c++ - 在C++上接收 “Error: expected an identifier”的 “==”

Question

我在“if(f_in); std::ifstream == int NULL);”上收到错误代码，其中“==”下划线表示错误，并显示“错误:预期为标识符”。我不确定该怎么办，所以被困了:/。这是代码:

#include <iostream>     // For cin/cout
#include <fstream>      // For file stream objects (ifstream)
#include <string>       // For the string data object

using namespace std;

int main()
{

   ifstream f_in;   // Creat an object for an 'input' file stream
   string       data;   // An object to hold our data read in form the file


  )

f_in.open("InputData.txt"); 

if (f_in); std::ifstream == int NULL); {
    fprintf(stderr, "Can't open input file in.list!\n");
    exit(1);
}


f_in >> data;
cout << "First data item from file is: ' " << data << endl;

f_in >> data;
cout << "Second data item from file is: ' " << data << endl;

getline(f_in, data);
cout << "Line from file is: '" << data << endl;

getline(f_in, data);
cout << "Next line from file is: '" << data << endl;

// We are finished with the file. We need to close it.
f_in.close();

cout << "Finished!\n";

return 0;
}

Answer 1

if (f_in); std::ifstream == int NULL);

您可以这样重写:

if (f_in)
    ;
std::ifstream == int NULL);

您会发现这没有任何意义。

也许你的意思是:

if (!f_in) {
    fprintf(stderr, "Can't open input file in.list!\n");
    exit(1);
}

要么

if (f_in == NULL) {
    fprintf(stderr, "Can't open input file in.list!\n");
    exit(1);
}