Can I use the ftw-function for class methods in C++? - c++

Question

I would like to use the ftw-function to recursivly traverse a filesystem structure. Additionally, the method shall be used inside of a class. Also, the entry-function, which is called by nftw(), belongs to the same class. That needs to be the case because the entry-function is supposed to change some class-members, dependent on the files that it finds.
When implementing such an approach, I get an error (see below). Is this an issue of syntax or is it not even possible to forward a pointer to a method to nftw()? In case it is not possible, do you know any alternative way to resursivly traverse a filesystem structure under linux?
class model
{
public:
boost::unordered_map<std::string, int> map;
int configure(const char *name)
{
// ...
ftw("DTModels/", this->ftw_entry, 15);
// ...
return = 0;
}
private:
int ftw_entry(const char *filepath, const struct stat *info, const int typeflag)
{
// Here I want to change the member 'map'
std::string filepath_s = filepath;
std::cout << "FTW_Entry: " << filepath_s << std::endl;
}
};
ERROR:
a pointer to a bound function may only be used to call the function
ftw("DTModels/", this->ftw_entry, 15);

Answer 1

I haven't used ftw in many many years and since you ask for an alternative, take a look at std::filesystem (C++17). Many pre-C++17 installations have it available via boost or experimental. If you use one of the pre-C++17 implementations, you many need to remove some of the stat lines from the below to make it work.
#include <iostream>
//#define I_HAVE_BOOST
#if __cplusplus >= 201703L
#include <filesystem>
namespace fs = std::filesystem;
#elif I_HAVE_BOOST
#include <boost/filesystem.hpp>
namespace fs = boost::filesystem;
#else
#include <experimental/filesystem>
namespace fs = std::experimental::filesystem;
#endif
auto& out = std::cout;
void show_dent(const fs::directory_entry& dent) {
static size_t indent=0;
std::string ind(indent, ' ');
fs::file_status lstat = dent.symlink_status();
if( fs::is_symlink(lstat) ) {
fs::path pp = fs::read_symlink(dent);
out << ind << dent << " -> " << pp << "\n";
++indent;
show_dent(fs::directory_entry(pp));
--indent;
} else {
if(fs::is_directory(dent)) {
fs::directory_iterator dit_end;
std::cout << "Directory " << dent << " includes the following files:\n";
++indent;
for(auto dit = fs::directory_iterator(dent); dit != dit_end; ++dit) {
show_dent(*dit);
}
--indent;
} else {
fs::file_status stat = dent.status();
out << ind << dent << "\n"
<< ind << " stat\n"
<< ind << " is_regular_file : " << fs::is_regular_file(stat) << "\n"
<< ind << " is_directory : " << fs::is_directory(stat) << "\n"
<< ind << " is_block_file : " << fs::is_block_file(stat) << "\n"
<< ind << " is_character_file: " << fs::is_character_file(stat) << "\n"
<< ind << " is_fifo : " << fs::is_fifo(stat) << "\n"
<< ind << " is_socket : " << fs::is_socket(stat) << "\n"
<< ind << " is_symlink : " << fs::is_symlink(stat) << "\n"
<< ind << " exists : " << fs::exists(stat) << "\n";
if( fs::is_regular_file(stat) ) {
out
<< ind << " file_size : " << fs::file_size(dent) << "\n";
}
}
}
}
int main(int argc, char* argv[]) {
std::vector<std::string> args(argv+1, argv+argc);
out << std::boolalpha;
for(const auto& file_or_dir : args) {
show_dent(fs::directory_entry(file_or_dir));
}
return 0;
}