Type 'foo' is not derived from type 'bar<class C>'

[Arne Schmitz]

Why do I get the following error:

error: type 'std::vector<TObject*, std::allocator<TObject*> >' is not
derived from type 'bar<TObject>'

with the following code:

#include <vector>

template<class TObject>
class bar
{

public:

typedef std::vector<TObject *> ElemVector;
typedef std::vector<TObject *>::size_type size_type;
// I even wanted to write:
// typedef ElemVector::size_type size_type;
};

I guess this is something obvious?

Arne

 

[Nate Barney]

Why do I get the following error:

error: type 'std::vector<TObject*, std::allocator<TObject*> >' is not
derived from type 'bar<TObject>'

with the following code:

#include <vector>

template<class TObject>
class bar
{

public:

typedef std::vector<TObject *> ElemVector;
typedef std::vector<TObject *>::size_type size_type;
// I even wanted to write:
// typedef ElemVector::size_type size_type;
};

I guess this is something obvious?

bar<TObject>::ElemVector is a typedef for a dependent type. That is,
the type of ElemVector depends on the type of TObject. Therefore, when
you do ElemVector::size_type, the compiler doesn't know that size_type
is a type. You have to tell it with 'typename', like this:

#include <vector>

template<class TObject>
class bar
{

public:

typedef std::vector<TObject *> ElemVector;
typedef typename ElemVector::size_type size_type;
};

 

[mlimber]

Why do I get the following error:

error: type 'std::vector<TObject*, std::allocator<TObject*> >' is not
derived from type 'bar<TObject>'

with the following code:

#include <vector>

template<class TObject>
class bar
{

public:

typedef std::vector<TObject *> ElemVector;
typedef std::vector<TObject *>::size_type size_type;

Make that:

// I even wanted to write:
// typedef ElemVector::size_type size_type;
};

I guess this is something obvious?

Not so obvious, but see this FAQ:

http://www.parashift.com/c++-faq-lite/templates.html#faq-35.18

Cheers! --M

 

[Noah Roberts]

bar<TObject>::ElemVector is a typedef for a dependent type. That is,
the type of ElemVector depends on the type of TObject. Therefore, when
you do ElemVector::size_type, the compiler doesn't know that size_type
is a type.

And that always results in the oddest of errors. So far, the only
compiler I have worked with that informed you that you "need typename"
is g++.

 

[Cy Edmunds]

bar<TObject>::ElemVector is a typedef for a dependent type. That is, the
type of ElemVector depends on the type of TObject. Therefore, when you do
ElemVector::size_type, the compiler doesn't know that size_type is a type.
You have to tell it with 'typename', like this:

#include <vector>

template<class TObject>
class bar
{

public:

typedef std::vector<TObject *> ElemVector;
typedef typename ElemVector::size_type size_type;
};

You're right of course but this certainly seems like a deficiency in the
language:

// to compiler: ElemVector is a type
typedef std::vector<TObject *> ElemVector;

// to compiler: ElemVector is STILL a type
typedef typename ElemVector::size_type size_type;

doh

 

[Pete Becker]

// to compiler: ElemVector is a type
typedef std::vector<TObject *> ElemVector;

// to compiler: ElemVector is STILL a type
typedef typename ElemVector::size_type size_type;

That's not the issue. The typename refers to size_type, not to
ElemVector. The problem is that vector can be specialized for TObject*,
and the specialization might not provide size_type. So you have to tell
the compiler that you really mean it, and that it's an error if
vector<TObject*> doesn't have a type named size_type.

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)

 

[Arne Schmitz]

Make that:

typedef typename std::vector<TObject *>::size_type  size_type;

Thanks to everyone who answered. I *always* fall for this mistake. This time
it looks a bit different, but it is the same nevertheless... I guess I
still have to meditate over the semantics still a while.

Arne