If the frame has exactly one child window, not counting the status and toolbar, this child is resized to take the entire frame client area. If two or more windows are present, they should be laid out explicitly either by manually handling wxEVT_SIZE or using sizers

A panel is a window on which controls are placed. It is usually placed within a frame. It contains minimal extra functionality over and above its parent class wxWindow; its main purpose is to be similar in appearance and functionality to a dialog, but with the flexibility of having any window as a parent.

#include <wx/wx.h>
#include <wx/image.h>

#ifndef MYFRAME_H
#define MYFRAME_H

#include <wx/statline.h>

class MyFrame: public wxFrame {
public:
 
    MyFrame(wxWindow* parent, int id, const wxString& title, const wxPoint& pos=wxDefaultPosition, const wxSize& size=wxDefaultSize, long style=wxDEFAULT_FRAME_STYLE);
protected:
    wxStaticLine* staticLine;
    wxPanel* panel;
};
#endif // MYFRAME_H

#include "MyFrame.h"

MyFrame::MyFrame(wxWindow* parent, int id, const wxString& title, const wxPoint& pos, const wxSize& size, long style):
    wxFrame(parent, id, title, pos, size, wxDEFAULT_FRAME_STYLE)
{
    panel = new wxPanel(this, -1);
    staticLine = new wxStaticLine(panel, -1);

	wxBoxSizer* sizer = new wxBoxSizer(wxHORIZONTAL);
    sizer->Add(staticLine, 1, wxALIGN_CENTER_VERTICAL, 0);
    panel->SetAutoLayout(true);
    panel->SetSizer(sizer);
}

#include <wx/wx.h>
#include <wx/image.h>
#include "MyFrame.h"

class MyApp: public wxApp {
public:
    bool OnInit();
};

IMPLEMENT_APP(MyApp)

bool MyApp::OnInit()
{
    wxInitAllImageHandlers();
    MyFrame* frame = new MyFrame(NULL, -1, "wxFrame");
    SetTopWindow(frame);
    frame->Show();
    return true;
}