You can use boost::filesystem:

struct space_info  // returned by space function
{
    uintmax_t capacity;
    uintmax_t free; 
    uintmax_t available; // free space available to a non-privileged process
};

space_info   space(const path& p);
space_info   space(const path& p, system::error_code& ec);

Example:

#include <boost/filesystem.hpp>
using namespace boost::filesystem;
space_info si = space(".");
cout << si.available << endl;

Returns: An object of type space_info. The value of the space_info object is determined as if by using POSIX statvfs() to obtain a POSIX struct statvfs, and then multiplying its f_blocks, f_bfree, and f_bavail members by its f_frsize member, and assigning the results to the capacity, free, and available members respectively. Any members for which the value cannot be determined shall be set to -1.

With C++17

You can use std::filesystem::space:

#include <iostream>  // only needed for screen output

#include <filesystem>
namespace fs = std::filesystem;

int main()
{
    fs::space_info tmp = fs::space("/tmp");

    std::cout << "Free space: " << tmp.free << '\n'
              << "Available space: " << tmp.available << '\n';
}

You can use Qt class QStorageInfo to acquire hardisk free space: First,you should include the header:

#include <QStorageInfo> 
#define GB (1024 * 1024 * 1024)
bool CheckHardiskFree(const QString &strDisk)
{
    QStorageInfo storage(strDisk);
    if(storage.isValid() && storage.isReady())
    {
     double useGb =(storage.bytesTotal()-storage.bytesAvailable()) * 1.0/ GB;
     double freeGb =storage.bytesAvailable() * 1.0 / GB;
     double allGb =storage.bytesTotal()* 1.0 / GB;
     return true;
    }
    return false;
}