效果图：
1.定义一个类名expr，基类选择dialog,也可以选择默认的类名与基类
单击下一步，完成。就完成了项目的创建。
2.在该项目文件下的界面文件中双击.ui文件，到界面布局
3.根据你所需要实现的功能添加的按钮并进行相应的布局
4.布局完成后，就是实现将相应按钮的功能，添加相应的槽函数
添加槽函数有两种方式：
第一种：使用connect（界面中的哪个按钮，发出了怎样的信号，谁，执行哪个函数）
第二种：直接在按钮上右击，转到槽，选择相应的信号之后确定，槽函数就绑好了。
5.代码段
在头文件中添加一个自定义的头文件floatnumber.h
代码内容：
#ifndef FLOATNUMBER_H#define FLOATNUMBER_H#endif#include <iostream>using namespace std;struct node{int flag;union{double a;char op;}num;};
expr.cpp中的代码：
#include "expr.h"#include "ui_expr.h"#include "floatnumber.h"#include <QPushButton>#include <QTextEdit>#include <QString>#include <QDialog>#include <QDebug>typedef struct node _tNode;#define NUM_OF_NUMBER 20#define KIND_NUMBER 0#define KIND_OPERATOR 1expr::expr(QWidget *parent) :QDialog(parent),ui(new Ui::expr){ui->setupUi(this);this->tmp = "";}expr::~expr(){delete ui;}void expr::on_btn_0_clicked(){if(this->tmp != " "){this->tmp += this->ui->btn_0->text();this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_1_clicked(){this->tmp += this->ui->btn_1->text();this->ui->lbl_display->setText(this->tmp);}void expr::on_btn_2_clicked(){this->tmp += this->ui->btn_2->text();this->ui->lbl_display->setText(this->tmp);}void expr::on_btn_3_clicked(){this->tmp += this->ui->btn_3->text();this->ui->lbl_display->setText(this->tmp);}void expr::on_btn_4_clicked(){this->tmp += this->ui->btn_4->text();this->ui->lbl_display->setText(this->tmp);}void expr::on_btn_5_clicked(){this->tmp += this->ui->btn_5->text();this->ui->lbl_display->setText(this->tmp);}void expr::on_btn_6_clicked(){this->tmp += this->ui->btn_6->text();this->ui->lbl_display->setText(this->tmp);}void expr::on_btn_7_clicked(){this->tmp += this->ui->btn_7->text();this->ui->lbl_display->setText(this->tmp);}void expr::on_btn_8_clicked(){this->tmp += this->ui->btn_8->text();this->ui->lbl_display->setText(this->tmp);}void expr::on_btn_9_clicked(){this->tmp += this->ui->btn_9->text();this->ui->lbl_display->setText(this->tmp);}void expr::on_btn_plus_clicked(){if(tmp != " "){this->tmp += this->ui->btn_plus->text();this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_sub_clicked(){if(tmp != " "){this->tmp += this->ui->btn_sub->text();this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_mul_clicked(){if(tmp != " "){this->tmp += this->ui->btn_mul->text();this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_div_clicked(){if(tmp != " "){this->tmp += this->ui->btn_div->text();this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_equ_clicked(){if(tmp != " "){expr::calculate();this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_point_clicked(){if(tmp != " "){this->tmp += this->ui->btn_point->text();this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_left_clicked(){if(tmp != " "){this->tmp += this->ui->btn_left->text();this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_right_clicked(){if(tmp != " "){this->tmp += this->ui->btn_right->text();this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_delet_clicked(){if(tmp != " "){tmp = tmp.left(tmp.length()-1);this->ui->lbl_display->setText(this->tmp);}}void expr::on_btn_clear_clicked(){if(tmp != " "){tmp.clear();this->ui->lbl_display->clear();this->ui->lbl_display->setText("0");}}void