c++ - 如何从主类中调用 C++ 中的函数？

Question

这是主文件:

// ConsoleApplication7.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include "targetver.h"

//void  __video_encode_example(const char *filename);

int _tmain(int argc, _TCHAR* argv[])
{

    //__video_encode_example("adi.avi");
    return 0;
}

我尝试使用以下方式调用该函数:

void  __video_encode_example(const char *filename);
__video_encode_example("adi.avi");

但是我遇到了一些错误。

这是带有我试图调用它的函数的类:

#define FF_API_AVCODEC_OPEN 1
#ifdef __cplusplus
   extern "C"

#include  "libavcodec\avcodec.h"
#include "libavutil\mathematics.h"

void video_encode_example(const char *filename)
 {
     AVCodec *codec;
     AVCodecContext *c= NULL;
     int i, out_size, size, x, y, outbuf_size;
     FILE *f;
     AVFrame *picture;
     uint8_t *outbuf;
     int n;

     printf("Video encoding\n");

     /* find the mpeg1 video encoder */
     codec = avcodec_find_encoder(CODEC_ID_MPEG1VIDEO);

     if (!codec) {
         fprintf(stderr, "codec not found\n");
         exit(1);
     }

     c = avcodec_alloc_context3(codec);
     picture= avcodec_alloc_frame();

     /* put sample parameters */
     c->bit_rate = 400000;
     /* resolution must be a multiple of two */
     c->width = 352;
     c->height = 288;
     /* frames per second */
     //c->time_base= (AVRational){1,25};
     c->time_base.num=1;c->time_base.den=25;

     c->gop_size = 10; /* emit one intra frame every ten frames */
     c->max_b_frames=1;
     c->pix_fmt = PIX_FMT_YUV420P;

     /* open it */
     if (avcodec_open(c, codec) < 0) {
         fprintf(stderr, "could not open codec\n");
         exit(1);
     }

     //f = fopen(filename, "wb");

     n = fopen_s(&f,filename, "wb");

     if (!f) {
         fprintf(stderr, "could not open %s\n", filename);
         exit(1);
     }

     /* alloc image and output buffer */
     outbuf_size = 100000;
     outbuf = (uint8_t*)malloc(outbuf_size);

     /* the image can be allocated by any means and av_image_alloc() is
      * just the most convenient way if av_malloc() is to be used */

     //av_image_alloc(picture->data, picture->linesize,
       //             c->width, c->height, c->pix_fmt, 1);
     picture->data[0] = (uint8_t*)av_malloc(1000000);
     picture->data[1] = (uint8_t*)av_malloc(1000000);
     picture->data[2] = (uint8_t*)av_malloc(1000000);
     picture->data[3] = (uint8_t*)av_malloc(1000000);
     picture->data[4] = (uint8_t*)av_malloc(1000000);
     picture->data[5] = (uint8_t*)av_malloc(1000000);
     picture->data[6] = (uint8_t*)av_malloc(1000000);
     picture->data[7] = (uint8_t*)av_malloc(1000000);

     /* encode 1 second of video */
     for(i=0;i<25;i++) {
         fflush(stdout);
         /* prepare a dummy image */
         /* Y */
         for(y=0;y<c->height;y++) {
             for(x=0;x<c->width;x++) {
                 picture->data[0][y * picture->linesize[0] + x] = x + y + i * 3;
             }
         }

         /* Cb and Cr */
         for(y=0;y<c->height/2;y++) {
             for(x=0;x<c->width/2;x++) {
                 picture->data[1][y * picture->linesize[1] + x] = 128 + y + i * 2;
                 picture->data[2][y * picture->linesize[2] + x] = 64 + x + i * 5;
             }
         }

         /* encode the image */
         out_size = avcodec_encode_video(c, outbuf, outbuf_size, picture);
         printf("encoding frame %3d (size=%5d)\n", i, out_size);
         fwrite(outbuf, 1, out_size, f);
     }

     /* get the delayed frames */
     for(; out_size; i++) {
         fflush(stdout);

         out_size = avcodec_encode_video(c, outbuf, outbuf_size, NULL);
         printf("write frame %3d (size=%5d)\n", i, out_size);
         fwrite(outbuf, 1, out_size, f);
     }

     /* add sequence end code to have a real mpeg file */
     outbuf[0] = 0x00;
     outbuf[1] = 0x00;
     outbuf[2] = 0x01;
     outbuf[3] = 0xb7;
     fwrite(outbuf, 1, 4, f);
     fclose(f);
     free(outbuf);

     avcodec_close(c);
     av_free(c);
     av_free(picture->data[0]);
     av_free(picture);
     printf("\n");
     #endif

问题是如何在main中声明和使用该函数？

现在，如果我编译它，我不会出现任何错误，但当我尝试在主函数中调用和使用该函数时，我遇到了一些错误。

编辑

这是我在尝试在 main 中声明和调用该函数时遇到的错误:

Error 1 error LNK2019: unresolved external symbol "void __cdecl __video_encode_example(char const *)" (?__video_encode_example@@YAXPEBD@Z) referenced in function wmain

还有:

Error 2 error LNK1120: 1 unresolved externals

我没有添加任何 header 或其他内容，这是具有上述函数的类，这就是我尝试从类中声明和调用该函数的方式。

Answer 1

我认为 extern "C" 定义不正确。之前使用

#ifdef __cplusplus 
extern "C" {
#endif

在函数定义之前和

#ifdef __cplusplus 
}
#endif

在它之后。它应该围绕函数定义，不包含#include。通常，行为良好的头文件无论如何都会包含这些行。您的 #endif 就在函数定义末尾之前。