STL and pointer/reference question

Hi all,

Im rather new to C++ (I´m changing from Java to C++), so please bear with me.
I have a question about STL and returning a vector array
from a static method.

How do I do it correctly, so that the function contructs
a vector that can be returned to the caller. I know if I
create a local object from a class and return it`s reference, it will point to null (if I´m rigth) after the
function call. So the object should be created with the new
operator. But how do I do it with template call??
I just get errors when I try to compile.

It should go something like this(please, do correct my syntax, pointers and references are used very wrong, I think):

static &vector<MyClass> loadFromFile()
{
vector<MyClass> *data = NULL;
try
{
// load from file and populate the vector
}
catch(...)
{ }

return &data;
}

help apprecitaed!

-mark


ps. could somebody tell me where to fing a decent
standard C++ API (yes, I`m migrating from Java to C++). All I have found are rather hard to use / read or incomplete.

Thank you.

One solution is to encapsulate the STL container within a structure. Create the object on the heap and pass its pointer.

Kuphryn

Quote:

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Originally posted by halmark6Z
How do I do it correctly, so that the function contructs
a vector that can be returned to the caller.

---

A vector has proper copy semantics, so there is no need to be using references or pointers.

Code:

---

#include <vector>
class MyClass
{
};

static std::vector<MyClass> loadFromFile(); // if you really need a static function

std::vector<MyClass> loadFromFile()
{
  std::vector<MyClass> data;
  try
  {
        // load from file and populate the vector
  }
  catch(...)
  { }
  return data;
}

int main()
{
  std::vector<MyClass> M;
  M = loadFromFile();
}

---

The code is much more easier than you thought it would be.

Quote:

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I know if I create a local object from a class and return it`s reference,

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Return it by value (as my code does above), not by reference.

Quote:

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it will point to null (if I´m rigth) after the
function call. So the object should be created with the new
operator.

---

You're right, you do come a Java background, since you do not need to create an object with "new" in C++. ;)

Quote:

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It should go something like this(please, do correct my syntax, pointers and references are used very wrong, I think):

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Again, no need to use pointers or references.

Quote:

---

ps. could somebody tell me where to fing a decent
standard C++ API (yes, I`m migrating from Java to C++). All I have found are rather hard to use / read or incomplete.

---

Get "Accelerated C++" by Koenig & Moo.

Regards,

Paul McKenzie

Thanks for your replies. Helped a lot, really!

But, could you explain little more futher this:

Quote:

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A vector has proper copy semantics, so there is no need to be using references or pointers.

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How it prevents a local copy from going out of scope?

again, many thanks!

The local data does go out of scope, but the function returns a copy of the local data. The copy can be assigned to something outside the function scope.

By the way, another option is to pass the vector into the function
as a parameter.

PHP Code:

---

#include <vector>
class MyClass
{
};

static void ChangeVector(std::vector<MyClass>& my_class_vect); // if you really need a static function

void ChangeVector(std::vector<MyClass>& my_class_vect )
{
   //the vector is a changing
}

int main()
{
   std::vector<MyClass> M;
   ChangeVector(M);
} 


---

Thanks for your replies. Helped a lot, really!

But, could you explain little more futher this:

Quote:

---

A vector has proper copy semantics, so there is no need to be using references or pointers.

---

How it prevents a local copy from going out of scope?

again, many thanks!

Quote:

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Originally posted by souldog
The local data does go out of scope, but the function returns a copy of the local data. The copy can be assigned to something outside the function scope.

By the way, another option is to pass the vector into the function
as a parameter.

PHP Code:

---

#include <vector>
class MyClass
{
};

static void ChangeVector(std::vector<MyClass>& my_class_vect); // if you really need a static function

void ChangeVector(std::vector<MyClass>& my_class_vect )
{
   //the vector is a changing
}

int main()
{
   std::vector<MyClass> M;
   ChangeVector(M);
} 


---

---

Quote:

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Originally posted by halmark6Z
Thanks for your replies. Helped a lot, really!

But, could you explain little more futher this:



How it prevents a local copy from going out of scope?

again, many thanks!

---

Consider the following:

Code:

---

int foo()
{
  int i = 0;
  return i;
}

int main()
{
    int myval = foo();
}

---

Note that the foo() function has created a local int, i, and returns it by value. The main() function calls foo() and assigns it to an int.

Compare this simple example using int's with the example using std::vector<> -- the only difference is the type returned. That is the beauty of C++. If your class provides a copy constructor and assignment operator, the class acts just like one of the basic types when returning by value. For vector, the assignment operator copies one vector to another, and the copy ctor creates a vector from an existing vector. So when you do a return of std::vector<>, you are invoking the copy constructor.

So in lay terms, you didn't question whether the local int in the foo()goes out of scope, right? So why question whether the vector goes out of scope? -- it follows the same rules of return by value. Now, if you return a pointer or reference to the local, then scope issues are a concern. My vector code does neither -- it returns by value, just like the int foo() example does.

