Switching Between windowed and full screen in OpenGL/GLFW 3.2

In the following, I'll describe a small but handy class, which deals with resizing a GLFW window and handles switch fullscreen window on and off.
All the used GLFW functions are well documented in the GLFW documentation.

#include <GL/gl.h>
#include <GLFW/glfw3.h>
#include <array>
#include <stdexcept>

class OpenGLWindow
{
private:

    std::array< int, 2 > _wndPos         {0, 0};
    std::array< int, 2 > _wndSize        {0, 0};
    std::array< int, 2 > _vpSize         {0, 0};
    bool                 _updateViewport = true;
    GLFWwindow *         _wnd            = nullptr;
    GLFWmonitor *        _monitor        = nullptr;

    void Resize( int cx, int cy );

public:

    void Init( int width, int height );
    static void CallbackResize(GLFWwindow* window, int cx, int cy);
    void MainLoop ( void );
    bool IsFullscreen( void );
    void SetFullScreen( bool fullscreen );
};

When creating the window, then the user function pointer (glfwSetWindowUserPointer) is set to the window management class. And the resize callback is set by glfwSetWindowSizeCallback. After the window is created its current size and position can be get by glfwGetWindowPos and glfwGetWindowSize.

void OpenGLWindow::Init( int width, int height )
{
    _wnd = glfwCreateWindow( width, height, "OGL window", nullptr, nullptr );
    if ( _wnd == nullptr )
    {
        glfwTerminate();
        throw std::runtime_error( "error initializing window" ); 
    }

    glfwMakeContextCurrent( _wnd );

    glfwSetWindowUserPointer( _wnd, this );
    glfwSetWindowSizeCallback( _wnd, OpenGLWindow::CallbackResize );

    _monitor =  glfwGetPrimaryMonitor();
    glfwGetWindowSize( _wnd, &_wndSize[0], &_wndSize[1] );
    glfwGetWindowPos( _wnd, &_wndPos[0], &_wndPos[1] );
    _updateViewport = true;
}

When the resize notification occurs, then the pointer to the window management class can be get by glfwGetWindowUserPointer:

static void OpenGLWindow::CallbackResize(GLFWwindow* window, int cx, int cy)
{
    void *ptr = glfwGetWindowUserPointer( window );
    if ( OpenGLWindow *wndPtr = static_cast<OpenGLWindow*>( ptr ) )
        wndPtr->Resize( cx, cy );
}

Any change of the window size is notified and the new window size is stored (glfwGetWindowSize):

void OpenGLWindow::Resize( int cx, int cy )
{
    _updateViewport = true;
}

When the window size has changed, then the viewport has to be suited to the window size (glViewport). This can be done in the main loop of the application:

void OpenGLWindow::MainLoop ( void )
{
    while (!glfwWindowShouldClose(_wnd))
    {
        if ( _updateViewport )
        {
            glfwGetFramebufferSize( _wnd, &_vpSize[0], &_vpSize[1] );
            glViewport( 0, 0, _vpSize[0], _vpSize[1] );
            _updateViewport = false;
        }

        // ..... render the scene

        glfwSwapBuffers(_wnd);
        glfwPollEvents();
    }
}  

If the current window is in full screen mode, can be achieved by asking for the monitor that the window uses for full screen mode (glfwGetWindowMonitor):

bool OpenGLWindow::IsFullscreen( void )
{
    return glfwGetWindowMonitor( _wnd ) != nullptr;
} 

To switch the full screen mode on and off, glfwSetWindowMonitor has to be called, either with the monitor for the full screen mode, or with nullptr:

void OpenGLWindow::SetFullScreen( bool fullscreen )
{
    if ( IsFullscreen() == fullscreen )
        return;

    if ( fullscreen )
    {
        // backup window position and window size
        glfwGetWindowPos( _wnd, &_wndPos[0], &_wndPos[1] );
        glfwGetWindowSize( _wnd, &_wndSize[0], &_wndSize[1] );
        
        // get resolution of monitor
        const GLFWvidmode * mode = glfwGetVideoMode(_monitor);

        // switch to full screen
        glfwSetWindowMonitor( _wnd, _monitor, 0, 0, mode->width, mode->height, 0 );
    }
    else
    {
        // restore last window size and position
        glfwSetWindowMonitor( _wnd, nullptr,  _wndPos[0], _wndPos[1], _wndSize[0], _wndSize[1], 0 );
    }

    _updateViewport = true;
}

There are a couple of issues with your code:

1. Assuming that glfwCreateWindow will set the resolution to width * height in fullscreen mode is not correct. The GLFW documentation states (emphasis mine):

   For full screen windows, the specified size becomes the resolution of the window's desired video mode. As long as a full screen window is not iconified, the supported video mode most closely matching the desired video mode is set for the specified monitor.

2. Assuming that the window size is specified in "pixels" is not correct either.Quoting the relevant part of the documentation again:

   While the size of a window is measured in screen coordinates, OpenGL works with pixels. The size you pass into glViewport, for example, should be in pixels. On some machines screen coordinates and pixels are the same, but on others they will not be. There is a second set of functions to retrieve the size, in pixels, of the framebuffer of a window.

Issues 1 and 2 can be solved by simply calling glfwGetFramebufferSize after the window was created. This leaves us with issue 3:

3. You call glViewport without having a current GL context - resulting in undefined behavior, and especially in not setting the viewport at all. Now that is actually an interesting one, because the initial viewport for the new context will be the full new window, so that your mistakes 1 and 2 have no direct effect. They still might have some effect later if your code relies on m_width and m_height containing useful values, though.