How can I reuse the same impl destructor in a class derived from a PIMPL base class?
I'm trying to create a class that inherits from a base class that uses the PIMPL idiom.
I have a base class base.h:
#include <memory>
class Base {
public:
class Impl;
Base(std::unique_ptr<Base::Impl> impl);
virtual ~Base();
private:
std::unique_ptr<Base::Impl> impl_;
};
which is implemented in base.cpp:
#include "base.h"
class Base::Impl {
public:
Impl() {}
~Impl() {}
};
Base::Base(std::unique_ptr<Base::Impl> impl) : impl_(std::move(impl)) {}
Base::~Base() {}
which is then derived in derived.cpp:
#include "base.h"
class Derived : public Base {
public:
Derived() : Base(nullptr) {}
~Derived() override {}
};
When I compile these three files, I get the following error:
In file included from /usr/include/c++/7/memory:80:0,
from base.h:1,
from derived.cpp:1:
/usr/include/c++/7/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = Base::Impl]’:
/usr/include/c++/7/bits/unique_ptr.h:268:17: required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = Base::Impl; _Dp = std::default_delete<Base::Impl>]’
derived.cpp:5:27: required from here
/usr/include/c++/7/bits/unique_ptr.h:76:22: error: invalid application of ‘sizeof’ to incomplete type ‘Base::Impl’
static_assert(sizeof(_Tp)>0,
which kind of makes sense, since there is no ~Impl() for the derived class to use. After I add the following snippet to derived.cpp before the Derived class definition:
class Base::Impl {
public:
Impl() {}
~Impl() {}
};
it compiles fine. Is there any way for derived.cpp to use the Impl destructor from base.cpp without adding it to some other shared file that both base.cpp and derived.cpp use? I've also noticed that this issue only arises because of the constructor taking in a unique_ptr to Base::Impl. If I replace that constructor with an empty constructor that just assigns a new Impl to Base::impl_, I don't have to include the destructor like I had to above. What's different in that situation?
c++ inheritance unique-ptr pimpl-idiom
asked Nov 19 '18 at 23:07
srujzssrujzs
6417
add a comment |
I'm trying to create a class that inherits from a base class that uses the PIMPL idiom.
I have a base class base.h:
#include <memory>
class Base {
public:
class Impl;
Base(std::unique_ptr<Base::Impl> impl);
virtual ~Base();
private:
std::unique_ptr<Base::Impl> impl_;
};
which is implemented in base.cpp:
#include "base.h"
class Base::Impl {
public:
Impl() {}
~Impl() {}
};
Base::Base(std::unique_ptr<Base::Impl> impl) : impl_(std::move(impl)) {}
Base::~Base() {}
which is then derived in derived.cpp:
#include "base.h"
class Derived : public Base {
public:
Derived() : Base(nullptr) {}
~Derived() override {}
};
When I compile these three files, I get the following error:
In file included from /usr/include/c++/7/memory:80:0,
from base.h:1,
from derived.cpp:1:
/usr/include/c++/7/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = Base::Impl]’:
/usr/include/c++/7/bits/unique_ptr.h:268:17: required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = Base::Impl; _Dp = std::default_delete<Base::Impl>]’
derived.cpp:5:27: required from here
/usr/include/c++/7/bits/unique_ptr.h:76:22: error: invalid application of ‘sizeof’ to incomplete type ‘Base::Impl’
static_assert(sizeof(_Tp)>0,
which kind of makes sense, since there is no ~Impl() for the derived class to use. After I add the following snippet to derived.cpp before the Derived class definition:
class Base::Impl {
public:
Impl() {}
~Impl() {}
};
it compiles fine. Is there any way for derived.cpp to use the Impl destructor from base.cpp without adding it to some other shared file that both base.cpp and derived.cpp use? I've also noticed that this issue only arises because of the constructor taking in a unique_ptr to Base::Impl. If I replace that constructor with an empty constructor that just assigns a new Impl to Base::impl_, I don't have to include the destructor like I had to above. What's different in that situation?
c++ inheritance unique-ptr pimpl-idiom
asked Nov 19 '18 at 23:07
srujzssrujzs
6417
The problem is the conversion ofnullptrinto astd::unique_ptr<Base::Impl>happening outside of Base.cpp whereImplis fully defined. This leads to the reason why I'm not trying to answer this: I can't provide a good solution. If onlyBaseknows how to make aImpl, how willDerivedever get anImplto supply toBase? This constructor seems minimally useful. Or maybe I do have a solution.... Lemme think.
– user4581301
Nov 19 '18 at 23:51
It does seem a bit strange, but for context, theDerivedclass in my use case is a mock, so I have no need to construct anImpl.
– srujzs
Nov 19 '18 at 23:57
Advice would be don't then. Add another constructor toBase.Base::Base() : impl_(nullptr) {}That said, I hate hate hate modifying a class to support a test. It means you are not testing the proper use-cases of the system under test and the test results may be questionable.