The only other concern is if your class does not implement a properly behaved copy constructor and assignment operator (i.e. the class uses the default copy and assignment when it shouldn't be using them, or you just coded user-defined versions of these functions incorrectly), then of course, returning by value still "works", but it will introduce a bug.

Regards,

Paul McKenzie

To add to Paul's reply:

Java and C# (AFAIK) don't actually copy objects: if you say "a = b" (where a and b have class type), then the object referred to by "a" has its reference count decremented, and the object referred to by "b"has its reference count incremented to reflect that it is now referred to by "a" as well. In C++, the same statement creates two independent copies of the same data - whatever data "a" originally contained is lost (overwritten). No reference counts involved. C++ doesn't do reference counting unless you implement it yourself. This is why copy constructors, copy assignment operators and destructors are so important - C++ makes you do your own resource management. Oh, and I should say that, IMHO although that may seem to be a disadvantage, it actually gives you a lot more flexibility than garbage collection once you're used to it.

I'm also new to C++ coming from Java.

One source of confusion is that Java has call by value only. C and Java has call by value only, Fortran has call by reference only, but Pascal and C++ has a mixed calling mechanism.

When someone from Java says she's passing something by reference she actually means she's passing a reference by value.

So in this case what the OP, coming from Java, actually asks is how can I create a collection in a function, and then pass it back to the calling function without copying. The reason is efficiency. If you create say 1 million integers in an STL container in a function you don't want it to be copied back to the calling function. What you want back is a pointer (a reference) to the collection.

http://www.sgi.com/tech/stl/table_of_contents.html
http://www.fifi.org/doc/gcc-3.0-base...espacestd.html

I find these sites very usefull

Quote:

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Originally posted by _uj
I'm also new to C++ coming from Java.

One source of confusion is that Java has call by value only. C and Java has call by value only, Fortran has call by reference only, but Pascal and C++ has a mixed calling mechanism.

When someone from Java says she's passing something by reference she actually means she's passing a reference by value.

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Actually the only call-by-value behavior in Java is for primitives, everything else is passed by reference (as there are only references in Java).
C has call by value or by reference (using pointers).
C++ has call by value or by reference (using either pointers or references).

If you need the efficiency of not copying the vector, pass one in.

Quote:

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Originally posted by kuphryn
One solution is to encapsulate the STL container within a structure. Create the object on the heap and pass its pointer.

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There is neither a sense nor necessity in doing that in this case...

Quote:

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Originally posted by halmark6Z
Thanks for your replies. Helped a lot, really!

But, could you explain little more futher this:



How it prevents a local copy from going out of scope?

again, many thanks!

---

In addition, you might want to take a look at this introduction to the 'vector' class...

Quote:

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Originally posted by CornedBee
Actually the only call-by-value behavior in Java is for primitives, everything else is passed by reference (as there are only references in Java).
C has call by value or by reference (using pointers).
C++ has call by value or by reference (using either pointers or references).

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I didn't expect to find this misconception here. The only parameter passing mechanism in Java is by value. Period. Pass by reference was considered error prone and ruled out by the founders of Java. C also has call by value only. Passing a pointer (reference) by value is NOT the same as pass by reference.

Quote:

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If you need the efficiency of not copying the vector, pass one in.

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That's one possibility. The other one is to create a new vector on the heap in the function and then return a pointer to it.

The references are passed by value, but the objects are passed by reference.

See this thread for a discussion on terminology
http://www.codeguru.com/forum/showth...hreadid=277180

Some objects are constant in that they do not allow their users to modify their state: all the primitive wrappers, String, the additional number classes. Even if they are passed by reference (as they are), you cannot modify them. It's like passing a pointer to an object in C++, where the object pointed to has a private assignment operator, no public properties and no methods that modify any internal values. You can change the pointer, but not the object. Still, the object was passed by reference.

But you can for example pass a StringBuffer to a function and modify it there. The modification will be visible outside the function because the StringBuffer was passed by reference. To pass an object "pseudo-by-value", you have to do something like

Code:

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StringBuffer sb = new StrinBuffer();
wantsByValue((StringBuffer)sb.clone());

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Quote:

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The other one is to create a new vector on the heap in the function and then return a pointer to it.

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Something I don't like at all. I can't stand functions that allocate stuff and return a pointer to it if there is no logical end function that deletes the memory.

Quote:

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Originally posted by CornedBee
The references are passed by value, but the objects are passed by reference.

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You may say so in everyday speech but technically it's NOT correct. This example is from p. 145 in the C++ book by Stroustrup,

void f(int val, int& ref)

The first argument is passed according to the pass-by-value mechanism, the second is passed using pass-by-reference. It doesn't matter what you pass, the whole difference is in the passing MECHANISM. In the example the arguments are int's but they could as well have been references (pointers to objects). It wouldn't have changed anything.

You just proved my point.

Quote:

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Originally posted by CornedBee
You just proved my point.

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Good! ;)