c++ - GCC 5.2.1 上的 EOF 与 basic_ios::eof()

Question

在使用 GCC 5.2.1 编译的程序中，ifstream 的状态在到达文件末尾时不会切换到 basic_ios::eof() — 即，条件 if(eof()) 在到达文件末尾后评估为 false — 而在 Visual Studio 2015 上编译的相同代码的行为符合预期:当到达文件末尾时，basic_ios::eof() 在 if 条件中的计算结果为 true。

我用 if(bad()) 替换了 if(eof()) ，然后用 if(fail()) 替换了，并且所有这些都评估为 false。但是，当我改为放置 EOF 宏时，if(EOF) 的计算结果为 true——就像 if(eof( ))在VS编译的程序上做。

std::basic_ios::eof() 无法处理使用 GCC 编译的程序的原因可能是什么？

PS:下面是程序代码

#include <string>
#include <iostream>
#include <fstream>
#include <regex>

using namespace std;
using namespace std::chrono;

int ex10()
{
    ifstream in{ "school.txt" };
    if (!in) cout << "The 'school.txt' file was not opened.\n";

    string line;

    regex header{ R"(^([\w]+)(\t{2}\w+\s*\w*)(\t\w+\s*\w*)(\t\w+\s*\w*)$)" };
    regex row{ R"(^(\w+\s*\w*)(\t{1,2}\d+)(\t{2}\d+)(\t{2}\d+)$)" };

    if (getline(in, line)) {
        smatch matches;
        if (!regex_match(line, matches, header))
            cerr << "Wrong header format.\n";
    }
    int linenum = 0;
    int boys = 0;
    int girls = 0;

    ofstream out{ "schoolCompressed.txt" };
    if (!out) cout << "The output file was not created.\n";
    string prevLine;
    int accumBoys;
    int accumGirls;
    int accumTot;

    while (getline(in, line)) {
        ++linenum;
        smatch matches;
        if (!regex_match(line, matches, row))
            cerr << "Row #" << linenum << " doesn't match the format.\n";


        int curr_boy = stoi(matches[2]);
        int curr_girl = stoi(matches[3]);
        int curr_total = stoi(matches[4]);

        if (curr_boy + curr_girl != curr_total)
            cerr << "Wrong children number in line #" << linenum << '\n';

        if (line[0] != prevLine[0]) {

            if (linenum != 1) out << prevLine[0] << "\t\t" << accumBoys << "\t\t"
                << accumGirls << "\t\t" << accumTot << '\n';
            accumBoys = curr_boy;
            accumGirls = curr_girl;
            accumTot = curr_total;

        }

        else if (line[0] == prevLine[0]) {

            accumBoys += curr_boy;
            accumGirls += curr_girl;
            accumTot += curr_total;
        }


        if (EOF && curr_boy == boys && curr_girl == girls) { out << line; return 0; } //this works on GCC
        //if (in.eof() && curr_boy == boys && curr_girl == girls) { out << line; return 0; } <= this works on VS 2015


        boys += curr_boy;
        girls += curr_girl;
        prevLine = line;
    }
    cerr << "Somehow the program didn't manage to complete its task :(.\n";
    return 1;
}
int main()
{
    ex10();
}

school.txt 文件的文本
KLASSE DRENGE PIGER ELEVER 经典款
0A 12 11 23
1A 7 8 15
1B 4 11 15
2A 10 13 23
3A 10 12 22
4A 7 7 14
4B 10 5 15
5A 19 8 27
6A 10 9 19
6B 9 10 19
7A 7 19 26
7G 3 5 8
7我7 3 10
8A 10 16 26
9A 12 15 27
0MO 3 2 5
0P1 1 1 2
0P2 0 5 5
10B 4 4 8
10CE 0 1 1
1MO 8 5 13
2CE 8 5 13
3DCE 3 3 6
4MO 4 1 5
6CE 3 4 7
8CE 4 4 8
9CE 4 9 13
休息 5 6 11
Alle klasser 184 202 386

Answer 1

EOF 是一个常量整数的宏，所以

 if (EOF && curr_boy == boys && curr_girl == girls)

正在“工作”。它只是没有按照您的预期进行，即检查文件末尾。

您应该仅在 io 操作失败时使用 eof()。doc规定

std::basic_ios::eof only reports the stream state as set by the most recent I/O operation.it does not examine the associated data source. For example, if the most recent I/O was a get(), which returned the last byte of a file, eof() returns false. The next get() fails to read anything and sets the eofbit. Only then eof() returns true

在你的代码中你有

while (getline(in, line)) {
   ...
   if (in.eof())
   ...
}

预期的行为是 GCC 的行为之一。要知道您位于文件末尾，您应该尝试进一步阅读。这是一个类似的问题

while (!in.eof()) {
   ...
   getline(in, line)
   ...
}

您应该将 curr_boy 和条件移出 while 循环并消除 eof 测试

int curr_boy = 0;
int curr_girl = 0;
while (getline(in, line)) {
}

if (curr_boy == boys && curr_girl == girls) { out << line; return 0; }