模块中简单类引发C2665错误：跨项目构造函数匹配失败

我正在学习C++模块与类的使用，编写了包含FirstClass和SecondClass的模块代码，模块编译正常。但在另一个项目中调用SecondClass sec(firstCl)时，触发了C2665错误，提示无法匹配构造函数参数，即使显式删除相关构造函数也无效。

模块代码 #

firstFile.cpp #

export module ModTest;

export import :Two;

export class FirstClass {
public:
    FirstClass();
    int firstClassFunc();
};

firstFile_impl.cpp #

module ModTest;

int FirstClass::firstClassFunc() { return 8; }

FirstClass::FirstClass()
{
    SecondClass sec(this);
}

secondFile.cpp #

export module ModTest:Two;

class FirstClass;

export class SecondClass {
public:
    SecondClass(FirstClass *parent){}
    SecondClass(const SecondClass&) noexcept = delete;
    SecondClass(const SecondClass&&) noexcept = delete;
    SecondClass& operator=(const SecondClass&) noexcept = delete;
    SecondClass& operator=(SecondClass&&) noexcept = delete;
};

调用代码 #

FirstClass *firstCl = new FirstClass();
SecondClass sec(firstCl);

错误信息 #

1>C:\ecworks_mep\_WINOHNEDLL\src\mainframe.cpp(358,17): error C2665: 'SecondClass::SecondClass': no overloaded function could convert all the argument types
1>(compiling source file '../src/mainframe.cpp')
1>    C:\ecworks_mep\moduleTest\moduleTest\secondFile.cpp(9,2):
1>    could be 'SecondClass::SecondClass(const SecondClass &&) noexcept'
1>        C:\ecworks_mep\_WINOHNEDLL\src\mainframe.cpp(358,17):
1>        'SecondClass::SecondClass(const SecondClass &&) noexcept': cannot convert argument 1 from 'FirstClass *' to 'const SecondClass &&'
1>            C:\ecworks_mep\_WINOHNEDLL\src\mainframe.cpp(358,18):
1>            Reason: cannot convert from 'FirstClass *' to 'const SecondClass'
1>    C:\ecworks_mep\moduleTest\moduleTest\secondFile.cpp(8,2):
1>    or       'SecondClass::SecondClass(const SecondClass &) noexcept'
1>        C:\ecworks_mep\_WINOHNEDLL\src\mainframe.cpp(358,17):
1>        'SecondClass::SecondClass(const SecondClass &) noexcept': cannot convert argument 1 from 'FirstClass *' to 'const SecondClass &'
1>            C:\ecworks_mep\_WINOHNEDLL\src\mainframe.cpp(358,18):
1>            Reason: cannot convert from 'FirstClass *' to 'const SecondClass'
1>    C:\ecworks_mep\moduleTest\moduleTest\secondFile.cpp(7,2):
1>    or       'SecondClass::SecondClass(FirstClass *)'
1>        C:\ecworks_mep\_WINOHNEDLL\src\mainframe.cpp(358,17):
1>        'SecondClass::SecondClass(FirstClass *)': cannot convert argument 1 from 'FirstClass *' to 'FirstClass *'
1>            C:\ecworks_mep\_WINOHNEDLL\src\mainframe.cpp(358,18):
1>            Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or parenthesized function-style cast
1>    C:\ecworks_mep\_WINOHNEDLL\src\mainframe.cpp(358,17):
1>    while trying to match the argument list '(FirstClass *)'
1>Done building project "mepOhneDll4.vcxproj" -- FAILED.

问题原因及修复 #

核心原因 #

编译器将两个上下文里的FirstClass判定为无关类型：

• 在模块分区:Two中，仅前向声明了class FirstClass;，但未导出这个前向声明；
• 主模块ModTest导出了完整的FirstClass类；
• 外部项目调用构造函数时，看到的FirstClass*是主模块导出的完整类型，而SecondClass构造函数参数里的FirstClass*对应:Two分区未导出的前向声明类型，二者被编译器视为完全不同的类型，因此无法匹配构造函数。

修复方案 #

在:Two分区中导出FirstClass的前向声明，确保外部代码看到的FirstClass类型与SecondClass构造函数参数的类型一致：

修改secondFile.cpp：

export module ModTest:Two;

export class FirstClass; // 新增export导出前向声明

export class SecondClass {
public:
    SecondClass(FirstClass *parent){}
    SecondClass(const SecondClass&) noexcept = delete;
    SecondClass(const SecondClass&&) noexcept = delete;
    SecondClass& operator=(const SecondClass&) noexcept = delete;
    SecondClass& operator=(SecondClass&&) noexcept = delete;
};

修改后，外部代码中的FirstClass*与SecondClass构造函数参数的类型完全匹配，错误即可解决。

内容的提问来源于stack exchange，提问作者mael15