c++ - VC++ 上的访问冲突错误

Question

为什么我在以下程序中收到“访问冲突错误读取”:

错误是在读取文件的while循环上。

#include <iostream>

class fileReader
{
public:
    FILE *fp;
    char** lines;
    fileReader()
    {
        fp = NULL;
    }
    fileReader(const char* path)
    {
        int i=0;
        fp = fopen(path,"r");
        while ( fgets(lines[i], 100, fp) )
            i++;
    }
};

int main(int argv, char** argc)
{
    const char* path = "D:\\PS4263-2.txt";
    fileReader *p = new fileReader(path);
    for (int i=0; i<2; i++)
        std::cout<<p->lines[i];
    return 0;
}

编辑

如答案所述，我将代码更改为(如下)，但我仍然遇到相同的错误。

#include <iostream>

class fileReader
{
public:
    FILE *fp;
    char** lines;
    fileReader()
    {
        fp = NULL;
    }
    fileReader(char* path)
    {
        int j=0;
        fp = fopen(path,"r");
        if (fp == NULL) 
            return;
        else 
        {
            lines = (char**) malloc(sizeof(char *)*56000);
            for (int i=0; i<56000; i++)
                lines[i] = (char*)malloc(sizeof(char)*1440);
            while ( fgets(lines[j], 1440, fp) )
                j++;
            fclose(fp);
        }
    }
};

int main(int argv, char** argc)
{
    char* path = "D:\\testfile.txt";
    fileReader *p = new fileReader(path);
    for (int i=0; i<2; i++)
        std::cout<<p->lines[i];
    return 0;
}

Answer 1

这段代码有很多问题。但主要是，问题在于您正在编写一些邪恶的 C/C++ 混合体。选择两种语言中的一种，然后使用它。

这是您的代码的修订版:

#include <iostream>

class fileReader
{
public:
    FILE *fp;
    char** lines;
    fileReader() : fp(NULL) // initialization of members happens here
    {
        //fp = NULL; // anything here happens *after* initialization
        lines = new char*[100]; // let's just assume max 100 lines. We have to allocate space for them
        for (int i = 0; i < 100; ++i) {
            lines[i] = new char[100]; // allocate space for the contents of each individual line
        }
    }
    fileReader(const char* path)
    {
        lines = new char*[100]; // let's just assume max 100 lines. We have to allocate space for them
        for (int i = 0; i < 100; ++i) {
            lines[i] = new char[100]; // allocate space for the contents of each individual line
        }

        int i=0;
        fp = fopen(path,"r");
        while ( fgets(lines[i], 100, fp) )
            i++;
    }
    ~fileReader() {
        // deallocate and close our members:
        fclose(fp);
        for (int i = 0; i < 100; ++i) {
            delete[] lines[i]; // delete the contents of each line
        }
        delete[] lines; // delete the lines array
    }
};

int main(int argv, char** argc)
{
    const char* path = "D:\\PS4263-2.txt";
    fileReader p(path); // don't use new unless you really really have to
    for (int i=0; i<2; i++)
        std::cout<<p.lines[i];
    return 0;
}

现在至少它可以工作了，如果每行包含少于 100 个字符并且有少于 100 行并且文件存在并且我们真正应该防范的其他十几种情况。特别是，我们在内存管理上花费了大量精力:为所有行数据分配和释放空间。

但是，如果我们真的开始编写 C++，我们只需做一些更改就可以做得更好。

#include <iostream> // we need this for the standard streams (cout)
#include <fstream> // we need proper C++ file streams too
#include <string> // C++ has strings. Don't waste your time on char pointers
#include <vector> // C++ has a dynamic array class. Don't use pointers as ad-hoc arrays

class fileReader
{
public:
    // FILE* fp; // no point in making this a class member, when it's only used in one function
    std::vector<std::string> lines; // use a vector of strings. Much easier to manage
    fileReader() // vectors are automatically initialized, no need to do anything
    {
    }
    fileReader(std::string path)
    {
        std::ifstream fp(path); // create an input file stream 
        std::string result; // store the contents of the current line here
        while (std::getline(fp, result)) {
                lines.push_back(result); // append the resulting line to the end of the vector
        }
    }
};

int main(int argv, char** argc)
{
    std::string path = "blah.txt";
    fileReader p(path); // don't use new unless you absolutely have to
    for (int i=0; i<2; i++)
        std::cout<<p.lines[i];
    return 0;
}

请注意，我们不再需要管理数组内存。当 vector 和字符串超出范围时，它们会自动清理。而且因为我们不再使用 new 来分配 fileReader，所以当 它 超出范围时它会自动被删除。这有效地启动了一个链式 react ，其成员开始自行清理:文件流关闭， vector 在要求其存储的字符串清理并关闭后释放其内存。整个程序折叠并关闭，我们无需编写一行代码来处理它。