Qdir的函数exist()总是返回true，即使目录不存在

我正在尝试对项目的第一部分进行防错，其中在指定的基本目录中创建了一个文件夹。基本目录由用户在QLineEdit中手动选择，或使用QFileDialog浏览计算机的目录。在使用以下命令在其中创建子文件夹之前，我正在尝试检查基本目录是否存在：

QString base_dir = ui->baseDir->text();
QString blank = "";
QString no_text1 = "Please enter a valid directory";
if( base_dir==no_text1 || base_dir==blank ) {
    if( !QDir( base_dir ).exists() ) {         //does base directory exist?
    ui->baseDir->setText( no_text1 );
    return;
    }
}

问题是，无论我在行编辑中键入一个有效的目录，还是一个无效的目录（即随机短语），第二个if语句总是返回false，这意味着exists（）总是返回true。第一个if语句工作正常。我只是用exists（）错了吗？

编辑：完整代码

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "functions.h"
#include <QtGui/QApplication>
#include <QFileDialog>
#include <fstream>
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}
MainWindow::~MainWindow()
{
    delete ui;
}


// This is the function that handles the directory search when the 'browse' button is pressed
void MainWindow::on_findDir_clicked()
{
    QString path;   //declaring the path to the base directory
    path = QFileDialog::getExistingDirectory(   //gathering the directory from QFileDialog class
        this, tr("Choose the project directory"),
        "/home",
        QFileDialog::ShowDirsOnly
        | QFileDialog::DontResolveSymlinks );
    ui->baseDir->setText( path );   //setting the retrieved path into the line edit box
}

// This function makes the project by creating a new directory with name 'project_name'
// in the base directory, and collects the OpenFOAM version number and simulation type
void MainWindow::on_create_clicked()
{
    QString project_name, foam_version, full_dir, slash, base_dir;
    base_dir = ui->baseDir->text();
    project_name = ui->projectName->text();     //getting the text from the 'projectName' field
    foam_version = ui->version->currentText();  //getting the selection from the 'version' drop-down box
    //first checking if the fields have input in them:
    QString blank = "";
    QString no_text0 = "Please enter project name";
    if( project_name==no_text0 || (project_name==blank) ) {
        ui->projectName->setText( no_text0 );
        return;
    }

    QString no_text1 = "Please enter a valid directory";
    if( base_dir==no_text1 || base_dir==blank ) {
        if( !QDir( base_dir ).exists() ) {         //does base directory exist?
        ui->baseDir->setText( no_text1 );
        return;
        }
    }

    slash = "/";        // needed to separate folders in the directory (can't use a literal)
    full_dir = base_dir.append(slash.append(project_name));
    if( !QDir(full_dir).exists() )  //check if directory already exists
        QDir().mkdir(full_dir);     //creating directory
    QString blockmesh_filename, suffix;
    suffix = "_blockmesh";
    slash = "/";    //must re-define
    blockmesh_filename = full_dir.append( slash.append( project_name.append(suffix) ) );
    std::ofstream create_file( blockmesh_filename.toStdString().c_str() );  //creating empty blockmesh file
}