error: no match for ‘operator==’ in ‘boiler::uniqueInstance == 0l’

Question

#include <iostream>
using namespace std;

class boiler
{
private:
    static boiler uniqueInstance;

    bool boilerEmpty;
    bool mixtureBoiled;

    boiler()
    {
        boilerEmpty = true;
        mixtureBoiled = false;
    }

public:
    static boiler getInstance()
    {
        if(uniqueInstance == NULL)
        {
            uniqueInstance = new boiler();
        }

        return uniqueInstance;
    }
};

The above code returns the error stated in the title.

anisha@linux-y3pi:~> g++ -Wall test.cpp
test.cpp: In static member function ‘static boiler boiler::getInstance()’:
test.cpp:22:26: error: no match for ‘operator==’ in ‘boiler::uniqueInstance == 0l’
test.cpp:24:34: error: no match for ‘operator=’ in ‘boiler::uniqueInstance = (operator new(2u), (<statement>, ((boiler*)<anonymous>)))’
test.cpp:5:1: note: candidate is: boiler& boiler::operator=(const boiler&)

why? Can't we compare an "object" to a NULL? Are there some syntax problems?

If you want to use `==` with your own class you need an `operator==` overload. — chris, May 04 '12 at 06:03
Might help you : http://stackoverflow.com/questions/1008019/c-singleton-design-pattern/1008289#1008289 — , May 04 '12 at 06:04
Also, this is also useful to understand why C++ is not Java : http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — , May 04 '12 at 06:07
@Muggen Actually, I am following Head first design pattersn book. Perhaps that's the reason, that book talks in Java, I talk in C++. — Aquarius_Girl, May 04 '12 at 06:09
@Muggen why did you remove the tags "static method". I do have a static method here. — Aquarius_Girl, May 04 '12 at 06:17
@Anisha, well I don't think that tag is really useful at this question. — , May 04 '12 at 06:19
The static method is central to this singleton design, but it has no relevance to the error itself. — juanchopanza, May 04 '12 at 06:38

juanchopanza · Accepted Answer · 2012-05-04T06:18:34.347

2

You probably need a pointer:

static boiler* uniqueInstance;

since then you are initializing it with new here:

uniqueInstance = new boiler ();

The compiler is telling you that it cannot compare an instance of boiler with an int (a long actually). This comparison doesn't exist. Pointers can be compared to integral types, which allows the comparison to 0.

Here the comparison to NULL serves as a means to check whether your pointer has been initialized already. It is not obvious how to do this with instances, there is no concept of invalid or uninitialized instance. You can compare an object to NULL by defining the appropriate operator==, but the comparison might not make sense, since NULL is often just another name for 0.

edited May 04 '12 at 06:18

answered May 04 '12 at 06:06

juanchopanza

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very grateful to you. This did the trick. But, why was the compiler complaining about the == operator? – Aquarius_Girl May 04 '12 at 06:07
@AnishaKaul I just added an explanation. – juanchopanza May 04 '12 at 06:10
That's a great explanation. Can you suggest what topics should I read to understand such kind of errors? – Aquarius_Girl May 04 '12 at 06:11
1

Well, read up on pointers, dynamically allocated pointers (using `new` has important implications), and as for the error itself, you can have a look at operator overloading. – juanchopanza May 04 '12 at 06:15

score 1 · Answer 2 · answered May 04 '12 at 06:07

You declare uniqueInstsance as a class instance:

 static boiler uniqueInstance;

but sometimes treat it as a pointer:

uniqueInstance = new boiler ();

I think you should declare it as a pointer so you can dynamically allocate the instance:

static boiler* uniqueInstance;

You'll also want to change getInstance() to return a pointer:

static boiler* getInstance() //...

Finally make sure you actually have a definition of uniqueInstance somewhere outside of the class declaration. In one spot in a .cpp file (not a header):

boiler* boiler::uniqueInstance;

yes, that was it. thanks. but the compiler was complaining about == operator, so I couldn't guess it. — Aquarius_Girl, May 04 '12 at 06:08

error: no match for ‘operator==’ in ‘boiler::uniqueInstance == 0l’

2 Answers2