CODEWITHSUNDEEP
c++qt
servetcolakoglu
servetcolakoglu

Reputation: 105

What is mean this convert?

I develop with QtFramework in c++. I encountered this converting method that is static_cast<void (QTcpSocket::*)(QAbstractSocket::SocketError)> (&QAbstractSocket::error). But I have not understand what is it mean?

Upvotes: 2

Views: 225

Answers (2)

code_fodder
code_fodder

Reputation: 16341

static_cast<void (QTcpSocket::*)(QAbstractSocket::SocketError)> (&QAbstractSocket::error)

where:

&QAbstractSocket::error - is the address of the function named error(...);

And it is casting it to void (QTcpSocket::*)(QAbstractSocket::SocketError) - which is a pointer to a member (of QTcpSocket) function that takes a parameter QAbstractSocket::SocketError and returns void.

So we are casting from address of a function which is a member of QAbstractSocket (perhaps a base class) to a function pointer which is a member of QTcpSocket (perhaps the derived class).

Note

As talamaki explains really nicely the reason for this (i.e. to pick the correct function during a connect(...); An alternative is to use the older Qt connect() style which you can explicitly choose the correct function at the cost of losing compiler-time errors if you get it wrong. I often think this is clearer (for example this question would probably not need to be asked). Here is an example:

// Here we can explicitly choose the correct function,
// and its clear to any reader what is going on.
connect(pTcpSocket, SIGNAL(error(QAbstractSocket::SocketError)),
        this,       SLOT(errorHandler(QAbstractSocket::SocketError)));

Upvotes: 1

talamaki
talamaki

Reputation: 5472

QTcpSocket inherits from the base class QAbstractSocket. There is a method SocketError error() and a signal void error(QAbstractSocket::SocketError socketError) in QAbstractSocket, i.e. with the same name.

In the presence of overloads the address of the name of the function in itself is ambiguous. You can use static_cast<>() to specify which function to use according to the function signature implied by the function pointer type.

The cast in your question selects the signal because void (QTcpSocket::*)(QAbstractSocket::SocketError) is a pointer to a function type in QTcpSocket which returns void and takes a parameter QAbstractSocket::SocketError.

For example to connect to QAbstractSocket::error signal using the function pointer syntax, you must specify the signal type in a static cast.

Upvotes: 2

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