Causes and solutions of QT signal and slot connection failure

introduction

The signal and slot mechanism is Qt Important foundation , By connecting the signal to the slot , It can realize the logical connection between different operations 、 Parameter transmission and timely feedback . Sometimes the program compiles successfully , however connect There is no connection between signal and slot , Why is that ？

Qt The main reasons for the failure of signal and slot connection are as follows ：

1. Slot function is not declared in class public slots（ or private slots or protected slots） in , therefore , The function you want to be a slot function is just a general member function .

2. There is parameter transfer between the signal and the slot , But the number or type of parameters of the two are inconsistent （ The number of parameters in the signal can be more than that in the slot function , But both have parameters , The type must correspond to ）;

3. There is parameter transmission between the signal and the slot , But use connect When slot function or signal or both are added with formal parameters , For example, the following two connect, The first connection has formal parameters , Will not connect successfully . The second one will connect successfully .

///connect 1
connect(sender, SIGNAL(sendMessage(int a)), receiver, SLOT(getMessage(int b)));
///connect 2
connect(sender, SIGNAL(sendMessage(int)), receiver, SLOT(getMessage(int)));

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4. There is a custom parameter transfer between the signal and the slot , But the type passed is not registered . What is a custom parameter ？ In passing int,double,char,bool,float It's OK to wait for the type , But delivery, such as QList class , Or a custom structure 、 Class isochronous , Connection will fail . The following error will be reported ：

At this point, we need to register this custom type , First include the header file

// Add a file reference 
#include <QMetaType>

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stay connect Previous registration ：

// Register parameter type 
qRegisterMetaType<QList<QString> > ("QList<double>");

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Then you can connect successfully ！