Code

//Complete code below :

#include "MainFrame.hpp"
#include "wx/event.h"
#include "wx/gdicmn.h"
#include "wx/gtk/textctrl.h"
#include "wx/gtk/textentry.h"
#include "wx/sizer.h"
#include "wx/textctrl.h"
#include <string>
#include <sstream>

template<typename T>
wxString toString(T val){
    std::stringstream ss;
    ss<<val;
    return ss.str();
}

enum {  BUTTON_0 = 1000,
        BUTTON_1 = 1001,
        BUTTON_2 = 1002,
        BUTTON_3 = 1003,
        BUTTON_4 = 1004,
        BUTTON_5 = 1005,
        BUTTON_6 = 1006,
        BUTTON_7 = 1007,
        BUTTON_8 = 1008,
        BUTTON_9 = 1009,
        BUTTON_ADD = 1010,
        BUTTON_SUB = 1011,
        BUTTON_MUL = 1012,
        BUTTON_DIV = 1013,
        BUTTON_00 = 1014,
        BUTTON_CLR = 1015,
        BUTTON_RES = 1016,
        BUTTON_DECM = 1017
    };

//event table
wxBEGIN_EVENT_TABLE(MainFrame, wxFrame)
    EVT_BUTTON(BUTTON_0, MainFrame::OnButton0)
    EVT_BUTTON(BUTTON_00, MainFrame::OnButton00)
    EVT_BUTTON(BUTTON_1, MainFrame::OnButton1)
    EVT_BUTTON(BUTTON_2, MainFrame::OnButton2)
    EVT_BUTTON(BUTTON_3, MainFrame::OnButton3)
    EVT_BUTTON(BUTTON_4, MainFrame::OnButton4)
    EVT_BUTTON(BUTTON_5, MainFrame::OnButton5)
    EVT_BUTTON(BUTTON_6, MainFrame::OnButton6)
    EVT_BUTTON(BUTTON_7, MainFrame::OnButton7)
    EVT_BUTTON(BUTTON_8, MainFrame::OnButton8)
    EVT_BUTTON(BUTTON_9, MainFrame::OnButton9)
    EVT_BUTTON(BUTTON_DECM, MainFrame::OnDecm)
    EVT_BUTTON(BUTTON_ADD, MainFrame::OnAdd)
    EVT_BUTTON(BUTTON_SUB, MainFrame::OnSub)
    EVT_BUTTON(BUTTON_MUL, MainFrame::OnMul)
    EVT_BUTTON(BUTTON_DIV, MainFrame::OnDiv)
    EVT_BUTTON(BUTTON_CLR, MainFrame::OnClr)
    EVT_BUTTON(BUTTON_RES, MainFrame::OnRes)
    EVT_KEY_DOWN(MainFrame::OnKeypad)
wxEND_EVENT_TABLE()

MainFrame::MainFrame() : wxFrame(nullptr, wxID_ANY, "Computerator"){
    pTopBoxSizer = new wxBoxSizer(wxVERTICAL);
    pTopPanel = new wxPanel(this, wxID_ANY);
    //add TopPanel to TopBoxSizer with proportion = 1 and make it expand on frame resize
    //also add 0px border to all sides
    pTopBoxSizer->Add(pTopPanel, 1, wxEXPAND | wxALL, 1);
    //add the TopBoxSizer to this
    this->SetSizer(pTopBoxSizer);

    //sub sizer in which we will add child objects
    //this sizer is a child of pTopPanel, i.e inside it
    pSubBoxSizer = new wxBoxSizer(wxVERTICAL); //we will add new widgets vertically
    pTopPanel->SetSizer(pSubBoxSizer); //wx widgets will query this sizer to get minimum size
    // pTopPanel->SetMinSize(wxSize(200, 300));

    //text control for entering operations
    pTextCtrl = new wxTextCtrl(pTopPanel, wxID_ANY, "", wxDefaultPosition, wxDefaultSize, wxTE_READONLY | wxTE_RIGHT);
    //horizontal sizer for TextControl
    pTextCtrlSizer = new wxBoxSizer(wxHORIZONTAL);
    pTextCtrlSizer->Add(pTextCtrl, 1, wxRIGHT | wxLEFT | wxTOP, 10);
    //add this sizer to SubBoxSizer
    pSubBoxSizer->Add(pTextCtrlSizer, 0, wxEXPAND);

