no operator "=" matches these operands, JUCE window demo application

Question

When I try to compile the demo file from This tutorial, I get a no operator "=" matches these operands error. I've tried including string (advice from similar questions), but that doesn't seem to make sense since it should already be included in JuceHeader.h.

Code issue:

void initialise (const String& commandLine) override
{
    // Add your application's initialisation code here..

    mainWindow = new MainWindow (getApplicationName());
}

Full Code:

#include "../JuceLibraryCode/JuceHeader.h"
#include <string>


//==============================================================================
class MainWindowTutorialApplication  : public JUCEApplication
{
public:
    //==============================================================================
    MainWindowTutorialApplication() {}

    const String getApplicationName() override       { return ProjectInfo::projectName; }
    const String getApplicationVersion() override    { return ProjectInfo::versionString; }
    bool moreThanOneInstanceAllowed() override       { return true; }

    //==============================================================================
    void initialise (const String& commandLine) override
    {
        // Add your application's initialisation code here..

        mainWindow = new MainWindow (getApplicationName());
    }

    void shutdown() override
    {
        // Add your application's shutdown code here..
        mainWindow = nullptr;
    }

    //==============================================================================
    void systemRequestedQuit() override
    {
        // This is called when the app is being asked to quit: you can ignore this
        // request and let the app carry on running, or call quit() to allow the app to close.
        quit();
    }

    void anotherInstanceStarted (const String& commandLine) override
    {
        // When another instance of the app is launched while this one is running,
        // this method is invoked, and the commandLine parameter tells you what
        // the other instance's command-line arguments were.
    }

    class MainWindow : public DocumentWindow
    {
    public:
        MainWindow(String name) : DocumentWindow(name,
            Colours::lightgrey,
            DocumentWindow::allButtons)
        {
            centreWithSize(300, 200);
            setVisible(true);
        }

        void closeButtonPressed() override
        {
            JUCEApplication::getInstance()->systemRequestedQuit();
        }

    private:
        JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR(MainWindow)
    };

private:
    std::unique_ptr<MainWindow> mainWindow;
};

//==============================================================================
// This macro generates the main() routine that launches the app.
START_JUCE_APPLICATION (MainWindowTutorialApplication)

Answer 1

The compiler sais you that you cannot assign a pointer to unique_ptr, and it's true: std::unique_ptr::operator=. You can only move another unique_ptr or assign nullptr. There are two possible solutions:

mainWindow.reset(new MainWindow(getApplicationName()));

or

mainWindow = std::make_unique<MainWindow>(getApplicationName());