Check if cmd arg file exist for g2o_viewer

[FelixSchladt]

Hi All,

thanks for this awesome library and the continued support.

We have been working with g2o and g2o_viewer in the last months and normally open the graph files with the viewer providing them as command line argument.

Unfortunately I am not perfect and sometimes provided an incorrect filename, which led to some wrong debugging as I rather questioned my code instead of the filename.

Unfortunately g2o_viewer does not give an error message if the provided file does not exists.
This would be something I would appreciate if this would be added.

Greetings Felix and Jan

[RainerKuemmerle]

Call fileExists instead? see https://github.com/RainerKuemmerle/g2o/blob/master/g2o/stuff/filesys_tools.h#L80C20-L80C30
Downside would be it also passes for a directory. We probably could use some filessystem calls instead directly.

[FelixSchladt]

The current approach with ifstream, does also not check if the file is a directory. Therefore we actually could use the function and would have the same result.

One approach would be to use stat to check the filetype:

bool is_regular_file(const std::string& path) {
    struct stat path_stat;
    stat(path.c_str(), &path_stat);
    return S_ISREG(path_stat.st_mode); // Return true if it's a regular file
}

This works just fine but will only work on posix compliant systems.
So far I did not find a good way to achieve this to work on windows as well as on posix without using preprocessor magic. Additionally I do not have any means to test on windows.

The right way in my opinion would be to use C++17's filesystem library, but for this the c++ version would require a bump.

So this might be a lean efficient compromise