lcm: Add ability to set the URL on construction

lcm/drake_lcm.cc

@@ -21,7 +21,11 @@ void Callback(const ::lcm::ReceiveBuffer* buffer,
 
 }  // namespace
 
-DrakeLcm::DrakeLcm() {}
+DrakeLcm::DrakeLcm() : DrakeLcm(std::string{}) {}
+
+DrakeLcm::DrakeLcm(std::string lcm_url)
+    : requested_lcm_url_(std::move(lcm_url)),
+      lcm_(requested_lcm_url_) {}
 
 DrakeLcm::~DrakeLcm() { receive_thread_.reset(); }
 
@@ -39,6 +43,10 @@ void DrakeLcm::StopReceiveThread() {
 
 ::lcm::LCM* DrakeLcm::get_lcm_instance() { return &lcm_; }
 
+std::string DrakeLcm::get_requested_lcm_url() const {
+  return requested_lcm_url_;
+}
+
 void DrakeLcm::Publish(const std::string& channel, const void* data,
                        int data_size, optional<double>) {
   DRAKE_THROW_UNLESS(!channel.empty());

lcm/drake_lcm.h

@@ -21,8 +21,18 @@ class DrakeLcm : public DrakeLcmInterface {
  public:
   DRAKE_NO_COPY_NO_MOVE_NO_ASSIGN(DrakeLcm);
 
+  /**
+   * Constructs using LCM's default URL (either the default hard-coded URL, or
+   * else LCM_DEFAULT_URL environment variable if it is set).
+   */
   DrakeLcm();
 
+  /**
+   * Constructs using the given URL.  If empty, it will use the default URL as
+   * per the no-argument constructor.
+   */
+  explicit DrakeLcm(std::string lcm_url);
+
   /**
    * A destructor that forces the receive thread to be stopped.
    */
@@ -63,6 +73,11 @@ class DrakeLcm : public DrakeLcmInterface {
    */
   ::lcm::LCM* get_lcm_instance();
 
+  /**
+   * Returns the LCM URL passed into the constructor; this can be empty.
+   */
+  std::string get_requested_lcm_url() const;
+
   void Publish(const std::string& channel, const void* data,
                int data_size, optional<double> time_sec) override;
 
@@ -74,6 +89,7 @@ class DrakeLcm : public DrakeLcmInterface {
 #pragma GCC diagnostic pop  // pop -Wdeprecated-declarations
 
  private:
+  std::string requested_lcm_url_;
   ::lcm::LCM lcm_;
   std::unique_ptr<LcmReceiveThread> receive_thread_{nullptr};
   std::list<HandlerFunction> handlers_;

lcm/test/drake_lcm_test.cc

@@ -225,6 +225,7 @@ TEST_F(DrakeLcmTest, SubscribeTest) {
 
 TEST_F(DrakeLcmTest, EmptyChannelTest) {
   DrakeLcm dut;
+  EXPECT_EQ(dut.get_requested_lcm_url(), "");
 
   MessageHandler handler;
   EXPECT_THROW(dut.Subscribe("", &handler), std::exception);
@@ -233,6 +234,14 @@ TEST_F(DrakeLcmTest, EmptyChannelTest) {
   EXPECT_THROW(Publish(&dut, "", message), std::exception);
 }
 
+
+TEST_F(DrakeLcmTest, UrlTest) {
+  const std::string custom_url = "udpm://239.255.66.66:6666?ttl=0";
+  const DrakeLcm dut(custom_url);
+
+  EXPECT_EQ(dut.get_requested_lcm_url(), custom_url);
+}
+
 }  // namespace
 }  // namespace lcm
 }  // namespace drake