C / C++ / MFC

Re: copying a file in C++17

6hrs 30mins ago

I wrote some code to copy a file:

#include <iostream>
#include <filesystem>

namespace fs = std::filesystem;

int main() {
    fs::path source_file("source.txt");
    fs::path destination_file("destination.txt");

    try {
        fs::copy_file(source_file, destination_file);
        std::cout << "File copied successfully!" << std::endl;
    }
    catch (const fs::filesystem_error& e) {
        std::cerr << "Error copying file: " << e.what() << std::endl;
    }

    return 0;
}

Is this a good way of doing it?

Anyone know what buffer sizes are being used behind the scenes?

Thanks.

Richard MacCutchan5hrs 59mins ago

5hrs 59mins ago

mike7411 wrote:
Anyone know what buffer sizes are being used behind the scenes?

It is not recorded, but chances are it is calculated based on the size of the source file and the disk sector size. If you want to find out how particular STL features work then you can find lots of useful information at C++ reference - cppreference.com[^] or C++ Standard Library Reference | Microsoft Learn[^].

printf without stdio.h

mike741122hrs 45mins ago

22hrs 45mins ago

I am in Microsoft Visual Studio Community 2022 (64-bit).

I noticed that I can use printf even without including stdio.h.

It works if I do this:

C++

#include <iostream>

Any idea why this works?

Thanks.

Mircea Neacsu22hrs 35mins ago

22hrs 35mins ago

The chain of include files can be long and tortuous. In this particular case:
iostream -> istream -> ostream -> ios -> cstdio -> stdio.h

Should you rely on compiler include chains? Depends if you expect your code to be compiled with a different compiler and how long the chain is. In this particular case, there is a strong probability that all C++ iostream operations will end up like C function calls so you are pretty safe.

Mircea

mike741111hrs 43mins ago

11hrs 43mins ago

I'm looking at the ios file, and I don't see where cstdio gets included.

I did a search in the file for cstdio, and it doesn't seem to be there.

Thanks.

Mircea Neacsu11hrs 24mins ago

11hrs 24mins ago

I skipped a step: ios -> xlocnum -> cstdio.

Mircea

Victor Nijegorodov22hrs 34mins ago

Victor Nijegorodov

22hrs 34mins ago

mike7411 wrote:
It works if I do this:
C++
include <iostream>

Any idea why this works?

I guess, because there is a declaration of printf or or this is #included

k505422hrs 4mins ago

k5054

22hrs 4mins ago

iostream provides very complex facilities to C++, and requires a great deal of supporting infrastructure. You can get a list of all the include files that a compilation unit needs by using the /showIncludes flag in your compiles (-H for gcc/clang). Using Compiler Explorer, I see over 100 additional files included, including new, string, istream, ostream, atomic and cstdio (which gets you printf), just to name a few. As others have pointed out, you can just go ahead and use printf for example, and not add #include <cstdio> to your source file. However, as compilers evolve, you may find that this is no longer the case, at which point your code will fail to compile. Its good practice to #include all the relevant headers. System headers, at least, will have guards against multiple inclusions, so there's no harm in being explicit about what headers your program, or module, requires.

"A little song, a little dance, a little seltzer down your pants"
Chuckles the clown

copying a file

mike74115-Dec-23 4:24

5-Dec-23 4:24

Can someone tell me if this code works to copy a binary file?

while ((ch = fgetc(source)) != EOF) {
        fputc(ch, target);
    }

What if the file contains a byte that is 255?
Won't it get converted to -1 and break the loop?

Thank you.

Re: copying a file

Mircea Neacsu5-Dec-23 4:32

5-Dec-23 4:32

Depends on declaration of ch. If it’s an int (as it should), all is fine as 255 != -1. If it’s char, it fails, with or without compiler warnings.

Mircea

Re: copying a file

Richard MacCutchan5-Dec-23 5:05

5-Dec-23 5:05

You should not use fgetc, fgets, or any other character based function to process binary files. You should use fread | Microsoft Learn[^] and fwrite | Microsoft Learn[^].

Re: copying a file

k505423hrs 24mins ago

k5054

23hrs 24mins ago

While your method may work, assuming that ch is an int, and so does not have the 255/-1 issue, your method is horribly inefficient. Richard's suggestion to use fread/fwrite produces much better performance.

