求了解ifstream 求解解决思路

求了解ifstream 求解
我的ifstream后面跟路径   怎么没用啊


  ifstream   ifs( "D:\a.txt ");
        if   (ifs   ==   false)   {
                cerr   < <   "Unable   to   open   file "   < <   endl;
                return   -1;
        }
这个文件存在  
但是我的结果     Unable   to   open   file    
有什么好的方法解决


------解决方案--------------------
If the constructor is not successful in opening the file, the object is still created although no file is associated to the stream buffer and the stream's failbit is set (which can be checked with inherited member fail).

用成员函数fail()
if (ifs.fail())

一个输出例子

C/C++ code

// ifstream constructor.
#include <iostream>
#include <fstream>
using namespace std;

int main () {

  ifstream ifs ( "test.txt" , ifstream::in );

  while (ifs.good())
    cout << (char) ifs.get();

  ifs.close();

  return 0;
}