C++ vector.push_back adds objects twice when adding an object once

Question

I am creating a DirectX program which needs to be able to dynamically add objects into a list (or vector) for use the render function. I need to be able to access these objects through the "[]" operator which is the reason i am using vectors instead of lists. The problem is that when I add an object using push_back, it stores that object twice. For example; If the vector is empty and I'm adding the first object, it will store that object into index 0 and into index 1. this creates problems for my render function which must loop through every object and render them. This results in each object being rendered twice.

Code which defines the vector(SpriteList2) located in header file:

class Level1 : public Dx11DemoBase// Define Level1 Class
{
public:
    vector<Sprite1> SpriteList2;//Define Vector
};

Code to add the objects located in cpp file:

bool Level1::LoadContent()//funtion in the Level1 class
{
    Sprite1 Sprite_1({0.0f, 0.0f, 3.0f});//Define Sprite Object
    SpriteList2.push_back(Sprite_1);//Add Sprite Object to Vector

    Sprite1 Sprite_2({ 100.0f, 00.0f, 2.0f });//Define Sprite Object
    SpriteList2.push_back(Sprite_2);//Add Sprite Object to Vector

    Sprite1 Sprite_3({ 100.0f, -50.0f, 1.0f });//Define Sprite Object
    SpriteList2.push_back(Sprite_3);//Add Sprite Object to Vector
    return true;
}

Code to loop through the objects located in cpp file:

void Level1::Render()//funtion in the Level1 class
{
    for (Sprite1 sprite : SpriteList2)//Loop through Vector
    {
        //Code to use the "sprite" object in rendering functions
        worldMat = sprite.GetWorldMatrix();//Retrieve values from each sprite object
    }
}

note The code which adds the objects (shown above) is located in a separate cpp file and the vector is defined in a header file.

I have tried with different types (ex:int) with the same result. This problem does not occur if the vector is defined outside of the header file but the vector has to be defined inside the header file so that the render function can access it.

Can't reproduce with this code. Please create a minimal and *complete* example that we can compile to reproduce the issue. — cdhowie, Dec 30 '14 at 06:03
"The problem is that when I add an object using push_back, it stores that object twice" - what are the chances that you are the first person to ever find this bug in the C++ standard library? Contrast that with the possibility that there's a bug in your _own_ code :-) — paxdiablo, Dec 30 '14 at 06:05
As others have said, you have a bug somewhere in code you aren't showing us. The vector class is definitely not storing every item twice for each time you insert it, you must be calling `push_back()` twice as often as you think you are. Or perhaps you have outright memory corruption in your program somewhere. — John Zwinck, Dec 30 '14 at 06:08
I think I might have found the source of the problem; the initializing functions may have called it multiple times before the main loop started. Thanks. — Conner Tenn, Dec 30 '14 at 06:27

paxdiablo · Accepted Answer · 2014-12-30T06:34:20.763

There is nothing in the code shown that would make objects be inserted twice, so it comes down to some old-fashioned debugging skills. For a start, change the loader function to be something like:

bool Level1::LoadContent() {
    std::cout << "Enter LoadContent with size " << SpriteList2.size() << '\n';

    Sprite1 Sprite_1({0.0f, 0.0f, 3.0f});//Define Sprite Object
    SpriteList2.push_back(Sprite_1);//Add Sprite Object to Vector

    Sprite1 Sprite_2({ 100.0f, 00.0f, 2.0f });//Define Sprite Object
    SpriteList2.push_back(Sprite_2);//Add Sprite Object to Vector

    Sprite1 Sprite_3({ 100.0f, -50.0f, 1.0f });//Define Sprite Object
    SpriteList2.push_back(Sprite_3);//Add Sprite Object to Vector

    std::cout << "Exit  LoadContent with size " << SpriteList2.size() << '\n';

    return true;
}

(using a different debug method than cout if you need to). This will hopefully make it obvious what order things are happening in. It's very unlikely that you'll see:

Enter LoadContent with size 0
Exit  LoadContent with size 6

Far more likely that you'll see some other combination which will show something external to that code is causing the problem, such as calling the function more than once, or affecting the vector externally to the function.

C++ vector.push_back adds objects twice when adding an object once

1 Answers1