    //create buttons
    wxSize buttonSize = wxDefaultSize;
    pButtonClr = new wxButton(pTopPanel, BUTTON_CLR, "Clr", wxDefaultPosition, buttonSize);
    pButtonDiv = new wxButton(pTopPanel, BUTTON_DIV, "/", wxDefaultPosition, buttonSize);
    pButtonMul = new wxButton(pTopPanel, BUTTON_MUL, "x", wxDefaultPosition, buttonSize);
    pButton00 = new wxButton(pTopPanel, BUTTON_00, "00", wxDefaultPosition, buttonSize);
    pButtonDecm = new wxButton(pTopPanel, BUTTON_DECM, ".", wxDefaultPosition, buttonSize);
    pButtonSub = new wxButton(pTopPanel, BUTTON_SUB, "-"); //these 3 buttons will be on the right side
    pButtonAdd = new wxButton(pTopPanel, BUTTON_ADD, "+");
    pButtonRes = new wxButton(pTopPanel, BUTTON_RES, "=");
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_0, "0", wxDefaultPosition, buttonSize)));
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_1, "1", wxDefaultPosition, buttonSize)));
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_2, "2", wxDefaultPosition, buttonSize)));
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_3, "3", wxDefaultPosition, buttonSize)));
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_4, "4", wxDefaultPosition, buttonSize)));
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_5, "5", wxDefaultPosition, buttonSize)));
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_6, "6", wxDefaultPosition, buttonSize)));
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_7, "7", wxDefaultPosition, buttonSize)));
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_8, "8", wxDefaultPosition, buttonSize)));
    Buttons.push_back(std::shared_ptr<wxButton>(new wxButton(pTopPanel, BUTTON_9, "9", wxDefaultPosition, buttonSize)));

    //add grid sizer for buttons
    pLeftKeypadSizer = new wxGridSizer(5, 3, 10, 10);
    pLeftKeypadSizer->Add(pButtonClr);
    pLeftKeypadSizer->Add(pButtonDiv);
    pLeftKeypadSizer->Add(pButtonMul);
    pLeftKeypadSizer->Add(Buttons[7].get());
    pLeftKeypadSizer->Add(Buttons[8].get());
    pLeftKeypadSizer->Add(Buttons[9].get());
    pLeftKeypadSizer->Add(Buttons[4].get());
    pLeftKeypadSizer->Add(Buttons[5].get());
    pLeftKeypadSizer->Add(Buttons[6].get());
    pLeftKeypadSizer->Add(Buttons[1].get());
    pLeftKeypadSizer->Add(Buttons[2].get());
    pLeftKeypadSizer->Add(Buttons[3].get());
    pLeftKeypadSizer->Add(pButton00);
    pLeftKeypadSizer->Add(Buttons[0].get());
    pLeftKeypadSizer->Add(pButtonDecm);

    //right keypad sizer for -/+/= buttons
    pRightKeypadSizer = new wxBoxSizer(wxVERTICAL);
    pRightKeypadSizer->Add(pButtonSub, 1, wxEXPAND | wxALL, 10);
    pRightKeypadSizer->Add(pButtonAdd, 2, wxEXPAND | wxALL, 10);
    pRightKeypadSizer->Add(pButtonRes, 2, wxEXPAND | wxALL, 10);

    //sizer for keypad
    pKeypadSizer = new wxBoxSizer(wxHORIZONTAL);
    pKeypadSizer->Add(pLeftKeypadSizer, 3, wxEXPAND | wxTOP, 10);
    pKeypadSizer->Add(pRightKeypadSizer, 1, wxEXPAND);

    //add KeypadSizer to SubBoxSizer
    pSubBoxSizer->Add(pKeypadSizer, 1, wxEXPAND);
    this->SetMinSize(pSubBoxSizer->GetMinSize());
    this->SetSize(pSubBoxSizer->GetSize());

    Centre();
    SetFocus();
}

void MainFrame::OnAdd(wxCommandEvent &event){
    currentOperand = RIGHT_OPERAND;
    currentOperation = ADD;
}

void MainFrame::OnSub(wxCommandEvent &event){
    currentOperand = RIGHT_OPERAND;
    currentOperation = SUB;
}

void MainFrame::OnMul(wxCommandEvent &event){
    currentOperand = RIGHT_OPERAND;
    currentOperation = MUL;
}

void MainFrame::OnDiv(wxCommandEvent &event){
    currentOperand = RIGHT_OPERAND;
    currentOperation = DIV;
}

void MainFrame::OnButton0(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('0');
    }else{
        rightOp.push_back('0');
    }
    UpdateRes();
}

