C++ std::vector::size() 改变它的状态 [英] C++ std::vector::size() changes its state

问题描述

我对 C++ 比较陌生，对一些奇怪的行为感到困惑.我得到一个包含 std::vector 的对象.然后，我用完全相同的复制行打印出它的大小两次:

I'm relatively new to c++ and am confused by some strange behavior. I get an object which contains an std::vector. Then, I print out its size twice, by exactly the same copied line:

    Pose& pose = getCurrentPose();
    std::cout << "nr1: " << pose.transforms.size() << " bones." << std::endl;
    std::cout << "nr2: " << pose.transforms.size() << " bones." << std::endl;

结果:

Nr1: 17 bones.
Nr2: 38294074 bones.

对此向量大小的任何进一步调用都会返回相同的巨大数字(17 应该是正确的).

Any further calls to this vector's size returns the same huge number (17 should be right).

我在迭代向量时也会出错.在我看来，它实际上并未调整大小，但某种端点已损坏.这里发生了什么，我该如何解决这个问题?

I also get errors when iterating over the vector. It seems to me that it hasn't actually resized, but that some kind of end-pointer got corrupted. What is happening here and how can I solve this problem?

getCurrentPose 大致如下所示:

Here is what getCurrentPose roughly looked like:

Pose& getCurrentPose() {
    Pose& accu;

    for (int b = 0; b < p.transforms.size(); b++) {
        BoneState bs;
        accu->transforms.push_back(bs);
    }

    for (int b = 0; b < p.transforms.size(); b++) {
        accu->transforms.at(b).loc += getLoc(b);
        accu->transforms.at(b).rot += getRot(b);
        accu->transforms.at(b).scale += getScale(b);
    }

    return accu;
}

据我所知，我也不是多线程的.这是一个可能相关的 OpenGL 应用程序.

I am also not multi-threading anywhere as far as I know. This is an OpenGL-application, which may be related.

推荐答案

我敢打赌 GetCurrentPose() 看起来很危险.

My bet is that GetCurrentPose() looks dangerously like this.

Pose & GetCurrentPose()
{
  Pose p;
  p = something_magic();
  return p;
}

或者，按照您自己的回答...

or, following your own answer...

Pose * GetCurrentPose()
{
  Pose p;
  p = something_magic();
  return &p;
}

两者都是导致混乱的万无一失的方法，返回对已离开作用域的对象的指针和引用.

Both are a surefire recipe for chaos, returning pointers and references to objects which have left scope.

正确的方法通常是按值返回.如果 Pose 对象由于某种原因不可复制，则您需要非常仔细地考虑整个设计.

The right approach for this is typically return-by-value. If Pose objects aren't copyable for some reason, you need to think through your whole design very carefully.

Pose GetCurrentPose()
{
  Pose p;
  p = something_magic();
  return p;  // theoretically this returns a copy of p, but the compiler can optimise out the copying step.
}

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