c++ - 在 C++ 中访问由 Bash 进程替换创建的 'virtual file'

Question

我有一个 C++ 可执行文件，在正常使用中，它以下列方式接受文件名作为参数选项:

executable -i myFile.txt

我想使用 Bash 进程替换来创建一个“虚拟文件”并按以下方式将信息(简单的逐行数据)发送到此可执行文件:

executable -i <(echo "${myData}")

但是，当我使用这个进程替换时，我的 C++ 程序没有访问信息。 C++程序中主要文件读取部分代码如下:

ifstream file1 (fileName1);
string line;
int currentLineNumber = 0;
if (verboseFlag == 1) {cout << "reading data from file " << fileName1 << "..." << endl;}
while (getline (file1, line)){
    currentLineNumber++;
    if (verboseFlag == 1) {cout << "line " << currentLineNumber << ": ";}
    istringstream linestream(line);
    string item;
    int itemNumber = 0;
    while (getline (linestream, item, ',')){
        itemNumber++;
        if (verboseFlag == 1) {cout << "item " << itemNumber << ": " << item << " ";}
        // data
            if (itemNumber == 1) {x[currentLineNumber]=atof(item.c_str());}
            if (itemNumber == 2) {y[currentLineNumber]=atof(item.c_str());}
    }
}
file1.close();

你能为我指出解决这个阅读问题的正确方向吗？是否有一些更好的方法既适用于正常文件读取，也适用于进程替换"file"读取？

我是这个流程替代的新手，非常感谢任何帮助。

---

编辑:

根据一些评论，下面是一个说明我遇到的问题的最小工作示例:

// definition of standard input/output stream objects
    #include <iostream>
// manipulate strings as though they were input/output streams
    #include <sstream>
// input and output operations
    #include <stdio.h>
// file input and output operations
    #include <fstream>
// manipulate C strings and arrays
    #include <string.h>
// classify and transform individual characters
    #include <ctype.h>
// Standard General Utilities Library
    #include <stdlib.h>
// getopts (handle command line options and arguments)
    #include <unistd.h>
// sstream (handle conversion from char* to double)
    #include <sstream>

using namespace std;

double returnDoubleFromPointerToChar(const char *cText){
    std::stringstream ss ( cText );
    double dText = 0;
    ss >> dText;
    return dText;
}

int returnNumberOfLinesInFile(const char *fileName1){
    int lineCount = 0;
    string line;
    ifstream file1(fileName1);
    while (std::getline(file1, line))
        ++lineCount;
    file1.close();
    return lineCount;
}

int main (int argc, char **argv){
    char *fileName1 = NULL; // input file name  (i) (required input)
    int verboseFlag = 0;        // verbose flag     (v)
    int index; // internal variable
    int c; // internal variable
    opterr = 0;

    // process command line arguments and options
        while ((c = getopt (argc, argv, "i:v")) != -1)
            switch (c){
                case 'i':
                    fileName1 = optarg;
                    break;
                case 'v':
                    verboseFlag = 1;
                    break;
                case '?':
                    if (
                        optopt == 'i'
                    ){
                        fprintf (stderr, "option -%c requires an argument.\n", optopt);
                    }
                    else if (isprint (optopt)){
                        fprintf (stderr, "unknown option `-%c'.\n", optopt);
                    }
                    else {
                        fprintf (stderr, "unknown option character `\\x%x'.\n", optopt);
                    }
                    return 1;
                default:
                    abort ();
        }
        for (index = optind; index < argc; index++) printf ("non option argument %s\n", argv[index]);
    if (verboseFlag == 1){
        cout << endl;
        cout << "input file name: " << fileName1 << endl;
    }
    // Determine the number of lines in the input file.
        int numberOfLinesInInputFile=returnNumberOfLinesInFile(fileName1);
        if (verboseFlag == 1) {cout << "number of lines in input file: " << numberOfLinesInInputFile << endl;}
    // number of data points
        int n=numberOfLinesInInputFile-1;
    // x variable
        double x[n];
    // y variable
        double y[n];
    // Access the data in the input file.
        ifstream file1 (fileName1);
        string line;
        int currentLineNumber = 0;
        if (verboseFlag == 1) {cout << "reading data from file " << fileName1 << "..." << endl;}
        while (getline (file1, line)){
            currentLineNumber++;
            if (verboseFlag == 1) {cout << "line " << currentLineNumber << ": ";}
            istringstream linestream(line);
            string item;
            int itemNumber = 0;
            while (getline (linestream, item, ',')){
                itemNumber++;
                if (verboseFlag == 1) {cout << "item " << itemNumber << ": " << item << " ";}
                // data
                    if (itemNumber == 1) {x[currentLineNumber]=atof(item.c_str());}
                    if (itemNumber == 2) {y[currentLineNumber]=atof(item.c_str());}
            }
            if (verboseFlag == 1) {cout << endl;}
        }
        file1.close();
    return 0;
}

