c++ - A core about string -

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i have function follow:

#define _glibcxx_fully_dynamic_string 1 typedef struct _coordinate{     int posx;     int posy;     _coordinate(int x = 0,int y = 0){posx = x; posy = y;}; }coordinate;  int find_way(int x, int y, int food_x, int food_y, vector <string>& grid, vector<coordinate>& co, vector<coordinate>& cur, nt& min){     if(cur.empty()){         if(!co.empty()){             return 1;         }         else{             return 0;         }     }     coordinate node = cur.back();     int nx = 0;     int ny = 0;     if((node.posx == food_x) && (node.posy == food_y)){         if(cur.size() < min){             co.clear();               for(int = 0; < cur.size(); i++){                 co.push_back(cur[i]);                 cout<<cur[i].posx<<" "<<cur[i].posy<<endl;             }             min = co.size();         }         cur.pop_back();         return find_way(x, y, food_x, food_y, grid, co, cur, min);     }     //down     if((node.posx < x-1) && (grid[node.posx+1].at(node.posy) != '%')){         nx = node.posx+1;         ny = node.posy;         grid[nx][ny] = '%';         cur.push_back(coordinate(nx,ny));         cout<<"down "<<nx<<" "<<ny<<endl;         cout<<"grid.size()="<<grid.size()<<", grid[nx].length()"<<grid[nx].length()<<endl;         find_way(x, y, food_x, food_y, grid, co, cur, min);         cur.pop_back();         grid[nx][ny] = '-';     }     //right     if((node.posy < y-1) && (grid[node.posx].at(node.posy+1) != '%')){         nx = node.posx;            ny = node.posy+1;         grid[nx][ny] = '%';         cur.push_back(coordinate(nx,ny));         cout<<"right "<<nx<<" "<<ny<<endl;         cout<<"grid.size()="<<grid.size()<<", grid[nx].length()"<<grid[nx].length()<<endl;         find_way(x, y, food_x, food_y, grid, co, cur, min);         cur.pop_back();         grid[nx][ny] = '-';     }     //left     if((node.posy > 0) && (grid[node.posx].at(node.posy-1) != '%')){         nx = node.posx;         ny = node.posy-1;         grid[nx][ny] = '%';         cur.push_back(coordinate(nx,ny));         cout<<"left "<<nx<<" "<<ny<<endl;         cout<<"grid.size()="<<grid.size()<<", grid[nx].length()"<<grid[nx].length()<<endl;         find_way(x, y, food_x, food_y, grid, co, cur, min);         cur.pop_back();         grid[nx][ny] = '-';     }     //up     if((node.posx > 0) && (grid[node.posx-1].at(node.posy) != '%')){         nx = node.posx-1;         ny = node.posy;         grid[nx][ny] = '%';         cur.push_back(coordinate(nx,ny));         cout<<"up "<<nx<<" "<<ny<<endl;         cout<<"grid.size()="<<grid.size()<<", grid[nx].length()"<<grid[nx].length()<<endl;         find_way(x, y, food_x, food_y, grid, co, cur, min);         cur.pop_back();         grid[nx][ny] = '-';     }     else{         cur.pop_back();         cout<<"pop_back"<<endl;         //find_way(x, y, food_x, food_y, grid, co, cur, min);     } }

when run program, crash occured, , coredump follow:

    (gdb) bt     #0  0x000000302af2e2ed in raise () /lib64/tls/libc.so.6     #1  0x000000302af2fa3e in abort () /lib64/tls/libc.so.6     #2  0x000000302d3b1138 in __gnu_cxx::__verbose_terminate_handler () /usr/lib64/libstdc++.so.6     #3  0x000000302d3af166 in __cxa_call_unexpected () /usr/lib64/libstdc++.so.6     #4  0x000000302d3af193 in std::terminate () /usr/lib64/libstdc++.so.6     #5  0x000000302d3af293 in __cxa_throw () /usr/lib64/libstdc++.so.6     #6  0x000000302d3af61d in operator new () /usr/lib64/libstdc++.so.6     #7  0x000000302d3901de in std::string::_rep::_s_create () /usr/lib64/libstdc++.so.6     #8  0x000000302d3908db in std::string::_m_mutate () /usr/lib64/libstdc++.so.6     #9  0x000000302d391e31 in std::string::_m_leak_hard () /usr/lib64/libstdc++.so.6     #10 0x000000302d391eb8 in std::string::at () /usr/lib64/libstdc++.so.6     #11 0x00000000004015b4 in find_way (x=7, y=20, food_x=5, food_y=1, grid=@0x7fbffff8b0, co=@0x7fbffff820, cur=@0x7fbffff7f0,          min=@0x7fbffff7ec) @ pacman-dfs.cpp:74     #12 0x0000000000401562 in find_way (x=7, y=20, food_x=5, food_y=1, grid=@0x7fbffff8b0, co=@0x7fbffff820, cur=@0x7fbffff7f0,          min=@0x7fbffff7ec) @ pacman-dfs.cpp:69     #13 0x0000000000401562 in find_way (x=7, y=20, food_x=5, food_y=1, grid=@0x7fbffff8b0, co=@0x7fbffff820, cur=@0x7fbffff7f0,          min=@0x7fbffff7ec) @ pacman-dfs.cpp:69

i want know why there call "operator new ()" when use function "string.at()", seems there no need apply block of memory, , why crash happened, thanks!

std::string.at() might call operator new if using reference counted implementation of std::string.

in reference counted implementation 2 strings might share same underlying character array. because at() on non-const string returns non-const reference in such circumstances implementation must allocate new character array because otherwise reference returned at() used modify both strings.


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