Sort function does not work with function object created on stack?

Question

#include<iostream>
#include<vector>
#include<algorithm>
class Integer
    {
public:
    int m;
    Integer(int a):m(a){};
    };
class CompareParts
    {
    public:
        bool operator()(const Integer & p1,const Integer & p2)
            {
            return p1.m<p2.m;
            }
    }obj1;
int main()
    {
    std::vector<Integer> vecInteger;
    vecInteger.push_back(Integer(12));
    vecInteger.push_back(Integer(13));
    vecInteger.push_back(Integer(5));
    vecInteger.push_back(Integer(7));
    vecInteger.push_back(Integer(9));
    Integer obj2();
    std::sort(vecInteger.begin(),vecInteger.end(),obj1);
    std::sort(vecInteger.begin(),vecInteger.end(),obj2);
    }

why is obj2 in second sort function leads to compiler error.

Be sure to add the compile error to the question. That will help diagnose the problem. — nathan
– nathan, Commented Aug 19, 2009 at 14:27
"Sort function does not with function object created on stack?" You're missing a word in the title. I had to click through to see you meant COMPILE rather than WORK. — Pod
– Pod, Commented Aug 19, 2009 at 14:28

legends2k · Accepted Answer · 2013-10-15 07:48:34Z

15

Integer obj2() isn't the definition of an object, it is the declaration of a function named obj2 returning an Integer (put it outside any function to understand why it is so). This occurs also sometimes with more complex constructions where it can be even more confusing. Some name this the most vexing parse.

Here is the promised example of a more complex case:

struct Foo {};
struct Bar { Bar(Foo); };

Bar quxx(Foo()); // quxx is a function

Here quxx is a function returning a Bar and taking (a pointer) to a function returning a Foo and without parameters. You could write the same declaration more clearly like this:

Bar quxx(Foo (*fn)()); // quxx is the same function as above

To get the definition of a variable initialized with the constructor taking a Foo, you can add a level of parenthesis:

Bar quux((Foo())); // quux is a variable

edited Oct 15, 2013 at 7:48

legends2k

33.2k28 gold badges131 silver badges238 bronze badges

answered Aug 19, 2009 at 14:26

AProgrammer

52.6k8 gold badges96 silver badges149 bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

Kim Gräsman Over a year ago

Yes, and the correct syntax is "Integer obj2;". Add to that the fact that you probably wanted to create a CompareParts object, and you should be closer to results.

Johannes Schaub - litb Over a year ago

Um, sure it's right. The point where a declaration appears is not necessary the point where the entity it declares lives. In this case, it declares a global function, using a local declaration.

anon Over a year ago

AProgrammer is correct - it's a function declaration. And yes, you cn declare a function inside another function.

anon Over a year ago

@Chickencha C++ takes from C the overriding rule that if something can possibly be parsed as a declaration, it will be parsed as a declaration.

Johannes Schaub - litb Over a year ago

Some comment from the Standard: 8.5 Initializers /8: "[Note: since () is not permitted by the syntax for initializer, X a(); is not the declaration of an object of class X, but the declaration of a function taking no argument and returning an X. The form () is permitted in certain other initialization contexts (5.3.4, 5.2.3, 12.6.2). ]" Hope that explains the matter.

|

Community · Accepted Answer · 2017-05-23 12:19:40Z

3

Because obj2 is a function. See this

edited May 23, 2017 at 12:19

CommunityBot

11 silver badge

answered Aug 19, 2009 at 14:29

Tadeusz Kopec for Ukraine

12.5k6 gold badges60 silver badges87 bronze badges

Comments

Jeff Leonard · Accepted Answer · 2009-08-19 14:35:47Z

1

obj2 is not a BinaryPredicate and is invalid as the third parameter to std::sort

obj2 needs to be something like

// Return whether first element is greater than the second
bool UDgreater ( int elem1, int elem2 )
{
   return elem1 > elem2;
}

or the functor type used by obj1.

answered Aug 19, 2009 at 14:35

Jeff Leonard

3,3147 gold badges31 silver badges27 bronze badges

Comments

KV Prajapati · Accepted Answer · 2009-08-19 14:44:21Z

There is no definition of no argument constructor.

Use, Integer obj2(0);

#include<iostream>
#include<vector>
#include<algorithm>
class Integer
{
     public:
     int m;
     Integer(int a):m(a){};
     bool operator()(const Integer p1,const Integer p2)
     {
      return p1.m<p2.m;
     }
};
class CompareParts
{    public:
     bool     operator()(const Integer  p1,const Integer p2)
     {
         return p1.m<p2.m;
         }
}obj1;

int main()
{
    std::vector<Integer> vecInteger;
    vecInteger.push_back(Integer(12));
    vecInteger.push_back(Integer(13));
    vecInteger.push_back(Integer(5));
    vecInteger.push_back(Integer(7));
    vecInteger.push_back(Integer(9));
    Integer obj2(0);
    std::sort(vecInteger.begin(),vecInteger.end(),obj1);
    std::sort(vecInteger.begin(),vecInteger.end(),obj2);

    return 0;
}

Jagannath · Accepted Answer · 2009-08-19 14:43:03Z

#include<iostream>
#include<vector>
#include<algorithm>

class Integer
{
public:
int m;
Integer(int a):m(a){};
};

class CompareParts {
public:
bool operator()(const Integer & p1,const Integer & p2)
{
return p1.m }
};

int main()
{
std::vector vecInteger;
vecInteger.push_back(Integer(12));
vecInteger.push_back(Integer(13));
vecInteger.push_back(Integer(5));
vecInteger.push_back(Integer(7));
vecInteger.push_back(Integer(9));

std::sort(vecInteger.begin(),vecInteger.end(),CompareParts()); 
typedef vector<Integer>::const_iterator Iter;
Iter beg = vecInteger.begin();
Iter end = vecInteger.end();

for (Iter iter = beg; iter != end; ++iter)
    cout << (*iter).m << " ";

cout << endl;

}

Output: 5 7 9 12 13

Collectives™ on Stack Overflow

Sort function does not work with function object created on stack?

5 Answers 5

9 Comments

Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

9 Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related