QSqlQuery last not working - c++

Question

I'm trying to get the size of an executed query in SQLite but when I use last and try to use last its always false
Here is the code I'm trying to execute
void createDB() {
QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE","CREATE_CON");
db.setDatabaseName("C:/Desktop/TestDb.db3");
db.open();
QSqlQuery q(db);
q.exec("CREATE TABLE IF NOT EXISTS Test(testCol TEXT PRIMARY KEY);");
}
int entries() {
QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE","SELECT_CON");
db.setDatabaseName("C:/Desktop/TestDb.db3");
db.open();
QSqlQuery q(db);
q.exec("SELECT * FROM Test;");
if(q.last() == false) {
qDebug()<<q.lastError().text();
qDebug()<<db.lastError().text();
}
return q.at()+1;
}
The error text I get is empty so I don't know what I'm doing wrong.
I can create the database just fine so my database instance is working as it should.
Operating system: Windows 10
I'm using: Qt 5.10.1
Compilator: MinGW

Answer 1

The behavior is correct, if your table is empty there will not be a last element so last() will be false.
I will take this question to show you that you must validate all possible errors. A program can sometimes work but the duty of a good programmer is to prevent it from always working.
#include <QApplication>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QDebug>
#include <QPushButton>
static bool createDB() {
QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE","CREATE_CON");
db.setDatabaseName("TestDb.db3");
if(!db.open()){
qDebug()<<db.lastError().text();
return false;
}
QSqlQuery q(db);
if(!q.exec("CREATE TABLE IF NOT EXISTS Test(testCol TEXT PRIMARY KEY);")){
qDebug()<<q.lastError().text();
return false;
}
return true;
}
static int entries() {
QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE","SELECT_CON");
db.setDatabaseName("TestDb.db3");
if(!db.open()){
qDebug()<< db.lastError().text();
return -1;
}
QSqlQuery q(db);
if(!q.exec("SELECT * FROM Test;")){
qDebug()<<q.lastError().text();
return -2;
}
if(!q.last()) {
return 0;
}
qDebug()<<"not empty";
return q.at()+1;
}
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
if(!createDB()){
return -1;
}
QPushButton button("call entries");
QObject::connect(&button, &QPushButton::clicked, [&](){
qDebug()<<entries();
});
button.show();
return a.exec();
}

Answer 2

Instead of :
q.exec("SELECT * FROM Test;");
if(q.last() == false) {
qDebug()<<q.lastError().text();
qDebug()<<db.lastError().text();
}
You should do:
if(!q.exec("SELECT * FROM Test;")) {
qDebug()<<q.lastError().text();
}
And if you want to know the size of the query, why don't you store it in a QString, and call whenever you want QString::length()?