英文:
Why is Person::Impl undefined?
问题
I was trying to do an exercise on Professional C++ 5th edition, and I met a problem. I have defined Person::Impl in Person.cpp, but MSVC always shows me it's undefined.
I have these files:
CMakeLists.txt
CMAKE_MINIMUM_REQUIRED(VERSION 3.26)
set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "2182bf5c-ef0d-489a-91da-49dbc3090d2a")
set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP ON)
set_property(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} PROPERTY VS_STARTUP_PROJECT ${CMAKE_PROJECT_NAME})
set(CMAKE_CXX_STANDARD 23)
project(9-4)
add_executable(${CMAKE_PROJECT_NAME})
target_sources(${CMAKE_PROJECT_NAME}
PUBLIC
FILE_SET all_my_modules TYPE CXX_MODULES FILES
main.cpp
Person.cppm
PRIVATE
Person.cpp
)
Person.cppm
export module person;
import <string>;
import <memory>;
export class Person{
public:
Person(const std::string_view firstName,const std::string_view lastName);
Person(const std::string_view firstName,const std::string_view lastName,const char firstUnitOfName);
Person()=default;
std::string getFirstName() const;
std::string getLastName() const;
char getFirstUnitOfName() const;
void setFirstName(const std::string_view firstName);
void setLastName(const std::string_view lastName);
void setFirstUnitOfName(const char c);
private:
class Impl;
std::unique_ptr<Impl> m_impl;
};
Person.cpp
module person;
import <string>;
using namespace std;
class Person::Impl
{
private:
string m_FirstName;
string m_LastName;
char m_FirstUnitOfName;
public:
Impl() = default;
Impl(const string_view firstName, const string_view lastName) : m_FirstName(firstName), m_LastName(lastName), m_FirstUnitOfName(firstName.front()){};
Impl(const string_view firstName, const string_view lastName,const char c) : m_FirstName(firstName), m_LastName(lastName), m_FirstUnitOfName(c){};
string getFirstName() const
{
return m_FirstName;
};
string getLastName() const
{
return m_LastName;
};
char getFirstUnitOfName() const
{
return m_FirstUnitOfName;
};
void setFirstName(const string_view firstName)
{
m_FirstName = firstName;
}
void setLastName(const string_view lastName)
{
m_LastName = lastName;
}
void setFirstUnitOfName(const char c)
{
m_FirstUnitOfName = c;
}
};
Person::Person(const std::string_view firstName, const std::string_view lastName) : m_impl{make_unique<Impl>(firstName, lastName)} {};
Person::Person(const string_view firstName, const string_view lastName,const char firstUnitOfName) : m_impl{make_unique<Impl>(firstName, lastName,firstUnitOfName)}{};
std::string Person::getFirstName() const{
return m_impl->getFirstName();
};
std::string Person::getLastName() const{
return m_impl->getLastName();
};
char Person::getFirstUnitOfName() const{
return m_impl->getFirstUnitOfName();
};
void Person::setFirstName(const std::string_view firstName){
m_impl->setFirstName(firstName);
};
void Person::setLastName(const std::string_view lastName){
m_impl->setLastName(lastName);
};
void Person::setFirstUnitOfName(const char c){
m_impl->setFirstUnitOfName(c);
};
Person::~Person() = default;
main.cpp
import <iostream>;
import <format>;
import <string>;
import person;
using namespace std;
int main(){
Person testman("Test","Man");
cout<<format("The man's firstname is {},lastname is {},first unit is {}.\n",testman.getFirstName(),testman.getLastName(),testman.getFirstUnitOfName());
testman.setFirstName("Man");
testman.setLastName("Test");
testman.setFirstUnitOfName('T');
cout<<format("The man's firstname is {},lastname is {},first unit is {}.\n",testman.getFirstName(),testman.getLastName(),testman.getFirstUnitOfName());
I have seen the book writer Marc's answer, but I sadly failed to find where it is wrong(not exactly after being tutoring, see below), because C++ modules seem to be too new, and there isn't any difference between mine and Marc's answer.
