效果示意图

思路

新建一个表格窗体dialog类，用于显示字段名，属性值，选择方式。并发出信号，在mainwindow中接收。主要还是用selectbyexpression函数进行选择，需要的就是一个字符串"字段名=属性值"，其实写死也行，只不过想让用户交互选择字段名和属性值。
核心功能的代码如下：

遍历得到当前图层的字段名

    QList< QgsMapLayer * > 	layers = m_mapcanvas->layers();
    QStringList items;
    QStringList values;

    //遍历得到字段名items和属性值values显示到界面
    foreach (QgsMapLayer *l , layers)
    {
    
        QgsVectorLayer *l1 = qobject_cast<QgsVectorLayer*>(l);
        items = l1->fields().names();

        QgsFeatureIterator featureIt=l1->getFeatures();
        featureIt.rewind();
        QgsFeatureIds ids;
        QgsFeature f;
        while(featureIt.nextFeature(f))
        {
    
            const QgsFields pFields=f.fields();
            ids<<f.id();
            for(int i=0;i<pFields.size();++i)
            {
    
                values.append(f.attribute(i).toString());
            }
        }
    }

mainwindow中用于属性选择要素的槽函数

//属性选择要素的槽函数
void MainWindow::slot_selectbyatt(QString name1,QString name2,QString name3)
{
    
    QList< QgsMapLayer * > 	layers = m_mapcanvas->layers();
    QString fieldname =name1;
    QString value=name2;
    foreach (QgsMapLayer *l , layers)
    {
    
        QgsVectorLayer *l1 = qobject_cast<QgsVectorLayer*>(l);
        const QString STR1 = fieldname.append("='").append(value).append("'");
        if(name3==QString::fromLocal8Bit("新建选择集"))
            l1->selectByExpression(STR1, QgsVectorLayer::SetSelection);
        if(name3==QString::fromLocal8Bit("添加到选择集"))
            l1->selectByExpression(STR1, QgsVectorLayer::AddToSelection);
        if(name3==QString::fromLocal8Bit("移出当前选择集"))
            l1->selectByExpression(STR1, QgsVectorLayer::RemoveFromSelection);
    }
}

mainwindow中弹出窗体的按钮函数

void MainWindow::on_actionxuanzhe_triggered()
{
    
    //属性选择要素按钮
    QList< QgsMapLayer * > 	layers = m_mapcanvas->layers();
    m_dlgselect =new selectbyattritube();

    //连接selectbyattritube类发出的信号和mainwindow接受的槽函数
    connect(m_dlgselect,SIGNAL(sig_selectbyshuxing(QString,QString,QString)),this,SLOT(slot_selectbyatt(QString,QString,QString)));

    QStringList items;
    QStringList values;

    //遍历得到字段名items和属性值values显示到界面
    foreach (QgsMapLayer *l , layers)
    {
    
        QgsVectorLayer *l1 = qobject_cast<QgsVectorLayer*>(l);
        items = l1->fields().names();

        QgsFeatureIterator featureIt=l1->getFeatures();
        featureIt.rewind();
        QgsFeatureIds ids;
        QgsFeature f;
        while(featureIt.nextFeature(f))
        {
    
            const QgsFields pFields=f.fields();
            ids<<f.id();
            for(int i=0;i<pFields.size();++i)
            {
    
                values.append(f.attribute(i).toString());
            }
        }
    }
    //显示窗体
    m_dlgselect->SetItems(items);
    m_dlgselect->Setvalues(values);
    m_dlgselect->show();
    m_dlgselect->raise();
    m_dlgselect->activateWindow();

}

窗体类的.h文件

#ifndef SELECTBYATTRITUBE_H
#define SELECTBYATTRITUBE_H

#include <QWidget>

namespace Ui {
    
class selectbyattritube;
}

class selectbyattritube : public QWidget
{
    
    Q_OBJECT

public:
    explicit selectbyattritube(QWidget *parent = nullptr);
    ~selectbyattritube();
    void SetItems(QStringList items);
    void Setvalues(QStringList items);
private slots:
    void on_pushButton_clicked();

    void on_pushButton_2_clicked();


signals:
    void sig_selectbyshuxing(QString ,QString,QString);

private:
    Ui::selectbyattritube *ui;
};

#endif // SELECTBYATTRITUBE_H

窗体类的.cpp文件

#include "selectbyattritube.h"
#include "ui_selectbyattritube.h"
#include "qgsmaplayer.h"
#include "qgsvectorlayer.h"


selectbyattritube::selectbyattritube(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::selectbyattritube)
{
    
    ui->setupUi(this);
    QStringList ll;
    ll<<QString::fromLocal8Bit("新建选择集") << QString::fromLocal8Bit("添加到选择集") << QString::fromLocal8Bit("移出当前选择集");
    ui->comboBox_3->addItems(ll);
}

selectbyattritube::~selectbyattritube()
{
    
    delete ui;
}

//属性选择OK按钮
void selectbyattritube::on_pushButton_clicked()
{
    
    QString name = ui->comboBox->currentText();
    QString shuxingvalue =ui->comboBox_2->currentText();
    QString selectway =ui->comboBox_3->currentText();
    emit sig_selectbyshuxing(name,shuxingvalue,selectway);

}

void selectbyattritube::on_pushButton_2_clicked()
{
    
    this->close();
}

//设置字段名
void selectbyattritube::SetItems(QStringList items)
{
    
    ui->comboBox->clear();
    ui->comboBox->addItems(items);
}

//设置属性值
void selectbyattritube::Setvalues(QStringList values)
{
    
    ui->comboBox_2->clear();
    ui->comboBox_2->addItems(values);
}