c++ - Linux 和 macOS 没问题，但 Windows 构建会抛出链接器错误 LNK2019

Question

我正在从另一个共享库中调用共享库中的类方法。应用程序在 Linux 和 macOS 上构建良好，但在 Windows 上我收到:

exportlib.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: class std::vector > __cdecl Algorithmslib::k_means(class std::vector > const &,unsigned __int64,unsigned __int64)" (__imp_?k_means@Algorithmslib@@QEAA?AV?$vector@VPoint@@V?$allocator@VPoint@@@std@@@std@@AEBV23@_K1@Z) referenced in function "private: void __cdecl Exportlib::setSuppPoints(void)" (?setSuppPoints@Exportlib@@AEAAXXZ) debug\exportlib.dll : fatal error LNK1120: 1 unresolved externals

我想不出可能导致错误的原因(仅在 Windows 上!)

---

带有导出类的共享库:

// Project file for exported shared library
// algorithmslib.pro

DEFINES += ALGORITHMSLIB_LIBRARY

根据定义导出或导入:

// algorithmslib_global.h

#if defined(ALGORITHMSLIB_LIBRARY)
#  define ALGORITHMSLIBSHARED_EXPORT Q_DECL_EXPORT
#else
#  define ALGORITHMSLIBSHARED_EXPORT Q_DECL_IMPORT
#endif

类声明:

// algorithmslib.h

#include "algorithmslib_global.h"

class ALGORITHMSLIBSHARED_EXPORT Algorithmslib : public QObject
{
    Q_OBJECT

public:
    Algorithmslib();
    std::vector<Point> k_means(const std::vector<Point>& data,
                               size_t k,
                               size_t number_of_iterations);
};

---

从另一个共享库中调用导出类的k_means方法:

// This is a second shared library which calls the exported class of the 1st shared library
// exportlib.cpp

#include "algorithmslib.h"

void Exportlib::setSuppPoints()
{
    Algorithmslib algorithmEngine;
    std::vector<Point> means = algorithmEngine.k_means(data, k, number_of_iterations);
}

---

我正在使用 Desktop_Qt_5_12_1_MSVC2017_64bit-Debug 套件进行编译:

---

我将共享库重命名为非常独特的名称，但仍抛出相同的链接器错误。因此，this issue不是我的情况

Answer 1

第一个共享库

我在第一个共享库中有一个struct:

// algorithmslib.h

struct Point {
    float x{0}, y{0}, z{0};
};

第二个共享库

我在我的第二个共享库中使用上面的 struct 是这样的:

标题:

// exportlib.h

class Point; // Note that I was using "class" here 
             // but the declaration inside algorithmslib.h is "struct"

size_t computeNumberOfClusters(const std::vector<Point> &data);

来源:

//exportlib.cpp

#include "algorithmslib.h"

std::vector<Point> data(number_of_points); // Point structure is declared in algorithmslib.h

size_t k = computeNumberOfClusters(data);
size_t number_of_iterations = 300;
Algorithmslib algorithmEngine;

std::vector<Point> means = algorithmEngine.k_means(data, k, number_of_iterations);

修复

我将第二个共享库的 header 从 class Point; 更改为 struct Point; 并解决了 Windows 上的链接器错误:

// exportlib.h

struct Point; // declared in algorithmslib.h as "struct" NOT "class"