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# Problem declaring extern std::vector

## 10 posts in this topic

Hi, I 've no issues declaring a standard type vector as extern as follows:

[code]
extern std::vector<int> testVector; // main.h
std::vector<int> testVector; // main.cpp
[/code]

However, if I use a vector containing a custom structure, like follows, I get an "expected class name" error. See below:

[code]
extern std::vector<myTempObjects::myTempObjectLib> testVector; // main.h
std::vector<myTempObjects::myTempObjectLib> testVector; // main.cpp
[/code]

[code]
// myTempObjects.h
class myTempObjects : public mainClass { // ERROR expected class name on mainClass
Q_OBJECT
public:
myTempObjects ();
~myTempObjects ();
struct myTempObjectLib {
vector3_t hairControlPoint;
int r,g,b;
};
[/code]

Would you guys please teach me what's wrong? Thanks
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A definition of mainClass must be present at the point where you define the myTempObjects class. In other words, you probably need to #include "mainClass.h", or similar.

This doesn't have anything to with the extern declaration of the vector, as far as I can tell.
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Well, the header is actually included. Weird.
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You'll have to post a little more code then. Beforehand, you should try to simmer it down to the smallest possible piece of code that exhibits the problem.

99% of the time, in doing this you'll discover the problem for yourself. Should that not be the case, post back here.
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Though judging from your symptoms, I'd guess that your problem is circular inclusion: the headers that you're declaring your vector in is directly or indirectly included by the header that your class definition occurs in. Generally the way to fix this is by using forward declarations rather than full inclusion of headers whenever you can.
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Could you make an example of what forward declaration means? Thanks
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If header A uses a class from header B, but only lets say a pointer, you dont need to include whole header B but only what is actually used from that header.

[CODE]
#include "B.h"
class AClass
{
BClass* pointer;
};
[/CODE]

you can simply do

[CODE]
class BClass; //There exists a class named BClass
class AClass
{
BClass* pointer;
}
[/CODE]

You only NEED to include the full header if you have a variable of that type which is not a pointer or reference, because in that case the class needs to know the size of the other class, for which it needs to full declearation. But since you use only a pointer, which is of fixed size, and do not use any of the functionality inside the class, all you need to do is to tell that the pointer is of type BClass so code outside the class with the pointer knows its type.
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In a.h:
[code]#pragma once
#include "b.h"

class B {
//...
};[/code]

In b.h:
[code]#pragma once
#include "a.h"

class A : public B { // <-- Possible problem!
// ...
}
};[/code]

Imagine a module that includes "a.h". It will expand the #include "b.h"' into the text of the file "b.h" and it will reach the line class A : public B' before it has seen B at all.

The solution I prefer is to not have header files that include each other like the two in my example. If at all possible, modules should provide services to the modules that use them, and not need to know anything about them. If you follow that advice, the inclusion graph is an acyclic directed graph and you won't run into the problem I just described.

In cases where two classes really need to know about each other, you can get around it using forward declarations. But this should really not happen very often.

Perhaps you have a header file that every module includes, where you can put things like global variables. That is a very tempting thing to do, but a really bad idea. Just don't do it.
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Thank you very much guys, I'm going to try these suggestions and report!
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Hi guys, I'm still at trouble with this. Following your suggestions, and taking a look at the initial code I posted, I made a forward class declaration like this:

[code]class myTempObjects[/code];

But then how do I declare the extern vector? I tried the following and I'm getting a "incomplete type 'myTempObjects' named in nested name specifier"

[code]std::vector<myTempObjects::myTempObjectLib> myTmpObject; [/code]
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As someone mentioned before, "You only NEED to include the full header if you have a variable of that type which is not a pointer or reference, because in that case the class needs to know the size of the other class, for which it needs to full declearation".

So if you're using a vector of the actual classes (and not a pointer to the classes), you need the entire class definition - a forward declaration won't do.

As someone else suggested, I think the best thing to do it to try to make the smallest possible test case that reproduces the error - then we might be able to figure out what's going on.
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