The whole error message is:
[build] C:\Environment\VisualStudio\2022\Community\VC\Tools\MSVC\14.35.32215\include\memory(3065,1): error C2027: Used the undefined type "Person::Impl" [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\Person.cppm(18,15): message: See the declaration of "Person::Impl" [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] C:\Environment\VisualStudio\2022\Community\VC\Tools\MSVC\14.35.32215\include\memory(3064,1): message: When compiling class template member functions "void std::default_delete<Person::Impl>::operator ()(_Ty *) noexcept const" [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] with
[build] [
[build] _Ty=Person::Impl
[build] ]
[build] C:\Environment\VisualStudio\2022\Community\VC\Tools\MSVC\14.35.32215\include\memory(3177,33): message: View a reference to the function template instantiation "void std::default_delete<Person::Impl>::operator ()(_Ty *) noexcept const" being compiled [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] with
[build] [
[build] _Ty=Person::Impl
[build] ]
[build] C:\Environment\VisualStudio\2022\Community\VC\Tools\MSVC\14.35.32215\include\memory(3101,1): message: View a reference to the class template instantiation "std::default_delete<Person::Impl>" being compiled [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build
<details>
<summary>英文:</summary>
I was trying to do an exercise on *Professional C++* 5th edition, and I met a problem. I have defined `Person::Impl` in `Person.cpp`, but [MSVC][1] always shows me it's undefined.
I have these files:
### *CMakeLists.txt*
```cmake
CMAKE_MINIMUM_REQUIRED(VERSION 3.26)
set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "2182bf5c-ef0d-489a-91da-49dbc3090d2a")
set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP ON)
set_property(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} PROPERTY VS_STARTUP_PROJECT ${CMAKE_PROJECT_NAME})
set(CMAKE_CXX_STANDARD 23)
project(9-4)
add_executable(${CMAKE_PROJECT_NAME})
target_sources(${CMAKE_PROJECT_NAME}
PUBLIC
FILE_SET all_my_modules TYPE CXX_MODULES FILES
main.cpp
Person.cppm
PRIVATE
Person.cpp
)
Person.cppm
export module person;
import <string>;
import <memory>;
export class Person{
public:
Person(const std::string_view firstName,const std::string_view lastName);
Person(const std::string_view firstName,const std::string_view lastName,const char firstUnitOfName);
Person()=default;
std::string getFirstName() const;
std::string getLastName() const;
char getFirstUnitOfName() const;
void setFirstName(const std::string_view firstName);
void setLastName(const std::string_view lastName);
void setFirstUnitOfName(const char c);
private:
class Impl;
std::unique_ptr<Impl> m_impl;
};
Person.cpp
module person;
import <string>;
using namespace std;
class Person::Impl
{
private:
string m_FirstName;
string m_LastName;
char m_FirstUnitOfName;
public:
Impl() = default;
Impl(const string_view firstName, const string_view lastName) : m_FirstName(firstName), m_LastName(lastName), m_FirstUnitOfName(firstName.front()){};
Impl(const string_view firstName, const string_view lastName,const char c) : m_FirstName(firstName), m_LastName(lastName), m_FirstUnitOfName(c){};
string getFirstName() const
{
return m_FirstName;
};
string getLastName() const
{
return m_LastName;
};
char getFirstUnitOfName() const
{
return m_FirstUnitOfName;
};
void setFirstName(const string_view firstName)
{
m_FirstName = firstName;
}
void setLastName(const string_view lastName)
{
m_LastName = lastName;
}
void setFirstUnitOfName(const char c)
{
m_FirstUnitOfName = c;
}
};
Person::Person(const std::string_view firstName, const std::string_view lastName) : m_impl{make_unique<Impl>(firstName, lastName)} {};
Person::Person(const string_view firstName, const string_view lastName,const char firstUnitOfName) : m_impl{make_unique<Impl>(firstName, lastName,firstUnitOfName)}{};
std::string Person::getFirstName() const{
return m_impl->getFirstName();
};
std::string Person::getLastName() const{
return m_impl->getLastName();
};
char Person::getFirstUnitOfName() const{
return m_impl->getFirstUnitOfName();
};
void Person::setFirstName(const std::string_view firstName){
m_impl->setFirstName(firstName);
};
void Person::setLastName(const std::string_view lastName){
m_impl->setLastName(lastName);
};
void Person::setFirstUnitOfName(const char c){
m_impl->setFirstUnitOfName(c);
};
Person::~Person() = default;
main.cpp
import <iostream>;
import <format>;
import <string>;
import person;
using namespace std;
int main(){
Person testman("Test","Man");
cout<<format("The man's firstname is {},lastname is {},first unit is {}.\n",testman.getFirstName(),testman.getLastName(),testman.getFirstUnitOfName());
testman.setFirstName("Man");
testman.setLastName("Test");
testman.setFirstUnitOfName('T');
cout<<format("The man's firstname is {},lastname is {},first unit is {}.\n",testman.getFirstName(),testman.getLastName(),testman.getFirstUnitOfName());
I have seen the book writer Marc's answer, but I sadly failed to find where it is wrong(not exactly after being tutoring, see below), because C++ modules seem to be too new, and there isn't any difference between mine and Marc's answer.
