How check a variable is null?

Hoe do I check that something's null, like in:

Code:

---

class Test
{
private:
    std::string name;
    Test prev;

    Test(Test previous, std::string n) : prev( previous ), name( n )
    {}

    Test(std::string n) : name( n ) {}

    const char* test()
    {
          //I want to do smth similar to this type of code:
        if ( prev != null ) return ...elided...;
          ..rest elided...
    }
}

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How do I check to see if "prev" was ever set?

NULL == 0

So prev would have to be a pointer for that to work.

Quote:

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Originally Posted by Raigne

NULL == 0

So prev would have to be a pointer for that to work.

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Right I know. :) I want to know how I can check if that variable was ever set. How do I do that? (Because if it wasn't set and I try to call a member on it, I'll get a runtime error)

So how do I know it's ok to call a member on it?

[QUOTE=6tr6tr;740392]Hoe do I check that something's null, like in:

Code:

---

class Test
{
private:
    std::string name;
    Test prev;

    Test(Test previous, std::string n) : prev( previous ), name( n )
    {}

    Test(std::string n) : name( n ) {}

    const char* test()
    {
          //I want to do smth similar to this type of code:
        if ( prev != null ) return ...elided...;
          ..rest elided...
    }
}

---

Unfortunately this code example will not even compile, because you have an instance of "Test" inside "Test" itself, in other words, an infinite recursion. You have to use a pointer or reference -- in this case, a pointer, since you want null checking.

The value of "null" is 0. So to see if a pointer is pointing "nowhere," just compare it with 0. In C we had the NULL constant -- in C++ it is preferred to just use 0.

A class instance is never not initialized (unless it is a POD), so you don't have to worry about an instance being null.

>> So how do I know it's ok to call a member on it?
It should always ok to call a member function (or use a member variable) of an instance of a class. It is only a pointer that could cause a problem in your program if you use it without being initialized.

Quote:

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Originally Posted by Daved

A class instance is never not initialized (unless it is a POD), so you don't have to worry about an instance being null.

>> So how do I know it's ok to call a member on it?
It should always ok to call a member function (or use a member variable) of an instance of a class. It is only a pointer that could cause a problem in your program if you use it without being initialized.

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OK, thanks! That makes sense now. It's very different from java. :)

Quote:

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Originally Posted by Daved

A class instance is never not initialized (unless it is a POD), so you don't have to worry about an instance being null.

>> So how do I know it's ok to call a member on it?
It should always ok to call a member function (or use a member variable) of an instance of a class. It is only a pointer that could cause a problem in your program if you use it without being initialized.

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Hmmm, my program's still not working. I'm now passing a pointer "Test* previous" and setting it equal to a pointer in th class. But when I check:

if ( test > 0 )

It always returns as true even if I pass "NULL" into the constructor. What am i doing wrong?

>> It's very different from java.
Yes, very different. If you want a pointer or reference, you have to explicitly state that by using * or &. Otherwise, C++ does a lot of stuff by value, and to be good at it you need to be able to do things by value in addition to by reference.

>> What am i doing wrong?
I don't know, can you post the code?

BTW, normally you use if (test != 0).

Don't use relational operators on pointers. Use !=.

But that's probably not the problem. Code?

By the way, in Java, all variables of object type are pointers (Java calls them references). In C++, variables are objects unless you explicitly make them pointers or references.

Quote:

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Originally Posted by CornedBee

Don't use relational operators on pointers. Use !=.

But that's probably not the problem. Code?

By the way, in Java, all variables of object type are pointers (Java calls them references). In C++, variables are objects unless you explicitly make them pointers or references.

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Code:

---

class Test
{

private:
      std::string name;
      Test* test;

public:
      Test(std::string n)
      {
            Test( n, NULL );
      }

      Test(std::string n, Test* other): name( n )
      {
            test = other;

            //This says it is zero when I call other constructor, so it's right
            if ( test == 0 ) printf( "null in constructor" );
      }

      const char* print()
      {
            //This does NOT print out, somehow it's no longer 0
            if ( test == 0 ) printf( "null in print" );
            return n;
      }


}


int main()
{
      Test t( "name" );

      //This says it's not null
      t.print();
      return 0;
}

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EDIT: OK, why is this?

Code:

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If I change the first constructor to this:

      Test(std::string n) :name( n );
      {
            test = NULL;
      }

then it works!! Why? What's the difference?

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You can't call one constructor from another constructor. You can make one constructor with a default value for test:

Code:

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Test(std::string n, Test* other = 0): name( n ), test(other)

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Otherwise you just have to initialize name and test in both constructors separately.

Quote:

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Originally Posted by Daved

You can't call one constructor from another constructor. You can make one constructor with a default value for test:

Code:

---

Test(std::string n, Test* other = 0): name( n ), test(other)

---

Otherwise you just have to initialize name and test in both constructors separately.

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Yep, thanks! I just figured that out. You can do it in java so I didn't realize there was no way to do it in C++. Too bad, it's very convenient.

Quote:

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Originally Posted by 6tr6tr

Yep, thanks! I just figured that out. You can do it in java so I didn't realize there was no way to do it in C++. Too bad, it's very convenient.

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Supposedly, the ability to chain constructors is coming in C++0x.

Quote:

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Originally Posted by 6tr6tr

Yep, thanks! I just figured that out. You can do it in java so I didn't realize there was no way to do it in C++. Too bad, it's very convenient.

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Yeah, but Java doesn't let you set default values, so C++ loses a point and so does Java. ;)

Quote:

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Supposedly, the ability to chain constructors is coming in C++0x.

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It's in the working paper.

Code:

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foo::foo() : foo(x, y, z) {}

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The bad part for Java programmers isn't that C++ doesn't let you chain constructors. It's that the syntax used for this in Java - with this replaced by the class name - is actually valid. It just doesn't really do anything.

Quote:

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Originally Posted by CornedBee

It's in the working paper.

Code:

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foo::foo() : foo(x, y, z) {}

---

The bad part for Java programmers isn't that C++ doesn't let you chain constructors. It's that the syntax used for this in Java - with this replaced by the class name - is actually valid. It just doesn't really do anything.

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Exactly! It had me confused for quite a while because it accepted my code without even throwing a warning.