Pass a reference to state_to_string.

We don't allow it to be null, so pass a reference instead of a pointer.
Also use make_unique rather than new.

src/GTP.cpp

@@ -641,7 +641,7 @@ bool GTP::execute(GameState & game, std::string xinput) {
         cmdstream >> tmp;   // eat printsgf
         cmdstream >> filename;
 
-        auto sgf_text = SGFTree::state_to_string(&game, 0);
+        auto sgf_text = SGFTree::state_to_string(game, 0);
 
         if (cmdstream.fail()) {
             gtp_printf(id, "%s\n", sgf_text.c_str());

src/SGFTree.cpp

@@ -397,11 +397,11 @@ std::vector<int> SGFTree::get_mainline() {
     return moves;
 }
 
-std::string SGFTree::state_to_string(GameState * pstate, int compcolor) {
-    std::unique_ptr<GameState> state(new GameState);
+std::string SGFTree::state_to_string(GameState& pstate, int compcolor) {
+    auto state = std::make_unique<GameState>();
 
     // make a working copy
-    *state = *pstate;
+    *state = pstate;
 
     std::string header;
     std::string moves;

src/SGFTree.h

@@ -51,7 +51,7 @@ class SGFTree {
     };
     FastBoard::square_t get_winner();
 
-    static std::string state_to_string(GameState * state, int compcolor);
+    static std::string state_to_string(GameState& state, int compcolor);
 
 private:
     void populate_states(void);