– user4581301
Nov 20 '18 at 0:08
Thinking didn't coalesces into a decent solution, by the way.
– user4581301
Nov 20 '18 at 0:09
add a comment |
I'm trying to create a class that inherits from a base class that uses the PIMPL idiom.
I have a base class base.h:
#include <memory>
class Base {
public:
class Impl;
Base(std::unique_ptr<Base::Impl> impl);
virtual ~Base();
private:
std::unique_ptr<Base::Impl> impl_;
};
which is implemented in base.cpp:
#include "base.h"
class Base::Impl {
public:
Impl() {}
~Impl() {}
};
Base::Base(std::unique_ptr<Base::Impl> impl) : impl_(std::move(impl)) {}
Base::~Base() {}
which is then derived in derived.cpp:
#include "base.h"
class Derived : public Base {
public:
Derived() : Base(nullptr) {}
~Derived() override {}
};
When I compile these three files, I get the following error:
In file included from /usr/include/c++/7/memory:80:0,
from base.h:1,
from derived.cpp:1:
/usr/include/c++/7/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = Base::Impl]’:
/usr/include/c++/7/bits/unique_ptr.h:268:17: required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = Base::Impl; _Dp = std::default_delete<Base::Impl>]’
derived.cpp:5:27: required from here
/usr/include/c++/7/bits/unique_ptr.h:76:22: error: invalid application of ‘sizeof’ to incomplete type ‘Base::Impl’
static_assert(sizeof(_Tp)>0,
which kind of makes sense, since there is no ~Impl() for the derived class to use. After I add the following snippet to derived.cpp before the Derived class definition:
class Base::Impl {
public:
Impl() {}
~Impl() {}
};
it compiles fine. Is there any way for derived.cpp to use the Impl destructor from base.cpp without adding it to some other shared file that both base.cpp and derived.cpp use? I've also noticed that this issue only arises because of the constructor taking in a unique_ptr to Base::Impl. If I replace that constructor with an empty constructor that just assigns a new Impl to Base::impl_, I don't have to include the destructor like I had to above. What's different in that situation?
c++ inheritance unique-ptr pimpl-idiom
asked Nov 19 '18 at 23:07
srujzssrujzs
6417
I'm trying to create a class that inherits from a base class that uses the PIMPL idiom.
I have a base class base.h:
#include <memory>
class Base {
public:
class Impl;
Base(std::unique_ptr<Base::Impl> impl);
virtual ~Base();
private:
std::unique_ptr<Base::Impl> impl_;
};
which is implemented in base.cpp:
#include "base.h"
class Base::Impl {
public:
Impl() {}
~Impl() {}
};
Base::Base(std::unique_ptr<Base::Impl> impl) : impl_(std::move(impl)) {}
Base::~Base() {}
which is then derived in derived.cpp:
#include "base.h"
class Derived : public Base {
public:
Derived() : Base(nullptr) {}
~Derived() override {}
};
When I compile these three files, I get the following error:
In file included from /usr/include/c++/7/memory:80:0,
from base.h:1,
from derived.cpp:1:
/usr/include/c++/7/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = Base::Impl]’:
/usr/include/c++/7/bits/unique_ptr.h:268:17: required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = Base::Impl; _Dp = std::default_delete<Base::Impl>]’
derived.cpp:5:27: required from here
/usr/include/c++/7/bits/unique_ptr.h:76:22: error: invalid application of ‘sizeof’ to incomplete type ‘Base::Impl’
static_assert(sizeof(_Tp)>0,
which kind of makes sense, since there is no ~Impl() for the derived class to use. After I add the following snippet to derived.cpp before the Derived class definition:
class Base::Impl {
public:
Impl() {}
~Impl() {}
};
it compiles fine. Is there any way for derived.cpp to use the Impl destructor from base.cpp without adding it to some other shared file that both base.cpp and derived.cpp use? I've also noticed that this issue only arises because of the constructor taking in a unique_ptr to Base::Impl. If I replace that constructor with an empty constructor that just assigns a new Impl to Base::impl_, I don't have to include the destructor like I had to above. What's different in that situation?
c++ inheritance unique-ptr pimpl-idiom
c++ inheritance unique-ptr pimpl-idiom
asked Nov 19 '18 at 23:07
srujzssrujzs
6417
asked Nov 19 '18 at 23:07
srujzssrujzs
6417
edited Nov 19 '18 at 23:39
srujzs
asked Nov 19 '18 at 23:07
srujzssrujzs
6417
asked Nov 19 '18 at 23:07
srujzssrujzs
6417
asked Nov 19 '18 at 23:07
srujzssrujzs
6417
6417
The problem is the conversion ofnullptrinto astd::unique_ptr<Base::Impl>happening outside of Base.cpp whereImplis fully defined. This leads to the reason why I'm not trying to answer this: I can't provide a good solution. If onlyBaseknows how to make aImpl, how willDerivedever get anImplto supply toBase? This constructor seems minimally useful. Or maybe I do have a solution.... Lemme think.
