Checking for an empty file in C++

问题

Is there an easy way to check if a file is empty. Like if you are passing a file to a function and you realize it's empty, then you close it right away? Thanks.

Edit, I tried using the fseek method, but I get an error saying 'cannot convert ifstream to FILE *'.

My function's parameter is

myFunction(ifstream &inFile)

回答1:

Perhaps something akin to:

bool is_empty(std::ifstream& pFile)
{
    return pFile.peek() == std::ifstream::traits_type::eof();
}

Short and sweet.

With concerns to your error, the other answers use C-style file access, where you get a FILE* with specific functions.

Contrarily, you and I are working with C++ streams, and as such cannot use those functions. The above code works in a simple manner: peek() will peek at the stream and return, without removing, the next character. If it reaches the end of file, it returns eof(). Ergo, we just peek() at the stream and see if it's eof(), since an empty file has nothing to peek at.

Note, this also returns true if the file never opened in the first place, which should work in your case. If you don't want that:

std::ifstream file("filename");

if (!file)
{
    // file is not open
}

if (is_empty(file))
{
    // file is empty
}

// file is open and not empty

回答2:

Ok, so this piece of code should work for you. I changed the names to match your parameter.

inFile.seekg(0, ios::end);  
if (inFile.tellg() == 0) {    
  // ...do something with empty file...  
}

回答3:

Seek to the end of the file and check the position:

 fseek(fileDescriptor, 0, SEEK_END);
 if (ftell(fileDescriptor) == 0) {
     // file is empty...
 } else {
     // file is not empty, go back to the beginning:
     fseek(fileDescriptor, 0, SEEK_SET);
 }

If you don't have the file open already, just use the fstat function and check the file size directly.

回答4:

char ch;
FILE *f = fopen("file.txt", "r");

if(fscanf(f,"%c",&ch)==EOF)
{
    printf("File is Empty");
}
fclose(f);

回答5:

use this: data.peek() != '\0'

I've been searching for an hour until finaly this helped!

回答6:

pFile = fopen("file", "r");
fseek (pFile, 0, SEEK_END);
size=ftell (pFile);
if (size) {
  fseek(pFile, 0, SEEK_SET);
  do something...
}

fclose(pFile)

回答7:

How about (not elegant way though )

int main( int argc, char* argv[] )
{
    std::ifstream file;
    file.open("example.txt");

    bool isEmpty(true);
    std::string line;

    while( file >> line ) 
        isEmpty = false;

        std::cout << isEmpty << std::endl;
}

回答8:

if (nfile.eof()) // Prompt data from the Priming read:
    nfile >> CODE >> QTY >> PRICE;
else
{
    /*used to check that the file is not empty*/
    ofile << "empty file!!" << endl;
    return 1;
}

来源：https://stackoverflow.com/questions/2390912/checking-for-an-empty-file-in-c

标签

c++

eof