Idea

class CTestEntity : CBaseEntity
{
    CHookHandle m_ThinkHook;

    void Think()
    {
        cout << "Think\n";
    }
public:
    CTestEntity()
    {
        m_ThinkHook = hooks.Think.Register(std::bind(CTestEntity::Think, this));
    }
}

// defined as

class hooks
{
    static CHook<void(void)> Think;
}

// called as

void GameThink()
{
    hooks.Think();
}