pointer in a class

[Shadowfiend]

Sorry, I made a mistake in my template declaration. Try:

namespace std { template <class T> class vector; }

namespace std { template <class T> class vector; }

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That compiles fine on my computer.

[mickey]

Sorry, I made a mistake in my template declaration. Try:

namespace std { template <class T> class vector; }

namespace std { template <class T> class vector; }

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That compiles fine on my computer.

Maybe it can be the solution; but then, my problem is in a other place; try to compile this ,please (It doens't compile on .net2005)

//main.cpp
#include <iostream>
#include "forward.h"
#include <vector>
using namespace std;
 
int main (int argc, char argv[]) {
 
	Forward f;	
	vector<float>* fvec = new vector<float>; //EDIT <----------------------------
 
	return EXIT_SUCCESS;
}
//forward.h
#ifndef FORWARD_H
#define FORWARD_H
 
namespace std { template <class T> class vector; } 
// #include <vector> // with only this, all compiles
 
class Forward {
	std::vector<float>* _vec;
public:
	Forward();
	~Forward();
};
 
#endif FORWARD_H
 
//forward.cpp
#include "forward.h"
#include <vector>
using namespace std;
 
Forward::Forward() {
 
	_vec = new vector<float>;
}
Forward::~Forward() {}

//main.cpp
#include <iostream>
#include "forward.h"
#include <vector>
using namespace std;

int main (int argc, char argv[]) {

	Forward f;	
	vector<float>* fvec = new vector<float>; //EDIT <----------------------------

	return EXIT_SUCCESS;
}
//forward.h
#ifndef FORWARD_H
#define FORWARD_H

namespace std { template <class T> class vector; } 
// #include <vector> // with only this, all compiles

class Forward {
	std::vector<float>* _vec;
public:
	Forward();
	~Forward();
};

#endif FORWARD_H

//forward.cpp
#include "forward.h"
#include <vector>
using namespace std;

Forward::Forward() {

	_vec = new vector<float>;
}
Forward::~Forward() {}

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[Shadowfiend]

Ah, that's right. std::vector takes an std::allocator as its second parameter. Well that just gets nasty...

Here's a place where this same issue is discussed: http://objectmix.com/c/251244-forwar...compliant.html

Looks like it's possible, but the advantage is negligible and there is not necessarily a guarantee that it will work.

[mickey]

Sorry, I did a mistake; what I want to show is that the code doens't still work; see the code I correct it....what I say is that, with your solution, if I put #define <vector> in such file that include even "forward.h", the compiler gets errors; does anyone know how to correct the last code above ,please?

[wysota]

Is it really worth the trouble? Use precompiled headers and your problem will disappear (along side a 100MB of your disk space).

[mickey]

Is it really worth the trouble? Use precompiled headers and your problem will disappear (along side a 100MB of your disk space).

When I start to do this forward declaration, I didn't think it was "particularly". What I'd like understand to this point is:
1. Why these all problems
2. why 100MB !!!
3. When Does the forward declaration is recommended

Thanks in advance.

[wysota]

1. Why these all problems

You should know, it's your code

2. why 100MB !!!

When you use precompiled headers this is more or less the amount of spaces those headers take after precompilation.

3. When Does the forward declaration is recommended

We've been all over this question... It's not recommended in this particular case, just include the vector and skip to some more interesting work.

[Shadowfiend]

Concerning (1) -- because STL container classes use enough template magic that forward-declaring them is a pain.

As for (3) -- forward declaration is mostly to speed up compilation times. Precompiled headers take care of that problem to some extent by only compiling the header part once, eliminating most of the overhead (so that the vector header doesn't need to be parsed every time a file is compiled that includes your class header that uses it).

In this case, obviously the effort to forward-declare a vector isn't worth the small time savings during compilation.