The whole error message is:
[build] C:\Environment\VisualStudio\2022\Community\VC\Tools\MSVC\14.35.32215\include\memory(3065,1): error C2027: Used the undefined type "Person::Impl" [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\Person.cppm(18,15): message: See the declaration of "Person::Impl" [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] C:\Environment\VisualStudio\2022\Community\VC\Tools\MSVC\14.35.32215\include\memory(3064,1): message: When compiling class template member functions "void std::default_delete<Person::Impl>::operator ()(_Ty *) noexcept const" [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] with
[build] [
[build] _Ty=Person::Impl
[build] ]
[build] C:\Environment\VisualStudio\2022\Community\VC\Tools\MSVC\14.35.32215\include\memory(3177,33): message: View a reference to the function template instantiation "void std::default_delete<Person::Impl>::operator ()(_Ty *) noexcept const" being compiled [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] with
[build] [
[build] _Ty=Person::Impl
[build] ]
[build] C:\Environment\VisualStudio\2022\Community\VC\Tools\MSVC\14.35.32215\include\memory(3101,1): message: View a reference to the class template instantiation "std::default_delete<Person::Impl>" being compiled [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\main.cpp(8,1): message: View class template instantiation of are compiling "STD: : unique_ptr < Person: : Impl, STD: : default_delete < Person: : Impl > >" [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] C:\Environment\VisualStudio\2022\Community\VC\Tools\MSVC\14.35.32215\include\memory(3065,25): error C2338: static_assert failed: 'can't delete an incomplete type' [C:\Users\leen\workspace\cpp\professional_c++_answer\chapter-9\9-4\build\9-4.vcxproj]
[build] Build finished with exit code -1
<hr/>
Added as being required after solved:
If you rewrite the above in the all old #include fashion, MSVC will show:
error C2600: "Person::~Person": Cannot define special compiler-generated member functions (must first be declared in the class)
It's like Forward declaration with unique_ptr.
答案1
得分: 1
你需要将析构函数的声明添加到Person类的定义中：
Person.cpp
export class Person{
public:
~Person();
[...]
这种情况类似于此代码，在预模块时代。如果析构函数没有包含在类定义中，编译器会假定你想要一个由编译器定义的隐式析构函数。这会导致错误，因为该析构函数需要在main.cpp的上下文中生成，而在那里类型Person::Impl是未知的。
我觉得微软的编译器接受你的代码而不在类定义中声明析构函数，但在模块实现中定义析构函数，这看起来很奇怪。这可能实际上是一个编译器bug（尽管我需要先查阅标准文档来确认）。
或者，你也可以将整个Person::Impl类定义移到主模块接口中，但不导出它。person的客户端仍然无法实例化该类，因为它被声明为private嵌套类，但它将为编译器提供足够的信息，以在main.cpp的上下文中实例化~Person()。
英文:
You need to add the declaration for the destructor to the class definiton of Person:
Person.cppm
export class Person{
public:
~Person();
[...]
The situation is similar to this code in a pre-modules world. If the destructor is not included in the class definition, the compiler will assume that you want a compiler-defined implicit destructor. This will fail, because that destructor would need to be generated in the context of main.cpp, where the type Person::Impl is not known.
The fact that MSVC accepted your code as-is, with the destructor definition in the module implementation, but no declaration of the destructor in the class definition looks weird to me. This might actually be a compiler bug (though I'll have to dig through the standard wording first to confirm).
Alternatively, you can also move the entire class definition for Person::Impl into the primary module interface without exporting it. Clients of person will still not be able to instantiate that class because of it being declared as a private nested class, but it will provide sufficient information for the compiler to instantiate ~Person() in the context of main.cpp.
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论