QTimer class and timeout() signal and slot not connecting

[di_zou]

I have a widget:

//foo.h
#include <QTimer>
...
 
class foo : public QWidget
{
...
private slots:
     void on_timer_timeout();
...
private:
     QTimer *timer;
...
}

//foo.h
#include <QTimer>
...

class foo : public QWidget
{
...
private slots:
     void on_timer_timeout();
...
private:
     QTimer *timer;
...
}

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and

//foo.cpp
#include "foo.h"
foo::foo(QWidget *parent)
	: QWidget(parent)
{
     ...
     timer = new QTimer(this);
     timer->start(5);
     ...
}
 
void foo::on_timer_timeout()
{
     //do stuff
}

//foo.cpp
#include "foo.h"
foo::foo(QWidget *parent)
	: QWidget(parent)
{
     ...
     timer = new QTimer(this);
     timer->start(5);
     ...
}

void foo::on_timer_timeout()
{
     //do stuff
}

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When I run the program that creates the widget foo, I get the error:
QMetaObject::connectSlotsByName: No matching signal for on_timer_timeout().

What is the problem?