CODEWITHSUNDEEP
c++qtlinkerlinker-errors
Megidd
Megidd

Reputation: 7938

Linux and macOS are fine, but Windows build throws linker error LNK2019

I'm calling a class method in a shared library from within another shared library. Application builds fine on Linux and macOS but on Windows I receive:

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

I ran out of ideas what might cause the error (only on Windows!)

Shared library with exported class:

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

DEFINES += ALGORITHMSLIB_LIBRARY

Export or import according to defines:

// algorithmslib_global.h

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

Class declaration:

// 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);
};

Calling k_means method of exported class from within another shared library:

// 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);
}

I'm compiling with Desktop_Qt_5_12_1_MSVC2017_64bit-Debug kit:

Compilers

I renamed the shared library to something very unique, and yet the same linker error is thrown. Therefore, this issue is NOT my case

Upvotes: 0

Views: 503

Answers (1)

Megidd
Megidd

Reputation: 7938

First shared lib

I have a struct inside 1st shared library:

// algorithmslib.h

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

Second shared lib

I was using the above struct inside my 2nd shared library like this:

header:

// 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);

source:

//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);

Fix

I changed the header for 2nd shared library from class Point; to struct Point; and linker error on Windows got resolved:

// exportlib.h

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

Upvotes: 1

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