c ++中使用指针的数组:访问返回的数组时出现分段错误 [英] arrays using pointers in c++: segmentation fault when accessing the returned array

问题描述

我是C ++的新手，我正在尝试使用指向指针的指针来构建3维数组.我敢肯定，这样做有更有效的方法，但是我现在正在尝试理解指针.

I'm new to C++ and I'm trying to build a 3 dimensional array using a pointer to a pointer to a pointer. I am sure there are more efficient ways in doing so, but I am trying to understand pointers at the moment.

作为示例代码，我最初有以下代码，可以很好地工作，分配，初始化和释放内存.

As example code, I originally had the following piece, which worked fine, allocating, initializing, and releasing the memory.

void builder(int aSize1, int aSize2, int aSize3)
{
    int i1, i2, i3;
    int ***frequencies;

    cout << "allocation started ..." << endl;
    frequencies = new int** [aSize1+1];
    for (i1=0; i1<=aSize1; i1++){
        frequencies[i1] = new int*[aSize2+1];
        for (i2 = 0; i2 <= aSize2; i2++)
        {
            frequencies[i1][i2] = new int [aSize3 + 1];
        }
    }
    cout << "allocation done" << endl;
    cout << " " << endl;

    cout << "before initialization" << endl;
    for (i1=0; i1<=aSize1; i1++){
        for(i2=0; i2<=aSize2; i2++){
            for(i3 = 0; i3 <= aSize3; i3++)
            {
                frequencies[i1][i2][i3]= (i1 * i2) % 10;
            }
        }
    }
    cout << "after initialization" << endl;
    cout << " " << endl;

    /* the "destroyer" part */

    cout << "deleting ..." << endl;

    for (i1=0; i1<=aSize1; i1++){
        for(i2=0; i2<=aSize2; i2++){
            delete [] frequencies[i1][i2];
        }
    }

    for (i1=0; i1<aSize1; i1++){
        delete [] frequencies[i1];
    }
    delete [] frequencies;
    cout << "deleting done" << endl;

}

我想通过将上面的代码分成几部分来提高赌注，以便可以在程序的main()函数中使用初始化的数组(只是看是否也可以在其中访问它们).因此，我最终完成了以下操作

I wanted to up the ante by splitting the code above into several parts, so that I could use the initialized array in the main() function of my program (just to see if I can access them there as well). So, I ended up doing the following

头文件:

void builder(int aSize1, int aSize2, int aSize3, int*** frequencies)
{
    int i1, i2, i3;
    //int ***frequencies;

    cout << "allocation started ..." << endl;
    frequencies = new int** [aSize1+1];
    for (i1=0; i1<=aSize1; i1++){
        frequencies[i1] = new int*[aSize2+1];
        for (i2 = 0; i2 <= aSize2; i2++)
        {
            frequencies[i1][i2] = new int [aSize3 + 1];
        }
    }
    cout << "allocation done" << endl;
    cout << " " << endl;

    cout << "before initialization" << endl;
    for (i1=0; i1<=aSize1; i1++){
        for(i2=0; i2<=aSize2; i2++){
            for(i3 = 0; i3 <= aSize3; i3++)
            {
                frequencies[i1][i2][i3]= (i1 * i2) % 10;
            }
        }
        cout << **(frequencies[i1]+2) << endl;
    }
    cout << "after initialization" << endl;
    cout << " " << endl;

}

void destroyer( int aSize1, int aSize2, int aSize3, int*** frequencies )
{
    int i1, i2;

    cout << "deleting ..." << endl;

    for (i1=0; i1<=aSize1; i1++){
        for(i2=0; i2<=aSize2; i2++){
            delete [] frequencies[i1][i2];
        }
    }

    for (i1=0; i1<aSize1; i1++){
        delete [] frequencies[i1];
    }
    delete [] frequencies;
    cout << "deleting done" << endl;
}

和我的main()，在这里我尝试不费吹灰之力地访问3d数组.

and my main() where I try to access the 3d array in a fruitless effort.

int main()
{
    int aSize1 = 10;
    int aSize2 = 10;
    int aSize3 = 10;
    int*** freq;

    builder(aSize1, aSize2, aSize3, freq);
    cout << "builder finished" << endl;
    cout << **(freq[1]+2) << endl;
    destroyer( aSize1, aSize2, aSize3, freq);
}

当我对此进行编译时，"builder"函数运行良好，但是每当我尝试访问main函数中的3d数组时，都会遇到分段错误.我希望它能奏效，因为我在书中已经读过它，如果使用指向函数的指针通过引用传递了某些东西，则该函数将具有操纵它的能力.另外，我希望我需要将3d数组取消引用3次(即*** freq)才能正确访问元素，但是编译器因为尝试这样做并脱口而出发怒

When I compile this, the "builder" function runs fine, but I get a segmentation fault whenever I try to access the 3d array in the main function. I would expect this to work because I have read it in my book that if something is passed by reference using a pointer to a function, the function would have the power to manipulate it. Also, I expected that I would need to dereference the 3d array 3 times (i.e. ***freq) in order to correctly access the elements, but the compiler gets mad at me for trying to do so and blurts out

myBuilder.cpp:42:17:错误:间接需要指针操作数('int'无效) cout<< ***(频率[i1] +1)<< endl;

myBuilder.cpp:42:17: error: indirection requires pointer operand ('int' invalid) cout << ***(frequencies[i1]+1) << endl;

我知道这是一个新问题，但是任何帮助将不胜感激！

I know this is a newb question, but any help will be appreciated!

推荐答案

builder和destroyer中的frequencies指针是main中freq指针的副本 .因此，在builder中进行设置不会更改main中的(未初始化)指针.您需要引用指针:

The frequencies pointer in builder and destroyer is a copy of the freq pointer in main. So setting it in builder does not change the (uninitialized) pointer in main. You want a reference to pointer instead:

void builder(int aSize1, int aSize2, int aSize3, int***& frequencies);

对于间接访问级别，请注意，如果freq是int***，则freq[1]是int**.

And for your levels of indirection, note that if freq is an int***, then freq[1] is an int**.

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