C++ 变量数据被覆盖

Question

我写 C/C++ 已经有几年了，现在我面临一个我似乎无法靠自己解决的问题。

给定以下结构：

struct InputData
{
    float diameter;
    float length;
    int vertIndex;
    struct InputData *parent;
    vector<InputData*> children;
    bool deadEnd;

    InputData(float dia, float lngth)
    {
        diameter = dia;
        length = lngth;
        vertIndex = NULL;
        parent = NULL;
        deadEnd = false;
    }
};

我首先定义了一些节点，以及它们的父/子关系：

InputData i0 = InputData(3.0f, 3.0f);
InputData i1 = InputData(2.0f, 2.0f);
InputData i2 = InputData(1.0f, 1.0f);
InputData i3 = InputData(1.0f, 1.0f);
InputData i4 = InputData(1.0f, 1.0f);
InputData i5 = InputData(1.01f, 0.5f);

i0.children.push_back(&i1);
i1.children.push_back(&i2);
i2.children.push_back(&i3);
i3.children.push_back(&i4);
i4.children.push_back(&i5);

i1.parent = &i0;
i2.parent = &i1;
i3.parent = &i2;
i4.parent = &i3;
i5.parent = &i4;

注意 i5 作为唯一节点没有任何子节点。

然后我继续使用这些数据做一些工作（从 main() 调用 BuildMeshVertices(&i0, &vertices)），并最终将一个子级添加到 i5：

void BuildMeshVertices(InputData* current, vector<SimpleVertex> *vertices)
{
    //Do work

    if(current->children.size() == 1)
    {
        BuildMeshVertices(current->children[0], vertices);
    }
    else if(current->children.size() == 0 && current->deadEnd == false)
    {
        InputData iDeadEnd = InputData(1.01f, 0.5f);
        iDeadEnd.deadEnd = true;
        iDeadEnd.parent = current;
        current->children.push_back(&iDeadEnd);     

        BuildMeshVertices(&iDeadEnd, vertices);
    }
}

之后一切都很好。 i0 有一个孩子 (i1)，i1 有一个孩子 (i2)，依此类推，i5 现在也有一个孩子。

我调用了另一个函数（BuildMeshIndices()），突然在这个函数的几行（第 63 行）中，新添加到 i5 的子节点的数据被覆盖了。 i5 仍然指向正确的孩子，但是这个孩子的数据突然乱码。

这是截图before and after（抱歉链接，但我不被允许使用IMG标签）

我不知道为什么会发生这种情况，但我感觉这与我糟糕的内存管理有关？

更新也不必这样做。例如，如果将子向量更改为值向量是首选的 C++ 方式，我更愿意这样做。我试图对答案发表评论，但我不确定你们是否看到了 cmets（根据常见问题解答，您需要 50 声望才能离开 cmets）？

以下是完整的源代码（去掉了所有不必要的内容，但足以重现错误）：

#include "stdafx.h"
#include <vector>

using std::vector;

struct InputData
{
    float diameter;
    float length;
    int vertIndex;
    struct InputData *parent;
    vector<InputData*> children;
    bool deadEnd;

    InputData(float dia, float lngth)
    {
        diameter = dia;
        length = lngth;
        vertIndex = NULL;
        parent = NULL;
        deadEnd = false;
    }
};

//--------------------------------------------------------------------------------------
// Vertex types
//--------------------------------------------------------------------------------------
struct SimpleVertex
{
    float Pos;

    SimpleVertex(float Position)
    {
        Pos = Position;
    }
};

void BuildMeshVertices(InputData* current, vector<SimpleVertex> *vertices)
{
    current->vertIndex = vertices->size();

    //Add vertices..

    if(current->children.size() == 1)
    {
        BuildMeshVertices(current->children[0], vertices);
    }
    else if(current->children.size() == 0 && current->deadEnd == false)
    {
        InputData iDeadEnd = InputData(1.01f, 0.5f);
        iDeadEnd.deadEnd = true;
        iDeadEnd.parent = current;
        current->children.push_back(&iDeadEnd);     