编辑:

我在下面添加了解决方案代码(根据其他人的评论):

// include WBM C++ library
//  #include "lib_cpp.c"
// definition of standard input/output stream objects
    #include <iostream>
// manipulate strings as though they were input/output streams
    #include <sstream>
// input and output operations
    #include <stdio.h>
// file input and output operations
    #include <fstream>
// manipulate C strings and arrays
    #include <string.h>
// classify and transform individual characters
    #include <ctype.h>
// Standard General Utilities Library
    #include <stdlib.h>
// getopts (handle command line options and arguments)
    #include <unistd.h>
// sstream (handle conversion from char* to double)
    #include <sstream>

using namespace std;

// example usage:
//  ./graph2d -i data.txt -o data.eps -v
//  ./graph2d -i data.txt -o graph.eps -t "training test error versus epochs" -x "epochs" -y "error measure" -v
//  ./graph2d -i data.txt -o graph.eps -t "training test error versus epochs" -x "epochs" -y "error measure" -a 70 -b 50 -c 22 -d 7 -v
//  ./graph2d -i <(echo "${dataForTrainingErrorVersusEpoch}") -o graph.eps -t "training test error versus epochs" -x "epochs" -y "error measure" -v

double returnDoubleFromPointerToChar(const char *cText){
    std::stringstream ss ( cText );
    double dText = 0;
    ss >> dText;
    return dText;
}

int main (int argc, char **argv){
    char *fileName1 = NULL; // input file name  (i) (required input)
    char *fileName2 = NULL; // output file name (o) (required input)
    char *graphTitleMain = NULL;    // graph title      (t)
    char *graphTitleAxisx = NULL;   // graph x axis title   (x)
    char *graphTitleAxisy = NULL;   // graph y axis title   (y)
    double axisyMaximum = DBL_MAX;  // y axis maximum   (a)
    double axisyMinimum = DBL_MAX;  // y axis minimum   (b)
    double axisxMaximum = DBL_MAX;  // x axis maximum   (c)
    double axisxMinimum = DBL_MAX;  // x axis minimum   (d)
    int verboseFlag = 0;        // verbose flag     (v)
    int index; // internal variable
    int c; // internal variable
    opterr = 0;

