输入为shp格式的多边形文件，输出为平滑后的shp格式文件，请用C++代码实现

实现这个任务需要使用一些地理信息系统 (GIS) 库，比如 GDAL 和 GEOS。下面是一个使用 C 语言、GDAL 和 GEOS 库实现多边形平滑的示例代码：

#include <stdio.h>
#include <gdal.h>
#include <ogr_api.h>
#include <ogr_geometry.h>
#include <geos_c.h>

int main(int argc, char *argv[])
{
    GDALAllRegister();
    OGRRegisterAll();
    GEOSContextHandle_t geos_context = OGR_G_CreateGEOSContext();

    // 打开输入和输出文件
    const char *input_filename = argv[1];
    const char *output_filename = argv[2];
    GDALDatasetH input_ds = GDALOpenEx(input_filename, GDAL_OF_VECTOR, NULL, NULL, NULL);
    if (input_ds == NULL) {
        fprintf(stderr, "无法打开输入文件 %s\n", input_filename);
        return 1;
    }
    OGRDataSourceH input_ds_ogr = GDALToOGRLayer(input_ds, GDALGetLayerByName(input_ds, "OGRGeoJSON"));
    GDALDriverH output_driver = GDALGetDriverByName("ESRI Shapefile");
    GDALDatasetH output_ds = GDALCreate(output_driver, output_filename, 0, 0, 0, GDT_Unknown, NULL);
    if (output_ds == NULL) {
        fprintf(stderr, "无法创建输出文件 %s\n", output_filename);
        return 1;
    }
    OGRLayerH output_layer = GDALToOGRLayer(output_ds, GDALCreateLayer(output_ds, "output", NULL, wkbUnknown, NULL));

    // 循环处理每个要素
    OGRLayerH input_layer = OGR_DS_GetLayer(input_ds_ogr);
    OGRFeatureH input_feature;
    while ((input_feature = OGR_L_GetNextFeature(input_layer)) != NULL) {
        OGRGeometryH input_geometry = OGR_F_GetGeometryRef(input_feature);

        // 对要素进行平滑处理
        GEOSGeometry *geos_input_geometry = OGR_G_ExportToGEOS(geos_context, input_geometry);
        GEOSGeometry *geos_output_geometry = GEOSSimplify(geos_context, geos_input_geometry, 0.01);
        OGRGeometryH output_geometry = OGR_G_CreateGeometryFromGEOS(geos_context, geos_output_geometry);

        // 将处理后的要素写入输出文件
        OGRFeatureH output_feature = OGR_F_Create(OGR_L_GetLayerDefn(output_layer));
        OGR_F_SetGeometry(output_feature, output_geometry);
        OGR_L_CreateFeature(output_layer, output_feature);

        // 释放内存
        OGR_F_Destroy(input_feature);
        OGR_F_Destroy(output_feature);
        OGR_G_DestroyGeometry(input_geometry);
        OGR_G_DestroyGeometry(output_geometry);
        GEOSGeom_destroy(geos_input_geometry);
        GEOSGeom_destroy(geos_output_geometry);
    }

    // 关闭文件和释放内存
    GDALClose(output_ds);
    GDALClose(input_ds);
    GEOS_finish(geos_context);

    return 0;
}

这个示例代码中，使用 GDAL 打开输入文件和创建输出文件，然后循环处理每个要素。对于每个要素，首先使用 GEOS 库将 OGRGeometryH 类型的几何对象转换