Priority Queue Compilation Error

I'm attempting to compile the code below, but the compiler gives the error

Struct.h:38:9: error: ‘priority_queue’ in namespace ‘std’ does not name a type

Several searches failed to reveal an answer so I'm hoping you guys can help out. The code was partially based on the sample code given at the c++ reference site.

struct aimedShot;
union moveFunc;
struct timeCommand;

struct aimedShot
{
    void (*move) (Dot*, SDL_Event&, double x, double y);
    double x;
    double y;
};

//Holds the kind of function used
union moveFunc
{
    void (*notAimed) (Dot*);
    aimedShot aimed;
};

//Dot to be operated on and the appropriate operator with time
struct timeCommand
{
    Dot* target;    
    moveFunc command;
    int time;
    bool type; //True indicates aimed (integer inputs), False indicates unaimed
 }; 

class CompareCommand
{
     public:
     bool operator()(timeCommand& c1, timeCommand& c2) //Return true if c1 comes first
    {
        return (c1.time < c2.time);
    }
};

typedef std::priority_queue< timeCommand, std::vector<timeCommand>, CompareCommand> commandTimeline;