Get File Size in C++

1. Use the std::filesystem::file_size Function to Get File Size in C++
2. Use std::filesystem::file_size With std::filesystem::path to Get File Size in C++
3. Use std::filesystem::file_size With error_code to Get File Size in C++
4. Use stat Function to Get File Size in C++

This article will demonstrate multiple methods of how to get file size in C++.

Use the std::filesystem::file_size Function to Get File Size in C++

std::filesystem::file_size is the C++ filesystem library function that retrieves the size of the file in bytes. std::filesystem::file_size takes path of the file as the function argument, which is of type std::filesystem::path. In the following example, though, we pass the string value of the file path, which is then used to construct the corresponding object and retrieve the file size.

#include <iostream>
#include <string>
#include <fstream>
#include <filesystem>

using std::cout; using std::cin;
using std::endl; using std::ifstream;

int main() {
    string path = "input.txt";

    cout << "size of file '" << path << "' = " <<
            std::filesystem::file_size(path) << endl;

    return EXIT_SUCCESS;
}

Output:

size of file 'inbox.txt' = 10440

Use std::filesystem::file_size With std::filesystem::path to Get File Size in C++

Alternative usage of std::filesystem::file_size function is to insert the file path value as the path type variable. At first, you would need to include the using std::filesystem::path statement before using the type. Next, we can declare the path object, as shown in the following example code, and initialize it with a string literal. The constructed variable is passed to the file_size function to get the file size.

#include <iostream>
#include <string>
#include <fstream>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <filesystem>

using std::cout; using std::cin;
using std::endl; using std::ifstream;
using std::string; using std::filesystem::path;
using std::error_code;

int main() {
    class path fd_path = "input.txt";

    cout << "size of file '" << fd_path << "' = " <<
            std::filesystem::file_size(fd_path) << endl;

    return EXIT_SUCCESS;
}

Output:

size of file 'inbox.txt' = 10440

Use std::filesystem::file_size With error_code to Get File Size in C++

Since std::filesystem::file_size utilizes underlying operating system services to retrieve the size of the file, there are good chances it may fail, raising some exception. To better handle each file_size function call, it is recommended to use the error_code object as the second argument and store the error message in case of failure. Consequently, we can evaluate the error_code object and implement error handling code using if..else statements.

#include <iostream>
#include <string>
#include <fstream>
#include <filesystem>

using std::cout; using std::cin;
using std::endl; using std::ifstream;
using std::string; using std::filesystem::path;
using std::error_code;

int main() {
    string path = "input.txt";
    error_code ec{};

    auto size = std::filesystem::file_size(path, ec);
    if (ec == error_code{})
        cout << "size of file '" << path << "' = " << size << endl;
    else
        cout << "Error accessing file '" << path
              << "' message: " << ec.message() << endl;

    return EXIT_SUCCESS;
}

Output:

size of file 'inbox.txt' = 10440

Use stat Function to Get File Size in C++

stat is the POSIX compliant function that is available across multiple operating systems. It’s often the function that the std::filesystem::file_size method underneath the hood. stat takes character string as the first argument to denote the path of the file address of struct stat type object as the second argument.

struct stat is a special predefined object that stores multiple file parameters after the successful stat function call. Note that we only need to access the st_size data member of the struct stat object, which stores file size as a number of bytes.

#include <iostream>
#include <string>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>

using std::cout; using std::cin;
using std::endl; using std::ifstream;
using std::string;

int main() {
    string path = "input.txt";
    struct stat sb{};

    if (!stat(path.c_str(), &sb)) {
        cout << "size of file '" << path << "' = "
                << sb.st_size << endl;
    } else {
        perror("stat");
    }

    return EXIT_SUCCESS;
}

Output:

size of file 'inbox.txt' = 10440

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