– user4581301
Nov 19 '18 at 23:51
It does seem a bit strange, but for context, theDerivedclass in my use case is a mock, so I have no need to construct anImpl.
– srujzs
Nov 19 '18 at 23:57
Advice would be don't then. Add another constructor toBase.Base::Base() : impl_(nullptr) {}That said, I hate hate hate modifying a class to support a test. It means you are not testing the proper use-cases of the system under test and the test results may be questionable.
– user4581301
Nov 20 '18 at 0:08
Thinking didn't coalesces into a decent solution, by the way.
– user4581301
Nov 20 '18 at 0:09
add a comment |
The problem is the conversion ofnullptrinto astd::unique_ptr<Base::Impl>happening outside of Base.cpp whereImplis fully defined. This leads to the reason why I'm not trying to answer this: I can't provide a good solution. If onlyBaseknows how to make aImpl, how willDerivedever get anImplto supply toBase? This constructor seems minimally useful. Or maybe I do have a solution.... Lemme think.
– user4581301
Nov 19 '18 at 23:51
It does seem a bit strange, but for context, theDerivedclass in my use case is a mock, so I have no need to construct anImpl.
– srujzs
Nov 19 '18 at 23:57
Advice would be don't then. Add another constructor toBase.Base::Base() : impl_(nullptr) {}That said, I hate hate hate modifying a class to support a test. It means you are not testing the proper use-cases of the system under test and the test results may be questionable.
– user4581301
Nov 20 '18 at 0:08
Thinking didn't coalesces into a decent solution, by the way.
– user4581301
Nov 20 '18 at 0:09
The problem is the conversion of
nullptr into a std::unique_ptr<Base::Impl> happening outside of Base.cpp where Impl is fully defined. This leads to the reason why I'm not trying to answer this: I can't provide a good solution. If only Base knows how to make a Impl, how will Derived ever get an Impl to supply to Base? This constructor seems minimally useful. Or maybe I do have a solution.... Lemme think.– user4581301
Nov 19 '18 at 23:51
The problem is the conversion of
nullptr into a std::unique_ptr<Base::Impl> happening outside of Base.cpp where Impl is fully defined. This leads to the reason why I'm not trying to answer this: I can't provide a good solution. If only Base knows how to make a Impl, how will Derived ever get an Impl to supply to Base? This constructor seems minimally useful. Or maybe I do have a solution.... Lemme think.– user4581301
Nov 19 '18 at 23:51
It does seem a bit strange, but for context, the
Derived class in my use case is a mock, so I have no need to construct an Impl.– srujzs
Nov 19 '18 at 23:57
It does seem a bit strange, but for context, the
Derived class in my use case is a mock, so I have no need to construct an Impl.– srujzs
Nov 19 '18 at 23:57
Advice would be don't then. Add another constructor to
Base. Base::Base() : impl_(nullptr) {} That said, I hate hate hate modifying a class to support a test. It means you are not testing the proper use-cases of the system under test and the test results may be questionable.– user4581301
Nov 20 '18 at 0:08
Advice would be don't then. Add another constructor to
Base. Base::Base() : impl_(nullptr) {} That said, I hate hate hate modifying a class to support a test. It means you are not testing the proper use-cases of the system under test and the test results may be questionable.– user4581301
Nov 20 '18 at 0:08
Thinking didn't coalesces into a decent solution, by the way.
– user4581301
Nov 20 '18 at 0:09
Thinking didn't coalesces into a decent solution, by the way.
– user4581301
Nov 20 '18 at 0:09
add a comment |
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The problem is the conversion of
nullptrinto astd::unique_ptr<Base::Impl>happening outside of Base.cpp whereImplis fully defined. This leads to the reason why I'm not trying to answer this: I can't provide a good solution. If onlyBaseknows how to make aImpl, how willDerivedever get anImplto supply toBase? This constructor seems minimally useful. Or maybe I do have a solution.... Lemme think.– user4581301
Nov 19 '18 at 23:51
It does seem a bit strange, but for context, the
Derivedclass in my use case is a mock, so I have no need to construct anImpl.– srujzs
Nov 19 '18 at 23:57
Advice would be don't then. Add another constructor to
Base.Base::Base() : impl_(nullptr) {}That said, I hate hate hate modifying a class to support a test. It means you are not testing the proper use-cases of the system under test and the test results may be questionable.– user4581301
Nov 20 '18 at 0:08
Thinking didn't coalesces into a decent solution, by the way.
– user4581301
Nov 20 '18 at 0:09