expr::calculate(){_tNode Node[NUM_OF_NUMBER];double num = 0;int i = 0;//字符串的位置int num_of_point = 0;//小数点后数字的个数int num_or_op = 0; //浮点数和操作符个数int isfloat = 0; //小数点int len = tmp.length(); //字符串长度while(i < len){if(tmp[i] >= '0' && tmp[i] <= '9'){if(!isfloat) //整数部分{num = num*10 + (tmp[i].toLatin1()-'0'); //tmp[i].toLatin1()将字符i转换为ASCII码++i;}else //小数部分{double per = 1;for(int k = 0;k < num_of_point;k++)per *= 0.1;num += (tmp[i].toLatin1()-'0') * per;num_of_point++;++i;}}else if(tmp[i] == '.') //读到小数点{isfloat = 1;num_of_point = 1;++i;}else //读到操作符{if(num) //读到操作符后说明一个完整的数字输入完毕，要将数字、小数点等清零{Node[num_or_op].flag = KIND_NUMBER;Node[num_or_op].num.a = num;num = 0;++num_or_op;isfloat= 0;num_of_point = 0;}Node[num_or_op].flag = KIND_OPERATOR;Node[num_or_op].num.op = tmp[i].toLatin1();++num_or_op;++i;}}if(num) //最后一个操作数{Node[num_or_op].flag = KIND_NUMBER;Node[num_or_op].num.a = num;++num_or_op;num = 0;}//将算数表达式改为后缀表达式QStack<_tNode>pNode; //构建一个_tNode类型的栈pNode，是一个保存操作符栈_tNode _Node[NUM_OF_NUMBER];int j = 0;for(int m = 0;m< num_or_op;){if(Node[m].flag) //符号入栈{if(pNode.isEmpty()){pNode.push(Node[m++]);}else{_tNode tem = pNode.top();int ret = process(tem.num.op,Node[m].num.op);switch(ret){case -1:pNode.push(Node[m++]);break;case 1:_Node[j++] = pNode.top();pNode.pop();break;default:pNode.pop();m++;break;}}}else{_Node[j++] = Node[m++];}}while(!pNode.isEmpty()) //弹出栈中剩余的元素{_tNode tem = pNode.top();if(tem.num.op != '(' &&tem.num.op != ')')_Node[j++] = tem;pNode.pop();}QStack<double> last; //构建一个数据栈double d1,d2;for(int n = 0;n < j;n++){if(_Node[n].flag) //遇到符号弹出数字进行计算{d2 = last.top();last.pop();d1 = last.top();last.pop();switch(_Node[n].num.op){case '+':d1 += d2;break;case '-':d1 -= d2;break;case '*':d1 *= d2;break;case '/':d1 /= d2;break;default:break;}last.push(d1);}else{last.push(_Node[n].num.a);}}qDebug() << d1;tmp += '=';tmp += QString::number(d1);}int expr::process(char a,char b) //优先级比较{char aim[7][8] = {{ ">><<<>>" },{ ">><<<>>" },{ ">>>><>>" },{ ">>>><>>" },{ "<<<<<=1" },{ ">>>>1>>" },{ "<<<<<1=" }};char sta[7] = { '+','-','*','/','(',')','#' };char result;int i,pa,pb;for (i = 0; i<6; i++){if (a == sta[i]){pa = i;}if (b == sta[i]){pb = i;}}result = aim[pa][pb];if(result == '>')return 1;else if(result == '<')return -1;else return 0;}
expr.h中的代码：
#ifndef EXPR_H#define EXPR_H#include <QDialog>#include <QStack>namespace Ui {class expr;}class expr : public QDialog{Q_OBJECTpublic:explicit expr(QWidget *parent = 0);~expr();void calculate();int process(char a,char b);private slots:void on_btn_0_clicked();void on_btn_1_clicked();void on_btn_2_clicked();void on_btn_3_clicked();void on_btn_4_clicked();void on_btn_5_clicked();void on_btn_6_clicked();void on_btn_7_clicked();void on_btn_8_clicked();void on_btn_9_clicked();void on_btn_plus_clicked();void on_btn_sub_clicked();void on_btn_mul_clicked();void on_btn_div_clicked();void on_btn_equ_clicked();void on_btn_point_clicked();void on_btn_left_clicked();void on_btn_right_clicked();void on_btn_delet_clicked();void on_btn_clear_clicked();private:QString tmp;Ui::expr *ui;};#endif // EXPR_H
注：算数表达式要考虑到符号的优先级问题，所以要运用到堆栈
要将QString类型的变量初始化为空时双引号之间不能有空格，否则在计算时强制退出，存在内存泄漏问题，你也可以将QString改为char类型也可以避免这个错误。
