当ifstream用键盘读取的字符串创建文件时，为什么会出现错误？

Question

I am doing exercises from Bjarne Stroustrup's book, Programming Principles and Practice Using C++ . I am on the first exercise in Chapter 10 , where it says to write a program that produces the sum of all the numbers in a file of whitespace-separated integers. I based my code below on what's used for Exercise 2 of Chapter 10.5 . I get an error when the ifstream object is created. Here is the code I am trying to run:

#include "../../std_lib_facilities.h"

int main(int argc, const char * argv[]) {
    // insert code here...

    cout << "Plese enter the input file name: " << endl;
    string iname;
    cin >> iname;
    ifstream ist {iname};
    if (!ist) error("Can't open input file ",iname);

    vector<int> numbers;
    int sum;
    int n;
    while(ist>>n) {
        numbers.push_back(n);
    }

    for (int i=0; i<numbers.size(); ++i) {
        sum += numbers[i];
    }
    cout << sum << endl;

    return 0;
}

Any input I enter is getting error. I tried myin, myin.txt or any other name. The error("Can't open input file ",iname); is from the library created by the author.

I know the file does exist in the same directory with main.cpp and created with TextEdit from Mac using the format for plain text.

Answer 1

[...] in the same directory with main.cpp [...]

It does not really matter where you put the input file relative to the source file. The file should be in the environment's current working directory when you run the program.

Answer 2

There must be some confusion in passing argument. You should try to pass absolute path of input file.

Below is your modified application. This will create One test file and use it instead of asking file name for Case 1. For case 2, it use file which doesn't exist.(Delete if present)

#include <cstdio>
#include <fstream>
#include <iostream>
#include <vector>
using namespace std;
template <typename T> void error(const T &t) { cout << t; }
template <typename T, typename... Args> void error(const T &t, Args... args) {
  cout << t << " ";
  error(args...);
  cout << "\n";
}

int main(int argc, const char *argv[]) {
  // insert code here...
  // cout << "Plese enter the input file name: " << endl;
  string iname = "a.txt";
  ofstream ofs{iname};
  ofs << 1 << " " << 2 << " " << 3 << " " << 4;
  ofs.close();
  //  cin >> iname;
  // part 1
  {
    cout << "Case1: Reading file a.txt which is just created\n";
    ifstream ist{iname};
    if (!ist)
      error("Can't open input file ", iname);
    if (ist.is_open()) {
      vector<int> numbers;
      int sum = 0;
      int n = 0;
      while (ist >> n) {
        numbers.push_back(n);
      }
      for (int i = 0; i < numbers.size(); ++i) {
        sum += numbers[i];
      }
      cout << sum << endl;
      ist.close();
    } else {
      error("can't open file to read", iname);
    }
  }
  // part 2
  {
    cout << "Case2:reading file which is not present\n";
    iname = "b.txt";
    std::remove(iname.c_str()); // delete if present
    ifstream ist{iname};
    if (!ist)
      error("Can't open input file ", iname);
    if (ist.is_open()) {
      vector<int> numbers;
      int sum = 0;
      int n = 0;
      while (ist >> n) {
        numbers.push_back(n);
      }
      for (int i = 0; i < numbers.size(); ++i) {
        sum += numbers[i];
      }
      cout << sum << endl;
      ist.close();
    } else {
      error("can't open file to read", iname);
    }
  }
  return 0;
}

Note: Construct of std::ifstream always create object. You need to its object either as you did or is_open() method.