c ++如何在运行时链接函数声明？

Question

我在共享库（* .dll）中有一个函数声明，并且定义在另一个应用程序中，这是最少的代码。但是如何告诉编译器在库加载到应用程序后在运行时找到符号set_x（在mylib.cpp中）。 extern关键字在链接时进行搜索。还是有其他方法可以调用application.cpp中定义的方法？

// mylib.cpp compiles to mylib.dll
class MyClass{
public:
    extern void set_x(int); // need to tell the compiler to search at runtime
}

extern "C"{
EXPORT_API void lib_func(MyClass* o){
    printf("setting o's x value");
    o->set_x(42);
}
}

// application.cpp compiles to application.exe
class MyClass{
private:
    int x = 0;
public:
    extern void set_x(int _x){ x = _x; }
}

typedef void (*func_ptr)(MyClass*);
...
func_ptr lib_func = (lib_func_ptr)dlopen_and_get_func_ptr("mydll.lib", "lib_func");
MyClass cls;
lib_func(&cls);
...

Answer 1

事实是，您正在混合C和C ++概念。即使2 建立在1 之上，他们也有非常不同的方式来处理这种情况。我要对^{C ++}进行分析。但是，无论选择哪种方法，代码中都有一些错误之处：^{<< [MyClass定义出现了两次}

并且有所不同

（也是-个人评论-我发现以“

My”开头的名称，让人挠头））该类未导出dll00.h

：

#pragma once

#if defined(_WIN32)
#  if defined(DLL00_EXPORTS)
#    define DLL00_EXPORT_API __declspec(dllexport)
#  else
#    define DLL00_EXPORT_API __declspec(dllimport)
#  endif
#else
#  define DLL00_EXPORT_API
#endif


class DLL00_EXPORT_API BaseClass {
public:
    virtual void setX(int x) = 0;  // Pure virtual
    // Some other method declarations might go here. Their implementation should be in dll00.cpp
};


DLL00_EXPORT_API void callSetX(BaseClass *pClass, int value);

dll00.cpp：

#define DLL00_EXPORTS
#include "dll00.h"

#include <iostream>

using std::cout;


void callSetX(BaseClass *pClass, int value) {
    if (!pClass) {
        cout << "NULL pointer!\n";
        return;
    }
    cout << "Setting x\n";
    pClass->setX(value);
}

main.cpp：

#include "dll00.h"
#include <iostream>

using std::cout;


class DerivedClass : public BaseClass {
public:
    void setX(int x) override;

private:
    int x;
};


void DerivedClass::setX(int x) {
    std::cout << "setX(" << x << ")\n";
    this->x = x;
}


int main() {
    DerivedClass *pInst = new DerivedClass;
    callSetX(pInst, 255);
    return 0;
}

输出：

[cfati@CFATI-5510-0:e:\Work\Dev\StackOverflow\q061152778]> sopr.bat
*** Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ***

[prompt]> "c:\Install\pc032\Microsoft\VisualStudioCommunity\2017\VC\Auxiliary\Build\vcvarsall.bat" x64
**********************************************************************
** Visual Studio 2017 Developer Command Prompt v15.9.21
** Copyright (c) 2017 Microsoft Corporation
**********************************************************************
[vcvarsall.bat] Environment initialized for: 'x64'

[prompt]> dir /b
dll00.cpp
dll00.h
main.cpp

[prompt]> cl /nologo /MD /DDLL /EHsc dll00.cpp  /link /NOLOGO /DLL /OUT:dll00.dll
dll00.cpp
   Creating library dll00.lib and object dll00.exp

[prompt]> cl /nologo /MD /EHsc main.cpp  /link /NOLOGO /OUT:main.exe dll00.lib
main.cpp
   Creating library main.lib and object main.exp

[prompt]> dir /b
dll00.cpp
dll00.dll
dll00.exp
dll00.h
dll00.lib
dll00.obj
main.cpp
main.exe
main.exp
main.lib
main.obj

[prompt]> main.exe
Setting x
setX(255)

有关从

。dll导出内容的更多详细信息，可以检查[SO]: Linker error when calling a C function from C++ code in different VS2010 project (@CristiFati's answer)。