Still celebrating National IT Professionals Day with 3 months of free Premium Membership. Use Code ITDAY17

x
Solved

typename T - statement return Element < T()

Posted on 2004-10-18
Medium Priority
282 Views

Question with regard to the typename T in this statement  'return Elem < T()'.  The typename per the example is 'int' hence the return statement equates to "return Elem < int()".    For Elem 3, we have " return 3 < int() " but I'm confused on what the default construction call for the built in type - in this case - int() equates to?   ie.  "return 3 < ???"

# include <iostream>
# include <string>
# include <iterator>
# include <algorithm>
# include <sstream>
# include <iomanip>
# include <vector>

using namespace std;

// [Predicate] object.
struct RemoveLessThanZero
{
template<typename T>
bool operator()( const T& Elem ) const
{
return Elem < T();
}
};

int main()
{
int Array[] = { 3, 2, 1, 0, -1, -2, -3, -4 };

// Calculate the size of the array.
static const std::size_t
NrElems( sizeof Array / sizeof* Array );

// Construct the vector with the 'Array' elements.
std::vector<int> V( Array, Array + NrElems );

// Print the contents of the vector to stdout.
std::copy( V.begin(), V.end(),
std::ostream_iterator<int>( std::cout, " " ) );

// Remove all negative values from the vector, with
// the help of our [Predicate] object.
V.erase( std::remove_if( V.begin(), V.end(),
RemoveLessThanZero() ), V.end() );

// Sort the vector elements prior to printing.
std::sort( V.begin(), V.end() );
std::cout << std::endl;

// Again, print the contents of the vector to stdout.
std::copy( V.begin(), V.end(),
std::ostream_iterator<int>( std::cout, " " ) );

return 0;
}
0
Question by:forums_mp
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

• Help others & share knowledge
• Earn cash & points
• 3
• 3
• 2
• +1

LVL 48

Accepted Solution

AlexFM earned 90 total points
ID: 12339317
I guess int() calls int constructor which creates int instance and sets it to 0. 3 < int() is like 3 < 0.

Line
i = int();

looks in Disassembly window by the following way:

22:       i = int();
004114DF   mov         dword ptr [ebp-4],0

This means, i = 0.

Other test:

class A
{
public:
A()
{
cout << "A" << endl;
}
};

A a;
a = A();

Result is:
A
A

0

LVL 39

Expert Comment

ID: 12339359
>>>> return Elem < int()".

That statement compiles but doesn't work for build-in types. So, i would call it a bug. T() creates a new int in your case, that isn't initialized --> the result is undefined.

To make it work, you have to define a member variable of type T that takes an 'empty' or 'zero' instance of type T. You can pass the empty value by the constructor or - more usual - by providing a separate init or set function. The statement above then turns to

return Elem < m_empty;

Regards, Alex

0

LVL 39

Expert Comment

ID: 12339406
>>>> I guess int() calls int constructor which creates int instance and sets it to 0

No, only VC Debugger initializes 'new' instances with zeroes. There is no 'int' constructor in C++.

Regards, Alex

0

Author Comment

ID: 12340283

>> So, i would call it a bug.
A bug considering the output made sense on repeated runs ..

>>> To make it work, you have to define a member variable of type T that takes an 'empty' or 'zero' instance of type T. You can pass the empty value by the constructor or - more usual - by providing a separate init or set function. The statement above then turns to

For instance.
0

LVL 15

Assisted Solution

efn earned 60 total points
ID: 12344159
It's deja vu all over again...

http://www.experts-exchange.com/Programming/Programming_Languages/Cplusplus/Q_21031400.html

Summarizing:  A plain old data object whose initializer is an empty set of parentheses should be zero-initialized, as the first Alex said.  However, there are compilers that don't do this right.

--efn
0

Author Comment

ID: 12346422

efn

The issue is not default initilization.  I understand that.  I realized though that I've interpreted the call to  V.erase/remove_if incorrectly.   The comparision is
3 < 0
2 < 0
:
:
-4 < 0

For some reason I thought (momentary insanity i suppose :)) about an element by element comparison and was trying to figure out how T() fit in..
0

LVL 48

Expert Comment

ID: 12346823
>> No, only VC Debugger initializes 'new' instances with zeroes.

Debugger ??
0

LVL 39

Expert Comment

ID: 12347258
>> Debugger ??

I meant VC Built in 'Debug Configuration'
0

LVL 48

Expert Comment

ID: 12397915
Program built by Microsoft VC++ compiler in Debug Configuration.
0

Featured Post

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

IntroductionThis article is the second in a three part article series on the Visual Studio 2008 Debugger.  It provides tips in setting and using breakpoints. If not familiar with this debugger, you can find a basic introduction in the EE article locâ€¦
Jaspersoft Studio is a plugin for Eclipse that lets you create reports from a datasource.  In this article, we'll go over creating a report from a default template and setting up a datasource that connects to your database.
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.
The viewer will be introduced to the member functions push_back and pop_back of the vector class. The video will teach the difference between the two as well as how to use each one along with its functionality.
Suggested Courses
Course of the Month8 days, 11 hours left to enroll