        BuildMeshVertices(&iDeadEnd, vertices);
    }
}

void BuildMeshIndices(InputData* current, vector<unsigned long> *indices)
{
    indices->push_back(current->vertIndex+2);
    indices->push_back(current->vertIndex+0);
    indices->push_back(current->vertIndex+1);
    indices->push_back(current->vertIndex+3);
    indices->push_back(current->vertIndex+0);
    indices->push_back(current->vertIndex+2);

    InputData *parent = current->parent;

    unsigned long vOffset;

    if(parent != NULL && parent->children.size() == 1)
    {   
        vOffset = (unsigned long)current->vertIndex;

        indices->push_back(vOffset+7);
        indices->push_back(vOffset+5);
        indices->push_back(vOffset+4);
        indices->push_back(vOffset+6);
        indices->push_back(vOffset+5);
        indices->push_back(vOffset+7);

        indices->push_back(vOffset+10);
        indices->push_back(vOffset+8);
        indices->push_back(vOffset+9);
        indices->push_back(vOffset+11);
        indices->push_back(vOffset+8);
        indices->push_back(vOffset+10);

        indices->push_back(vOffset+15);
        indices->push_back(vOffset+13);
        indices->push_back(vOffset+12);
        indices->push_back(vOffset+14);
        indices->push_back(vOffset+13);
        indices->push_back(vOffset+15);

        indices->push_back(vOffset+18);
        indices->push_back(vOffset+16);
        indices->push_back(vOffset+17);
        indices->push_back(vOffset+19);
        indices->push_back(vOffset+16);
        indices->push_back(vOffset+18);
    }

    if(current->children.size() == 1 && current->deadEnd == false)
    {
        BuildMeshIndices(current->children[0], indices);
    }
}

int _tmain(int argc, _TCHAR* argv[])
{
    InputData i0 = InputData(3.0f, 3.0f);
    InputData i1 = InputData(2.0f, 2.0f);
    InputData i2 = InputData(1.0f, 1.0f);
    InputData i3 = InputData(1.0f, 1.0f);
    InputData i4 = InputData(1.0f, 1.0f);
    InputData i5 = InputData(1.01f, 0.5f);

    i0.children.push_back(&i1);
    i1.children.push_back(&i2);
    i2.children.push_back(&i3);
    i3.children.push_back(&i4);
    i4.children.push_back(&i5);

    i1.parent = &i0;
    i2.parent = &i1;
    i3.parent = &i2;
    i4.parent = &i3;
    i5.parent = &i4;

    // Create vertex buffer
    vector<SimpleVertex> vertices;

    BuildMeshVertices(&i0, &vertices);

    // Create index buffer
    vector<unsigned long> indices;

    BuildMeshIndices(&i0, &indices);

    return 0;
}

Answer 1

您正在将指向堆栈对象的指针推送到您的向量中。一旦执行离开范围，堆栈对象将被销毁并且内存将被重用，从而产生虚假值。试试

InputData *iDeadEnd = new InputData(1.01f, 0.5f);
iDeadEnd->deadEnd = true;
iDeadEnd->parent = current;
current->children.push_back(iDeadEnd);

那么您必须在适当的时候释放该内存。

Answer 2

将原始指针更改为smart pointers，您将解决内存管理问题。

您不需要将所有 boost 复制到您的项目中，只需要复制所需的标题即可。

#include <vector>
#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>

struct InputData
{
    float diameter;
    float length;
    unsigned long vertIndex;
    boost::weak_ptr<InputData> parent;
    std::vector< boost::shared_ptr<InputData> > children;
    bool deadEnd;

    InputData(float dia, float lngth, boost::weak_ptr<InputData> p = boost::weak_ptr<InputData>(), bool de = false)
        : diameter(dia), length(lngth), vertIndex(0), parent(p), deadEnd(de) {}
};

struct SimpleVertex
{
    float Pos;

    SimpleVertex(float position) : Pos(position) {}
};

void BuildMeshVertices(boost::shared_ptr<InputData> current, std::vector<SimpleVertex>& vertices)
{
    current->vertIndex = vertices.size();

    //Add vertices..
    if(current->children.size() == 1)
    {
        BuildMeshVertices(current->children[0], vertices);
    }
    else if(current->children.size() == 0 && current->deadEnd == false)
    {
          // this was a stack variable, so the pointer became invalid when going out of ambit.
        boost::shared_ptr<InputData> iDeadEnd( new InputData(1.01f, 0.5f, current, true) );
        current->children.push_back(iDeadEnd);         

