How to extract single file, by name, to memory?

[gbook]

I want to extract a single file, which I know the name of, to memory. How would I do that?

[rikyoz]

Hi!
Using BitArchiveReader you can extract the file to a memory buffer; bit7z uses std::vector<unsigned char> for buffers.

Example:

#include "bit7z.hpp"
#include "bitarchivereader.hpp"

int main () {
    try {
        bit7z::Bit7zLibrary lib{ "C:/Program Files/7-Zip/7z.dll" };
        bit7z::BitArchiveReader archive{ lib, "myfile.7z", bit7z::BitFormat::SevenZip };
        bit7z::buffer_t outBuffer{}; // buffer_t is just an alias for std::vector<unsigned char>
        archive.extractTo(outBuffer);
    } catch ( const bit7z::BitException& ex ) { /* do something with ex.what() */ }
}

[rikyoz]

Sorry, I missed the requirement of extracting by name!
Unfortunately, there's no direct way using BitArchiveReader, as extractTo only supports an index parameter to choose which file to extract from the archive (at least for now, but I'll probably add an overload for this use case).

However, there's a more clean way using BitFileExtractor:

bit7z::BitFileExtractor extractor{lib, bit7z::BitFormat::SevenZip};
// The second parameter of extractMatching can be a wildcard pattern,
// or simply the name of the file to be extracted.
bit7z::buffer_t outBuffer{};
extractor.extractMatching("myfile.7z", "archived_file.txt", outBuffer);

Alternatively, you can achieve the same result, in a slightly more complex way, using BitArchiveReader as follows:

bit7z::BitArchiveReader archive{lib, "myfile.7z", bit7z::BitFormat::SevenZip};
auto archivedFile = archive.find("archived_file.txt");
if (archivedFile != archive.cend()) { // The check is unnecessary if you're sure the file is inside the archive.
    bit7z::buffer_t outBuffer{};
    archive.extractTo(outBuffer, archivedFile->index());
}

[gbook]

That works! I'm able to extract a single file to memory.

One other question... if I've now altered the file in memory and I want to put the file back into the archive, and overwrite the original, how would I do that?

[rikyoz]

One other question... if I've now altered the file in memory and I want to put the file back into the archive, and overwrite the original, how would I do that?

You can use BitArchiveEditor as follows:

bit7z::BitFileExtractor extractor{lib, bit7z::BitFormat::SevenZip};
bit7z::buffer_t buffer{};
extractor.extractMatching("myfile.7z", "archived_file.txt", buffer);

// Modify the buffer here

bit7z::BitArchiveEditor editor{lib, "myfile.7z", bit7z::BitFormat::SevenZip};
editor.updateItem("archived_file.txt", buffer);
editor.applyChanges(); // Any change to the archive is applied only when calling applyChanges().

If instead you use BitArchiveReader, please be aware that it keeps the archive file opened until it exits its scope:

bit7z::buffer_t buffer{};
{
    bit7z::BitArchiveReader archive{lib, "myfile.7z", bit7z::BitFormat::SevenZip};
    auto archivedFile = archive.find("archived_file.txt");
    if (archivedFile != archive.cend()) { 
        archive.extractTo(outBuffer, archivedFile->index());
    }
    // This scope is needed to make BitArchiveReader close the original archive at the end of it,
    // and let BitArchiveEditor modify the archive, otherwise it will fail to write the changes.
}

if (!buffer.empty()) {
    // Modify the buffer here
    
    bit7z::BitArchiveEditor editor{lib, "myfile.7z", bit7z::BitFormat::SevenZip};
    editor.updateItem("archived_file.txt", buffer);
    editor.applyChanges();
}

You can also store the index of the file in the archive when you read it, and use that to update it using BitArchiveEditor. This approach has the benefit of making the editor avoid searching the item to be updated in the archive:

std::uint32_t fileIndex{};
bit7z::buffer_t buffer{};
{
    bit7z::BitArchiveReader archive{lib, "myfile.7z", bit7z::BitFormat::SevenZip};
    auto archivedFile = archive.find("archived_file.txt");
    if (archivedFile != archive.cend()) {
        fileIndex = archivedFile->index();
        archive.extractTo(outBuffer, fileIndex);
    }

}

if (!buffer.empty()) {
    // Modify the buffer here
    
    bit7z::BitArchiveEditor editor{lib, "myfile.7z", bit7z::BitFormat::SevenZip};
    editor.updateItem(fileIndex, buffer);
    editor.applyChanges();
}

[gbook]

It worked!

[rikyoz]

Perfect! 👍