How is value passing implemented within the Qt library?

In Qt (C++), passing data by value usually means copying the object when passing it to a function or assigning it. Qt widely uses the "copy-on-write" mechanism, which optimizes performance.

For example, many Qt classes (QString, QList, etc.) do not immediately duplicate data when copied but use internal data sharing. Actual copying occurs only when the object is modified (copy-on-write).

Example of passing QString by value:

void processString(QString str) {
    // str is a copy, but copying is efficient thanks to copy-on-write
    str.append(" modified");
}

QString original = "Hello";
processString(original);
// original remains unchanged

Thus, passing by value in Qt is efficient due to internal optimizations, although from a C++ language perspective, it is a normal object copy.