Using a vector to store multiple instances of a class

Question

To warm up with SMFL, I am creating a simple Block Breaker game. For the blocks and balls, I would like to store them as "instances" in a vector so that they can be added/removed with ease.

However, the compiler spits out an error when attempting to access the vector.

block.h

class Block {
private:
    bool dead;

public:
    Block();
    bool isDead();
    static std::vector<Block*> blockList;

    float x;
    float y;
};

main.h

#include "Block.h"

Block playerBlock;

main.cpp

#include "main.h"

int main()
{
    playerBlock.x = 50;
    playerBlock.y = 20;
    Block::blockList.push_back(&playerBlock);
    return 0;
}

When compiling, Xcode spits out the following error: Undefined symbols for architecture i386: "Block::blockList", referenced from: _main in main.o ld: symbol(s) not found for architecture i386 clang: error: linker command failed with exit code 1 (use -v to see invocation)

Any help is welcome. This is not all of my src but it is all of the src related to the problem at hand.

Community · Accepted Answer · 2017-05-23 12:37:40Z

1

In order to get your code working, you have to add the following initialization statement for your Block::blockList. In your case, it has to be in main.cpp (or your block.cpp if that does exist), since it needs to be outside of the class declaration for all static members (other than some integral types such as int):

std::vector<Block*> Block::blockList;

Some links to similar questions:

edited May 23, 2017 at 12:37

CommunityBot

1

answered Mar 15, 2014 at 22:07

Philip Allgaier

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Community · Accepted Answer · 2017-05-23 12:37:40Z

You declared your static member only in a header file, but now you need to actually define it in an implementation file. So in your main.cpp, below the #include, add:

std::vector<Block*> Block::blockList;

Some side notes:

Defining the static member should happen only in 1 translation unit (~= .cpp file)
Be cautious with static members as their initialisation order is undefined, so once you start adding them all over the place and putting one inside the other, things may/will go haywire at some point.
You're storing "naked" pointers in the vector, which is valid but as your program expands you will run into ownership issues (i.e. who is in charge of maintaining/deleting what memory?) so you will want to look into std::shared_ptr and/or std::unique_ptr sooner rather than later.

std::shared_ptr http://en.cppreference.com/w/cpp/memory/shared_ptr std::unique_ptr http://en.cppreference.com/w/cpp/memory/unique_ptr

Keep all of your blocks in one single std::vector> for example and keep shared_ptr or weak_ptr references to them where appropriate. Aside from the reference links read up a bit on examples of shared_ptr online, e.g. https://stackoverflow.com/questions/3476938/example-to-use-shared-ptr

Good luck!

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