Why QObject::disconnect(const QMetaObject::Connection &connection) takes const argument with intention to modify it?

Question

static bool QObject::disconnect(const QMetaObject::Connection &connection)

This method is meant to disconnect an existing Connection object with intention to modify it. So why is the function argument declared as a const reference?

In the source code implementation (qtbase/src/corelib/kernel/qobject.cpp), you can find the unavoidable const_cast:

const_cast<QMetaObject::Connection &>(connection).d_ptr = 0;

What is the advantage of marking the function argument as const when the function's purpose is to modify it?

Answer 1

The reasons are purely historical. The API, as originally conceived, was thought not to need to modify the Connection instance while disconnecting it. This has proven to be ill-conceived indeed. Binary compatibility reasons forced the API to remain unchanged. Neither the const-removal in the argument type nor mutable-addition in the field type would be binary-compatible changes. Relevant excerpt:

You cannot [...] For existing classes [...] For existing functions of any type [...] change its signature. This includes [...] changing any of the types of the arguments in the parameter list, including changing the const/volatile qualifiers [...]

You cannot [...] For non-static members [...] change the type of the member, except for signedness [...]

The original implementation of the new disconnect API did not modify the d_ptr. This caused a memory leak when the Connection instance outlived the sender object. The fix had to modify the Connection's non-mutable fields, and this const_cast was the only way to do it.

The original implementation is from 2011, and predates Qt 5 release, but the fix went into Qt 5.0.1, in 2012: at that point the binary compatibility had to be retained.