using an std::vector of std::unique_ptr

Question

I am new to using smart pointers and have only used unique_ptr so far. I am creating a game and I am using a vector of unique_ptr to store my game states.

Here is my vector code:

std::vector<std::unique_ptr<GameState>> gameStates;

Here is my question. Are the following functions going to work to control my vector:

void GameStateManager::popGameState(){
    if (getCurrentGameState() != nullptr)
        gameStates.pop_back();
}

void GameStateManager::pushGameState(GameState *gameState){
    gameStates.push_back(std::unique_ptr<GameState>(gameState));
}

GameState *GameStateManager::getCurrentGameState(){
    return gameStates.back().get();
}

My worries where that using raw pointers as arguments and to return the current game state would eliminate the point of using smart pointers. Is this a good way to do this?

Raw pointers are fine as long as they don't own memory, but do look into move semantics particularly for pushGameState. — user703016
– user703016, Commented Jun 18, 2014 at 11:00
That popGameState is weird. What does the if mean there? If you do not want to pop from an empty vector check that with if(!gameStates.empty()). — Csq
– Csq, Commented Jun 18, 2014 at 11:06
If someone does GameState g; gameStateManger.pushGameState(&g); things will break. To prevent that pushGameState should require a unique_ptr. — nwp
– nwp, Commented Jun 18, 2014 at 11:20
Is GameState polymorphic? If not, from its name, it sounds more like you want a std::vector<GameState>, with no pointers what so ever. — James Kanze
– James Kanze, Commented Jun 18, 2014 at 11:23

ikh · Accepted Answer · 2014-06-18 11:24:20Z

1

Try this:

void GameStateManager::pushGameState(std::unique_ptr<GameState> gameState){
    gameStates.push_back(std::move(gameState));
}

std::unique_ptr cannot be copied, but it can be moved.

I think it has some benefits.. For example, Look at this code.

std::unique_ptr<GameState> pgs;
...
gsm.pushGameState(pgs); // error!
gsm.pushGameState(std::move(pgs)); // you should explicitly move it

If you use raw pointers ...,

void GameStateManager::pushGameState(GameState *gameState) { ... }

{
    std::unique_ptr<GameState> pgs;
    ...
    gsm.pushGameState(pgs.get()); // IT'S NOT COMPILE ERROR! you can make some mistakes like this..
    gsm.pushGameState(pgs.release()); // you can use it, but I think you will make a mistake sometime, finally.
} // if you use `pgs.get()`, the `GameState` is deleted here, though `gameStates` still contains it.

edited Jun 18, 2014 at 11:24

answered Jun 18, 2014 at 11:01

ikh

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4 Comments

Dennis Harrop Over a year ago

Thankyou for your suggestion could you please explain how this would work as I do not understand fully the use of the double reference.

user703016 Over a year ago

You may want to read What is move semantics?

ikh Over a year ago

@DennisHarrop please see edit, which explain the benefit of use that. If you want to know about rvalue-reference, see William's link. (and please choose >o<)

Joseph Mansfield Over a year ago

I don't see a good reason for using an rvalue reference here. Why is it not just taken by value?

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