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C++ Overview

Add the module to your Build.cs:

PublicDependencyModuleNames.Add("HostMigrationCore");

Saving state​

UPROPERTY(SaveGame)
int32 Score = 0;

UPROPERTY(SaveGame)
TObjectPtr<AActor> CarriedItem; // survives if the item has a Host Migration Id

Add UMigrationIdComponent to actors that should survive:

AMyPickup::AMyPickup()
{
	MigrationId = CreateDefaultSubobject<UMigrationIdComponent>(TEXT("MigrationId"));
}

Events​

Implement IHostMigrationEvents (the same interface Blueprint uses):

UCLASS()
class AMyGameState : public AGameStateBase, public IHostMigrationEvents
{
	GENERATED_BODY()

	virtual void OnHostMigrationSave_Implementation() override;
	virtual void OnHostMigrationRestored_Implementation() override;

	UPROPERTY(SaveGame)
	float RemainingTime = 0.f;
};

For data SaveGame cannot express, use IHostMigratable.

The subsystem​

#include "HostMigrationSubsystem.h"

UHostMigrationSubsystem* Migration = GetGameInstance()->GetSubsystem<UHostMigrationSubsystem>();
Migration->OnHostChanged.AddDynamic(this, &ThisClass::HandleHostChanged);

// Leaving the match
Migration->LeaveSession(TEXT("/Game/Maps/MainMenu"));

See the API Reference.