@newsread1.mlpsca01.us.t o.verio.net...
Tony Johansson wrote:
I have the following Array template class see below. I execute these three statements statement 1: Array<int> x(5); statement 2: cin >>x[1]; statement 3: Array<int>::element_type y = x[1]; but I can't understand the last one which is Array<int>::element_type y = x[1]; We have a typedef T element_type; in the template Array class see below and in my example I use int as the type parameter for T so that would mean that it says Array<int>::int y = x[1]; this is even more confusing for me.
No, it doesn't mean it says "Array<int>::int y = x[1];". It means what it says,
Array<int>::element_type y = x[1];
or you can rewrite it to say
int y = x[1];
'element_type' does not mean 'int' by itself. It only means 'int' if used _along_ with the class in which it's defined. For example, in Array<string>, 'element_type' would mean 'string'. So, if you write
Array<string>::element_type
it would be the same as writing
string
If the Array template class for example had a static variable with name number then you would use Array<int>::number; to access this static variable.
Yes.
V
I can't understand how Array<int>::element_type is the same as int in my example.
Try a simple substitution for this struct template:
template<class T> struct A { typedef T type; };
If I instantiate A<int>, what happens? It's as if I declare a type
struct A<int> /* incorrect syntax, but will do */ { typedef int type; };
(anywhere 'T' was used inside the 'A' definition, I put 'int'). Now, what does 'A<int>::type' stand for? It's typedef'ed to be the same as 'int'. So, anywhere I want to use 'int' I _can_ use 'A<int>::type' because they are defined to mean the same thing.
or that Array<string>::element_type is the same as string Some question that might help me to understand this. How does the scope operation be understood here?
It's not "the scope operation". It's what you may call a "class-wide member access operation". A typedef inside a class is a class-wide member. It has a name which you can access using the '::'. The name of that member in my example is 'type'. A way to access it is to supply the name of the _class_ with the scope resolution operator:
A<int> :: type ^^^^^^ ^^ ^^^^ Name of the class Scope resolution Member name
What meaning have T in the typedef T element_type;?
Just like with any other typedef, 'element_type' is now the synonym for whatever 'T' is. For any particular instantiation of the template, 'T' means something specific. So, in any particular instantiation of your 'Array' template, 'element_type' will mean something in particular.
I want to understand the scope operatorn in such way that there is some variable in the Array template class that has name element_type.
I don't understand the sentence above.
The fully name of the Array template class when you have int as the parameter is Array<int>
It's called "instantiation", not "fully name of the Array template class".
Can you give me some more explanation about Array<int>::element_type that might help me to understand this.
See above. And, yes, I can give you more if you ask more specific questions.
V
I think I start to understand it now. I just check with you Victor hope you
read this.
Assume there exist a valid type x that I use to get a specialisation of the
template Array.
If you instantiate the template class Array and pass type x as parameter you
get the instantiation name Array<x>.
If I want to access a class-wide member in the Array template I use the ::
In my case I have a class-wide member with name element_type so I access
this element_type by giving statement
Array<x>::element_type
this whole expession can always be substituted with the type that this
template was called with in my example here it's x because we have this
typedef T element_type
Does my thinking seems to be about correct?
//Tony
Tony Johansson wrote: [..] Assume there exist a valid type x that I use to get a specialisation of the template Array. If you instantiate the template class Array and pass type x as parameter you get the instantiation name Array<x>. If I want to access a class-wide member in the Array template I use the :: In my case I have a class-wide member with name element_type so I access this element_type by giving statement Array<x>::element_type this whole expession can always be substituted with the type that this template was called with in my example here it's x because we have this typedef T element_type
Does my thinking seems to be about correct?
Yes... I am trying to find some kind of hidden pothole in that or
some additional condition that has to be satisfied, but it all seems
to be fine.
I recommend looking at the standard library containers and how they
define 'value_type' members. That's a common requirement for all of
them, and you should be able to rely on that, for example.
V
"Array<int>::element_type" is really "typename
Array<int>::element_type"
and in fact the newer compilers issue warnings asking you to write it
this way.
It is a hint to the compiler to look for a type definition rather than
a data/function member.
neerajk wrote: "Array<int>::element_type" is really "typename Array<int>::element_type"
and in fact the newer compilers issue warnings asking you to write it this way.
Really? Which ones? Please post a complete compilable piece of code
that illustrates what you're talking about.
V This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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