void MainFrame::OnButton1(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('1');
    }else{
        rightOp.push_back('1');
    }
    UpdateRes();
}

void MainFrame::OnButton2(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('2');
    }else{
        rightOp.push_back('2');
    }
    UpdateRes();
}

void MainFrame::OnButton3(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('3');
    }else{
        rightOp.push_back('3');
    }
    UpdateRes();
}

void MainFrame::OnButton4(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('4');
    }else{
        rightOp.push_back('4');
    }
    UpdateRes();
}

void MainFrame::OnButton5(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('5');
    }else{
        rightOp.push_back('5');
    }
    UpdateRes();
}

void MainFrame::OnButton6(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('6');
    }else{
        rightOp.push_back('6');
    }
    UpdateRes();
}

void MainFrame::OnButton7(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('7');
    }else{
        rightOp.push_back('7');
    }
    UpdateRes();
}

void MainFrame::OnButton8(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('8');
    }else{
        rightOp.push_back('8');
    }
    UpdateRes();
}

void MainFrame::OnButton9(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('9');
    }else{
        rightOp.push_back('9');
    }
    UpdateRes();
    std::cout<<"clear"<<std::endl;
}

void MainFrame::OnButton00(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp.push_back('0');
        leftOp.push_back('0');
    }else{
        rightOp.push_back('0');
        rightOp.push_back('0');
    }
    UpdateRes();
}

void MainFrame::OnDecm(wxCommandEvent &event){
    if(currentOperand == LEFT_OPERAND){
        leftOp += ".";
    }else{
        rightOp += ".";
    }
    UpdateRes();
}

void MainFrame::OnRes(wxCommandEvent &event){
    if(currentOperand == RIGHT_OPERAND){
        if(currentOperation == ADD){
            result = stod(leftOp) + stod(rightOp);
        }else if(currentOperation == SUB){
            result = stod(leftOp) - stod(rightOp);
        }else if(currentOperation == MUL){
            result = stod(leftOp) * stod(rightOp);
        }else{
            result = stod(leftOp) / stod(rightOp);
        }
        pTextCtrl->Clear();
        pTextCtrl->AppendText(toString(result));

        //reset operand and update the result as leftOp
        currentOperand = LEFT_OPERAND;
        leftOp = toString(result);
        rightOp.clear();
    }
}

void MainFrame::OnClr(wxCommandEvent &event){
    leftOp.clear();
    rightOp.clear();
    result = 0;
    pTextCtrl->Clear();
}

void MainFrame::UpdateRes(){
    if(currentOperand == LEFT_OPERAND){
        pTextCtrl->Clear();
        pTextCtrl->AppendText(toString(leftOp));
    }else{
        pTextCtrl->Clear();
        pTextCtrl->AppendText(toString(rightOp));
    }
}

void MainFrame::OnKeypad(wxKeyEvent &event){
    std::cout<<"I called"<<std::endl;
    if(event.GetKeyCode() == WXK_NUMPAD0) OnButton0(e);
    else if(event.GetKeyCode() == WXK_NUMPAD1) OnButton1(e);
    else if(event.GetKeyCode() == WXK_NUMPAD2) OnButton2(e);
    else if(event.GetKeyCode() == WXK_NUMPAD3) OnButton3(e);
    else if(event.GetKeyCode() == WXK_NUMPAD4) OnButton4(e);
    else if(event.GetKeyCode() == WXK_NUMPAD5) OnButton5(e);
    else if(event.GetKeyCode() == WXK_NUMPAD6) OnButton6(e);
    else if(event.GetKeyCode() == WXK_NUMPAD7) OnButton7(e);
    else if(event.GetKeyCode() == WXK_NUMPAD8) OnButton8(e);
    else if(event.GetKeyCode() == WXK_NUMPAD9){
        std::cout<<"Again I called"<<std::endl;
        OnButton9(e);
    }
    else if(event.GetKeyCode() == WXK_NUMPAD_ADD) OnAdd(e);
    else if(event.GetKeyCode() == WXK_NUMPAD_SUBTRACT) OnSub(e);
    else if(event.GetKeyCode() == WXK_NUMPAD_MULTIPLY) OnMul(e);
    else if(event.GetKeyCode() == WXK_NUMPAD_DIVIDE) OnDiv(e);
    else if(event.GetKeyCode() == WXK_NUMPAD_DECIMAL) OnDecm(e);
    else if(event.GetKeyCode() == WXK_RETURN) OnRes(e);

}