判断文件是否存在

方法一：C++中比较简单的一种办法（使用文件流打开文件）

 1 #include <iostream>
 2 #include <fstream>
 3 
 4 using namespace std;
 5  
 6 #define FILENAME "*.dat"  // 指定文件名
 7 
 8 int main( void )
 9 {
10      fstream _file;
11      _file.open(FILENAME, ios::in);
12      if(!_file)
13      {
14           cout<<FILENAME<<"没有被创建!"<<endl;
15      }
16      else
17      {
18           cout<<FILENAME<<"已经存在!"<<endl;
19      }
20 
21      cin.get();
22      return 0;
23 }

方法二：利用C语言库函数（_access）

函数原型：

　　int _access( const char *path, int mode ）

函数参数：

　　l path：文件路径

　　l mode：读写属性

返回值（MSDN）：

Each of these functions returns 0 if the file has the given mode. The function returns –1 if the named file does not exist or is not accessible in the given mode; in this case, errno is set as follows:

EACCES Access denied: file’s permission setting does not allow specified access.

ENOENT Filename or path not found.

EINVAL Invalid parameter.

函数功能（MSDN）：

When used with files, the _access function determines whether the specified file exists and can be accessed as specified by the value of mode（见下图表）. When used with directories, _access determines only whether the specified directory exists; in Windows NT, all directories have read and write access.

/* ACCESS.C: This example uses _access to check the
 * file named "ACCESS.C" to see if it exists and if
 * writing is allowed.
 */

#include  <io.h>
#include  <stdio.h>
#include  <stdlib.h>
 
void main( void )
{
   /* Check for existence */
   if( (_access( "ACCESS.C", 0 )) != -1 )
   {
      printf( "File ACCESS.C exists " );
      /* Check for write permission */
      if( (_access( "ACCESS.C", 2 )) != -1 )
         printf( "File ACCESS.C has write permission " );
   }
}

输出：
>>File ACCESS.C exists.
>>File ACCESS.C has write permission

方法三：使用Windows API函数FindFirstFile(...)

　　(1) 检查某一文件是否存在：

#include "windows.h"
int main(int argc, char *argv[])
{
      WIN32_FIND_DATA  FindFileData;
      HANDLE hFind;
      printf ("Target file is %s. ", argv[1]);
      hFind = FindFirstFile(argv[1], &FindFileData);
      if (hFind == INVALID_HANDLE_VALUE)
      {

           printf ("Invalid File Handle. Get Last Error reports %d ", GetLastError ());

      }
      else
      {
           printf ("The first file found is %s ", FindFileData.cFileName);
           FindClose(hFind);
      }

      return  0;
}

　　(2) 检查某一目录是否存在：

// 目录是否存在的检查：
BOOL CheckFolderExist(const string &strPath)
{
 　　WIN32_FIND_DATA  FindFileData;
    BOOL bValue = false;
    HANDLE hFind = FindFirstFile(strPath.c_str(),  &FindFileData);
    if ((hFind != INVALID_HANDLE_VALUE) && (wfd.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY))
    {
    　　bValue = TRUE;  
    }
    FindClose(hFind);
    return bValue;
}

方法四：使用boost库中filesystem类库的exists函数

#include <boost/filesystem/operations.hpp>
#include <boost/filesystem/path.hpp>
#include <boost/filesystem/convenience.hpp>
 
using namespace boost::filesystem;
 
int GetFilePath(std::string &strFilePath)
{
    string strPath;
    int nRes = 0;
    //指定路径           
    strPath = "C:";

    path full_path( initial_path() );
    full_path = system_complete( path(strPath, native ) );
    //判断各级子目录是否存在，不存在则需要创建
    if ( !exists( full_path ) )
    {
        bool bRet = create_directories(full_path);
        if (false == bRet)
        {
            return -1;
        }
    }
    strFilePath  =  full_path.native_directory_string();
    return 0;
}