    // process command line arguments and options
        while ((c = getopt (argc, argv, "i:o:t:x:y:a:b:c:d:v")) != -1)
            switch (c){
                case 'i':
                    fileName1 = optarg;
                    break;
                case 'o':
                    fileName2 = optarg;
                    break;
                case 't':
                    graphTitleMain = optarg;
                    break;
                case 'x':
                    graphTitleAxisx = optarg;
                    break;
                case 'y':
                    graphTitleAxisy = optarg;
                    break;
                case 'a':
                    axisyMaximum = returnDoubleFromPointerToChar(optarg);
                    break;
                case 'b':
                    axisyMinimum = returnDoubleFromPointerToChar(optarg);
                    break;
                case 'c':
                    axisxMaximum = returnDoubleFromPointerToChar(optarg);
                    break;
                case 'd':
                    axisxMinimum = returnDoubleFromPointerToChar(optarg);
                    break;
                case 'v':
                    verboseFlag = 1;
                    break;
                case '?':
                    if (
                        optopt == 'i' ||
                        optopt == 'o' ||
                        optopt == 't' ||
                        optopt == 'x' ||
                        optopt == 'y' ||
                        optopt == 'a' ||
                        optopt == 'b' ||
                        optopt == 'c' ||
                        optopt == 'd'
                    ){
                        fprintf (stderr, "option -%c requires an argument.\n", optopt);
                    }
                    else if (isprint (optopt)){
                        fprintf (stderr, "unknown option `-%c'.\n", optopt);
                    }
                    else {
                        fprintf (stderr, "unknown option character `\\x%x'.\n", optopt);
                    }
                    return 1;
                default:
                    abort ();
        }
        for (index = optind; index < argc; index++) printf ("non option argument %s\n", argv[index]);
    if (verboseFlag == 1){
        cout << endl;
        cout << "input file name: " << fileName1 << endl;
        cout << "output file name: " << fileName2 << endl;
    }
    // x variable
        vector<int> x;
    // y variable
        vector<int> y;
    // Access the data in the input file.
        ifstream file1 (fileName1);
        string line;
        int currentLineNumber = 0;
        if (verboseFlag == 1) {cout << "reading data from file " << fileName1 << "..." << endl;}
        while (getline (file1, line)){
            currentLineNumber++;
            if (verboseFlag == 1) {cout << "line " << currentLineNumber << ": ";}
            istringstream linestream(line);
            string item;
            int itemNumber = 0;
            while (getline (linestream, item, ',')){
                itemNumber++;
                if (verboseFlag == 1) {cout << "item " << itemNumber << ": " << item << " ";}
                // data
                    if (itemNumber == 1) {x.push_back(atof(item.c_str()));}
                    if (itemNumber == 2) {y.push_back(atof(item.c_str()));}
            }
            if (verboseFlag == 1) {cout << endl;}
        }
        file1.close();
        int numberOfLinesInInputFile = currentLineNumber + 1;
    // number of data points
        int n=numberOfLinesInInputFile;
    // graph
        if (verboseFlag == 1){
            cout << "graph main title: " << graphTitleMain << endl;
            cout << "graph x axis title: " << graphTitleAxisx << endl;
            cout << "graph y axis title: " << graphTitleAxisy << endl;
        }
        // Create a new canvas.
            TCanvas *c1 = new TCanvas(graphTitleMain, graphTitleMain); // #u
        // Create a new graph.
            //TGraph *graph = new TGraph(n, x, y);
            TGraph *graph = new TGraph(n, &x[0], &y[0]);
        // Set the graph titles.
            graph->SetTitle(graphTitleMain);
            graph->GetXaxis()->SetTitle(graphTitleAxisx);
            graph->GetYaxis()->SetTitle(graphTitleAxisy);
        // Set the marker styles.
            graph->SetMarkerColor(2); // red
            graph->SetMarkerStyle(kFullCircle); // circle
            graph->SetMarkerSize(1); // default size
        // Set the graph range, if ranges have been specified in command line options.
            if (
                axisyMaximum != DBL_MAX &&
                axisyMinimum != DBL_MAX
            ){
                if (verboseFlag == 1){
                    cout << "graph y axis minimum: " << axisyMinimum << endl;
                    cout << "graph y axis maximum: " << axisyMaximum << endl;
                }
                graph->GetYaxis()->SetRangeUser(axisyMinimum, axisyMaximum);
            }
            if (
                axisxMaximum != DBL_MAX &&
                axisxMinimum != DBL_MAX
            ){
                if (verboseFlag == 1){
                    cout << "graph x axis minimum: " << axisxMinimum << endl;
                    cout << "graph x axis maximum: " << axisxMaximum << endl;
                }
                graph->GetXaxis()->SetRangeUser(axisxMinimum, axisxMaximum);
            }
        // Draw the canvas, then draw the graph and then save the canvas to an image file.
            c1->Draw();
            graph->Draw("ALP");
            // disable ROOT messages
                gErrorIgnoreLevel = 5000;
            if (verboseFlag == 1) {cout << "saving file " << fileName2 << "..." << endl;}
            c1->SaveAs(fileName2);
    if (verboseFlag == 1) {cout << endl;}
    return 0;
}

Answer 1

首先，我删减了你程序的三分之二，它仍然显示出问题。这明显更接近于最小值:

#include <iostream>
#include <fstream>

using namespace std;

int returnNumberOfLinesInFile(const char *fileName1){
  int lineCount = 0;
  string line;
  ifstream file1(fileName1);
  while (std::getline(file1, line))
    ++lineCount;
  file1.close();
  return lineCount;
}

int main (int argc, char **argv){
  char *fileName1 = argv[1];
  cout << "input file name: " << fileName1 << endl;
  int numberOfLinesInInputFile=returnNumberOfLinesInFile(fileName1);
  cout << "number of lines in input file: " << numberOfLinesInInputFile << endl;

  ifstream file1(fileName1);
  string line;
  cout << "File contents: " << endl;
  while (getline (file1, line)){
    cout << "line: " << line << endl;
  }
  file1.close();
  return 0;
}

这里的问题是您打开文件两次。 <(process substitution)只运行一次命令并流式传输结果。如果您想再次读取输出，Bash 不会擅自再次运行该命令，因为该命令除了吐出文本之外还可以做很多其他事情。

确保您的程序只打开并读取一次内容，它就可以运行。这可能需要您稍微重写您的逻辑，或者只是懒惰并立即将其全部读入内存。