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PrestoChung

Passing std::list object to function argument

9 posts in this topic

I'm trying to get a dynamic list for visible objects to pass to my render function, so each loop the list is iterated and each object in the list is rendered in my scene. My function prototype looks like
void render(Player& Player, list Wlist);
but my error says
syntax error : identifier 'list'
maybe I am passing it wrong since list is a template, I have tried list<> Wlist, list<Wire> Wlist but no go.
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Assuming that this is a 'list of Wire', then list<Wire> is what you want. However, you'll also need to include the appropriate header (list), and either qualify list with std:: or use an appropriate using directive or declaration. (Or, you could create a typedef for std::list<Wire>, which would probably be the best option.)

Also, you'll probably want to pass the list by constant reference rather than by value (as is, a copy of the list will be made each time you call the function).
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Turned out I was missing the namespace std in my header file.

Is it common to have using namespace std; in both the header and the source file?

And thanks for catching the pass by reference, I started off that way but lost it thinking it was part of the problem. I thought it wouldn't matter since I wasn't making any changes to the Wire objects but I forgot that a new copy would be made each time.

Edit:
Fixed :)
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Quote:
Is it common to have using namespace std; in both the header and the source file?
You shouldn't put using directives or declarations at file scope in header files, as they will also affect any file that includes that file. (It may not really matter if it's a one-person project, but it's a bad habit to get into nonetheless, IMO.)

As for using 'using' statements elsewhere (within functions, in source files), that's up to you, but my own preference is to qualify explicitly (e.g. std::list). It's not that much extra typing, it makes your code more clear (IMO), and if you use typedefs (which is advisable), you won't see it in your code much anyway.
Quote:
Ouch I get a fatal linking error when passing by reference.

[code]
fatal error LNK1000: Internal error during IncrCalcPtrs
Are you using Visual Studio? That error looks familiar, although I can't remember what it's related to. You might try doing a clean rebuild and see if it recurs.

In any case, the error isn't related (directly, at least) to whether that particular argument is passed by value or reference; passing by constant reference is the right thing to do here, so you shouldn't change your code in order to work around that error (assuming that changing your code back to the way it was previously would even have that effect).
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Quote:
Original post by jyk

As for using 'using' statements elsewhere (within functions, in source files), that's up to you, but my own preference is to qualify explicitly (e.g. std::list). It's not that much extra typing, it makes your code more clear (IMO), and if you use typedefs (which is advisable), you won't see it in your code much anyway.


So it would be advisable to put something like:


class Wire;

typedef std::list<Wire> aList;


in my header file?


Quote:

Are you using Visual Studio? That error looks familiar, although I can't remember what it's related to. You might try doing a clean rebuild and see if it recurs.

In any case, the error isn't related (directly, at least) to whether that particular argument is passed by value or reference; passing by constant reference is the right thing to do here, so you shouldn't change your code in order to work around that error (assuming that changing your code back to the way it was previously would even have that effect).


You're right the problem was my main loop was calling
wireList.push_back(wire1);
every frame.

Isn't there some kind of 'do once' functionality in C++? I used this (in mainloop) as a temporary measure

static int once = 0;
if (once == 0) {
wireList.push_back(wire1);
wireList.push_back(wire2);
once = 1;
}

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This leads me to my next problem. I can now add and remove objects to draw from a list, but how do I 'spawn' more objects in an open-ended way?

I mean that whenever I make a new object it looks like


static Wire wire1(5);

static Wire wire2(13, 26, 13, 2);


where I have to hard-code in the object names 'wire1', 'wire2' etc. So how could I make an indeterminate number of objects wire 4 ,...,wire n?

I have not used dynamic memory allocation before but maybe that is what I need to do, something like


void spawnWire {
wireList.push_back(new Wire);
}
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You don't need to dynamically allocate here, unless Wire objects need non-trivial ownership semantics or are expensive to copy:

void spawnWire() {
wireList.push_back(Wire()); // Creates an anonymous Wire instance and adds it.
}

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Cool thanks.

I'm trying to make the list now instead of the derived class (Wire) of the base class (Visible) I have made for visible objects but the
list<Visible>::iterator visIter;
only seems to be calling the virtual function from the base(I got an error when I tried to use an abstract class with the list template):

for (visIter = visList.begin() ; visIter != visList.end() ; visIter++)	{
visIter->drawMe();
}


My classes look like so

class Visible
{
public:
GLfloat posX, posY, posZ;

Visible();

virtual void drawMe() {}
};


class Wire: public Visible
{
public:
int size;

Wire(int Size);
Wire(int atX, int atY, int atZ, int Size);

void drawMe();
};
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Quote:
So it would be advisable to put something like:


class Wire;

typedef std::list<Wire> aList;


in my header file?
That's the general idea (although you may run into problems if you're trying to forward declare Wire - you'll probably need to include the appropriate header if you're not doing so already).
Quote:
Isn't there some kind of 'do once' functionality in C++? I used this (in mainloop) as a temporary measure

static int once = 0;
if (once == 0) {
wireList.push_back(wire1);
wireList.push_back(wire2);
once = 1;
}
I doubt many languages have a special provision for 'just doing something once'. That's a fairly specific need (although probably not uncommon), and is generally something you'd code up yourself, just as you've done here (note however that a bool would probably be more appropriate than an int here).

There might be better (or at least more elegant) ways of accomplishing what you're trying to accomplish, but without knowing the context it's hard to make specific suggestions.
Quote:
I'm trying to make the list now instead of the derived class (Wire) of the base class (Visible) I have made for visible objects but the
list<Visible>::iterator visIter;
only seems to be calling the virtual function from the base
Sounds like slicing.

Typically, in this context you would store your objects by pointer rather than by value. If you're not sure why this is, try searching for 'c++ inheritance polymorphism'.
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Whoops.
Quote:
Original Post by rip-off
You don't need to dynamically allocate here, unless Wire objects need non-trivial ownership semantics or are expensive to copy...

^ or you need polymorphic behaviour.
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