用户输入文件路径

Question

The program is supposed to open a text file whose path is user-input. Next, it counts the lines contained in the file and outputs them. Here's what I tried:

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main()
{
string path = NULL;
string garbage = NULL;
int cnt = 0;
cout << "Enter file path: ";
cin >> path;

ifstream inFile(path.c_str());
if (inFile)
{
    while (!inFile.eof())
    {
        getline(inFile, garbage);
        ++cnt;
    }
}
inFile.close();

cout << endl;

cout << path << " has " << cnt << " lines";


cin.ignore();
cin.get();
return 0;
}

This is what I get:

Program: C:\\Windows\\SYSTEM32\\MSVCP120D.dll File: c:\\program files (x86)\\microsoft visual studio 12.0\\vc\\include\\xstring Line: 1168 Expression: invalid null pointer

Note: The course I'm following has only shown me the basics of the methods used by ifstream and ofstream, like open, close and eof. So I would appreciate a solution with only these, as I'm sure you know many ways of doing this.

Answer 1

The class std::string is a reasonable container class, like std::vector but with an API that has a number of extra string-oriented functions.

In a particular, its use does not resemble old-fashioned C-style string handling in terms of char* , which is where I assume you got the idea of trying to use NULL as an initializer. (in modern C++ you should use the C++ keyword nullptr to create a null pointer, rather than the old C style macro NULL )

what string path = NULL; actually does is interpret NULL as a const char* , and then tries to read the C-style string at the location NULL points to so as to copy it into path . Since NULL is null rather than actually pointing to a string, you get the error message you cite.

What you really want to do is to simply use the default constructor via string path; which initializes path to be an empty string.

Answer 2

Don't use the meaningless NULL they're not pointers:

string path;
string garbage;