错误：C++无法解析的外部符号

Question

我目前正在使用C++文件中的其他人在我的C++项目中编写视觉  Studio   2008（实际上，它是作为多边形交集库在线提供的）。

我下载的两个文件是gcp.h和gcp.c.所以我将gcp.h和gcp.c复制到我的项目目录中。链接器给了我这个错误：

unresolved external symbol: void __cdecl gpc_polygon_clip(enum gpc_op,struct gpc_polygon *,struct gpc_polygon *,struct gpc_polygon *)

下面是如何写我的代码，使用该库：

static int poly_intersection(Convex_Polygon& poly1,Convex_Polygon& poly2,Convex_Polygon& rp) 
{ 
    if(!poly1.is_convex() || !poly2.is_convex()) 
     return 1; 

    // Construct 1st convex polygon 
    gpc_vertex *gvp1 = new gpc_vertex[poly1.size()]; 
    gpc_vertex v1; 
    for (unsigned int n = 0; n < poly1.size(); n++) 
    { 
     v1.x = poly1.vertex(n).x(); 
     v1.y = poly1.vertex(n).y(); 
     gvp1[n] = v1; 
    } 

    gpc_vertex_list gvl1; 
    gvl1.num_vertices = poly1.size(); 
    gvl1.vertex = gvp1; 

    gpc_vertex_list *gvlp1 = new gpc_vertex_list[1]; 
    gvlp1[0] = gvl1; 

    gpc_polygon gp1; 
    gp1.num_contours = 1; 
    gp1.hole = 0; 
    gp1.contour = gvlp1; 

    // Construct 2st convex polygon 
    gpc_vertex *gvp2 = new gpc_vertex[poly2.size()]; 
    gpc_vertex v2; 
    for (unsigned int n = 0; n < poly2.size(); n++) 
    { 
     v2.x = poly2.vertex(n).x(); 
     v2.y = poly2.vertex(n).y(); 
     gvp2[n] = v2; 
    } 

    gpc_vertex_list gvl2; 
    gvl2.num_vertices = poly2.size(); 
    gvl2.vertex = gvp2; 

    gpc_vertex_list *gvlp2 = new gpc_vertex_list[1]; 
    gvlp2[0] = gvl2; 

    gpc_polygon gp2; 
    gp2.num_contours = 1; 
    gp2.hole = 0; 
    gp2.contour = gvlp2; 

    // Do convex polygon intersection 
    gpc_polygon result; 
    gpc_polygon_clip(GPC_INT, &gp1, &gp2, &result); 

而且为了清楚起见，我用了两个文件的内容是

#gcp.h 

#define GPC_EPSILON (DBL_EPSILON) 
#define GPC_VERSION "2.32" 

/* 
=========================================================================== 
          Public Data Types 
=========================================================================== 
*/ 

typedef enum      /* Set operation type    */ 
{ 
    GPC_DIFF,       /* Difference      */ 
    GPC_INT,       /* Intersection      */ 
    GPC_XOR,       /* Exclusive or      */ 
    GPC_UNION       /* Union        */ 
} gpc_op; 

typedef struct      /* Polygon vertex structure   */ 
{ 
    double    x;   /* Vertex x component    */ 
    double    y;   /* vertex y component    */ 
} gpc_vertex; 

typedef struct      /* Vertex list structure    */ 
{ 
    int     num_vertices; /* Number of vertices in list  */ 
    gpc_vertex   *vertex;  /* Vertex array pointer    */ 
} gpc_vertex_list; 

typedef struct      /* Polygon set structure    */ 
{ 
    int     num_contours; /* Number of contours in polygon  */ 
    int    *hole;   /* Hole/external contour flags  */ 
    gpc_vertex_list *contour;  /* Contour array pointer    */ 
} gpc_polygon; 

typedef struct      /* Tristrip set structure   */ 
{ 
    int     num_strips; /* Number of tristrips    */ 
    gpc_vertex_list *strip;  /* Tristrip array pointer   */ 
} gpc_tristrip; 


/* 
=========================================================================== 
         Public Function Prototypes 
=========================================================================== 
*/ 

void gpc_read_polygon  (FILE   *infile_ptr, 
           int    read_hole_flags, 
           gpc_polygon  *polygon); 

void gpc_write_polygon  (FILE   *outfile_ptr, 
           int    write_hole_flags, 
           gpc_polygon  *polygon); 

void gpc_add_contour   (gpc_polygon  *polygon, 
           gpc_vertex_list *contour, 
           int    hole); 

void gpc_polygon_clip  (gpc_op   set_operation, 
           gpc_polygon  *subject_polygon, 
           gpc_polygon  *clip_polygon, 
           gpc_polygon  *result_polygon); 

void gpc_tristrip_clip  (gpc_op   set_operation, 
           gpc_polygon  *subject_polygon, 
           gpc_polygon  *clip_polygon, 
           gpc_tristrip *result_tristrip); 

void gpc_polygon_to_tristrip (gpc_polygon  *polygon, 
           gpc_tristrip *tristrip); 

void gpc_free_polygon  (gpc_polygon  *polygon); 

void gpc_free_tristrip  (gpc_tristrip *tristrip); 

#endif 

在我自己的C++头文件开头的方法之前，我把#include“gpc.h”，我认为它会照顾它。但它没有奏效。

Answer 1

如果使用在一个cpp文件C函数，包括C头文件象下面这样：

extern "C" { 
    #include "gpc.h" 
} 

因为在C和C++函数符号是不同的。