        BuildMeshVertices(iDeadEnd, vertices);
    }
}

void BuildMeshIndices(boost::shared_ptr<InputData> current, std::vector<unsigned long>& indices)
{
    unsigned long vi = current->vertIndex;
    unsigned long  ioffset[] = { vi+2, vi, vi+1, vi+3, vi, vi+2};
    indices.insert(indices.end(), ioffset, ioffset+6);

    boost::shared_ptr<InputData> parent = current->parent.lock();
    if (parent && parent->children.size() == 1)
    {   
        unsigned long offs = current->vertIndex;
          unsigned long voffset[] = 
          { offs+7, offs+5, offs+4, offs+6, offs+5, offs+7,
            offs+10, offs+8, offs+9, offs+11, offs+8, offs+10,
            offs+15, offs+13, offs+12, offs+14, offs+13, offs+15,
            offs+18, offs+16, offs+17, offs+19, offs+16, offs+18 };
          indices.insert(indices.end(), voffset, voffset+24);
    }

    if(current->children.size() == 1 && current->deadEnd == false)
    {
        BuildMeshIndices(current->children[0], indices);
    }
}

int main()
{
    boost::shared_ptr<InputData> i0( new InputData(3.0f, 3.0f) );
    boost::shared_ptr<InputData> i1( new InputData(2.0f, 2.0f) );
    boost::shared_ptr<InputData> i2( new InputData(1.0f, 1.0f) );
    boost::shared_ptr<InputData> i3( new InputData(1.0f, 1.0f) );
    boost::shared_ptr<InputData> i4( new InputData(1.0f, 1.0f) );
    boost::shared_ptr<InputData> i5( new InputData(1.01f, 0.5f) );

    i0->children.push_back(i1);
    i1->children.push_back(i2);
    i2->children.push_back(i3);
    i3->children.push_back(i4);
    i4->children.push_back(i5);

    i1->parent = i0;
    i2->parent = i1;
    i3->parent = i2;
    i4->parent = i3;
    i5->parent = i4;

    // Create vertex buffer
    std::vector<SimpleVertex> vertices;
    BuildMeshVertices(i0, vertices);

    // Create index buffer
    std::vector<unsigned long> indices;
    BuildMeshIndices(i0, indices);

    return 0;
}

认为你还有一半 C、一半 C++ 的脏代码……你应该选择一种语言。

Answer 3

您应该使用动态内存来处理指针。 InputData会在你退出BuildMeshVertices函数的时候被销毁，所以数据会被垃圾或者你会得到一个内存异常。

你应该做类似的事情

InputData * iDeadEnd = new InputData(1.01f, 0.5f);

而不是

InputData iDeadEnd = InputData(1.01f, 0.5f);

Answer 4

您正在堆栈上实例化 iDeadEnd，并获取指向堆栈地址的指针！当函数终止并且堆栈展开时，iDeadEnd 的数据会出现乱码。

InputData *iDeadEnd = new InputData(1.01f, 0.5f);
iDeadEnd->deadEnd = true;
iDeadEnd->parent = current;
current->children.push_back(iDeadEnd);         

BuildMeshVertices(iDeadEnd, vertices);

您现在遇到的问题是 iDeadEnd 的内存在您完成后需要显式释放。

Answer 5

在您的 BuildMeshVertices 函数退出的那一刻，iDeadEnd（i5 的子级）被解构，因为您在堆栈上声明了它，并且通过退出该函数，整个堆栈帧将失效并且所有对象都被解构。您要么想要动态分配 iDeadEnd，要么从根本上重新考虑如何定义树。你最好让每个结构都保存一个 InputData 向量（不是 InputData*），然后按如下方式设置它们：

InputData i0 = InputData(3.0f, 3.0f);
i0.children.push_back( InputData( 2.0f, 2.0f ) );
i0.children[0].children.push_back( InputData( 1.0f, 1.0f ) );

等

由于显而易见的原因，这远非理想。不过，定义元素树从来都不是最有趣的事情。