For example, given an 8M file, looping through your code 100 times took about 32 seconds. Using a fread/fwrite using a 4K buffer, the same 100 lops took just under 1 second to complete. This was on a PI-3, and successive runs remained stable. I'd expect an 8M file to sit comfortably in the file cache, so the difference is totally down to the difference between extracting a single char at a time, and reading a lot of characters at a time.

"A little song, a little dance, a little seltzer down your pants"
Chuckles the clown

Myth of a Myth?

jschell4-Dec-23 4:38

jschell

4-Dec-23 4:38

From CP newsletter

The Myth of Smart Pointers – Logikal Blog[^]

Basically complaining about what unique_ptr<t> is defined as.

Myself reading it and looking at the code it seems to me that the author does not understand the difference between the C++ specification and the C++ compiler/runtime.

Which has always been a problem for some people using C/C++ pointers since C (before C++ existed.)

The author seems to be suggesting that the specification is guaranteeing that the memory will be collected. But without even reading the spec I seriously doubt it says that. I suspect it is noting that the it might be collected so one should not rely on it.

There are multiple other places where one can do similar things and it might or might not work depending on compiler, binary and even execution flow (which is really fun to debug.)

Perhaps I am mistaken in what I am reading?

Re: Myth of a Myth?

Richard MacCutchan4-Dec-23 5:57

4-Dec-23 5:57

Given the fact that he is breaking all the rules by branching off into QML (whatever that is, and by his own comments something to be avoided) and Javscript, then you cannot expect C++ to know what is happening outside.

Re: Myth of a Myth?

Mircea Neacsu4-Dec-23 6:05

how to add another primary menu - C + in Qt

4-Dec-23 6:05

jschell wrote:
The author seems to be suggesting that the specification is guaranteeing that the memory will be collected. But without even reading the spec I seriously doubt it says that. I suspect it is noting that the it might be collected so one should not rely on it.

Of course it doesn't guarantee. Where the memory is coming from and where it is going to, is outside the scope of the language specification. From C++11 to C++23 there was some kind of support for garbage collection using std::declare_reachable, but it was deleted[^].

Mircea

Salvatore Terress1-Dec-23 7:34

Salvatore Terress

1-Dec-23 7:34

Voluntarily removed - posted in wrong forum...

modified 2 days ago.

Re: how to add another primary menu - C + in Qt

Andre Oosthuizen1-Dec-23 23:18

Andre Oosthuizen

1-Dec-23 23:18

37 Questions asked in 2 months starts leaning towards a "Help Vampire". You have not shown us in your code above what you have tried apart from 4 lines of code, which oddly looks AI generated. Please tell us what you have researched so far on how to add second primary menus. If this errors, show the error and we will gladly assist from there.

Re: how to add another primary menu - C + in Qt

Richard MacCutchan1-Dec-23 23:22

1-Dec-23 23:22

A number of previous incarnations, one of which was at Discussions - Linux Programming Discussion Boards[^]

Invalidate rectangle

Calin Negru1-Dec-23 1:41

Calin Negru

1-Dec-23 1:41

I’m trying to display an animated bitmap in a win32 application with GDIplus. Last time when I talked about this here I was told I should invalidate a rectangle to force the App to refresh/draw another frame. I’m trying to understand the theory for now. I will come up with code in a day or two if necessary. My question is do you create and invalidate between beginpaint and endpaint a rectangle that has no particular purpose or does it have to be a rectangle that actually does something.
I’m having a difficult time understanding the connection between a rectangle and the rendering process of a win32 api window

CPallini1-Dec-23 2:01

CPallini

1-Dec-23 2:01

Roughly speaking, the invalidated rectagle will be redrawn in the next WM_PAINT call (other parts of the client area are not redrawn). So, if you need to update a portion of the client area then invalidate the corresponding rectangle and then call UpdateWindow().
In simple scenarios you can invalidate the entire client area and then call UpdateWindow().
See, for instance: Invalidating the Client Area - Win32 apps | Microsoft Learn[^].

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

Calin Negru1-Dec-23 2:40

Calin Negru

1-Dec-23 2:40

Basically I need a rectangle taking the whole window for my game, I think I understand, I’ll post a followup if I’ll run into trouble setting things. Thank you.

CPallini1-Dec-23 2:56

CPallini

1-Dec-23 2:56

You are welcome.

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

Richard MacCutchan1-Dec-23 3:13