MyString.exe中0x0FC9E559（ucrtbased.dll）的未處理異常：0xC0000005：訪問沖突寫入位置0x00000000

Question

我的字符串類中有一個問題：“ strcpy函數中的未處理異常”。 我沒有很多指針方面的經驗。 請給我您的建議。 先感謝您 ！

使用的IDE：Visual Studio 2017

MyString.exe中0x0FC9E559（ucrtbased.dll）的未處理異常：0xC0000005：訪問沖突寫入位置0x00000000。

MyString.cpp：

#pragma warning(disable:4996)
#include "MyString.h"
#include "cstring"
#include <iostream>
using namespace std;

MyString::MyString()
{
    length = 0;
    content = NULL;
}

MyString::MyString(int length, const char* content) 
{
    this->length = length;
    this->content = new char[this->length + 1];
    strcpy(this->content, content);
}

MyString::MyString(const char* content)
{
    length = strlen(content);
    this->content = new char[length + 1];
    strcpy(this->content, content);
}

void MyString::setLength(int length) 
{
    this->length = length;
}

const int MyString::getLength() 
{
    return length;
}

void MyString::setContent(char* content) 
{
    strcpy(this->content, content); // Unhandled exception !!!
}

const char* MyString::getContent() 
{
    return content;
}

ostream& operator << (ostream& out, const MyString& string)
{

        out << "Content:\n" << string.content << "\n";
        out << "Length:\n" << string.length << "\n";

    return out;
}

const MyString operator+(MyString& string1, MyString& string2)
{
    MyString concatString;
    concatString.setLength(string1.length + string2.length);
    strcat(string1.content, string2.content);
    concatString.setContent(string1.content);

    return concatString;
}

MyString::~MyString()
{
    delete[] content;
}

MyString.h：

#include <iostream>
using namespace std;

class MyString
{
private:
    int length;
    char* content;

public:

    friend ostream& operator << (ostream& out, const MyString& anotherString);

    MyString(); // Constructor fara parametrii
    MyString(int, const char*); // Constructor cu 2 parametrii
    MyString(const char*); // Constructor cu 1 parametru

    friend const MyString operator+(MyString&, MyString&);

    // setters and getters
    void setLength(int);
    const int getLength();
    void setContent(char*);
    const char* getContent();

    // destructor
    ~MyString();
};

Main.cpp：

#include <iostream>
#include "MyString.h"
using namespace std;

int main() {

    MyString string1("---");
    MyString string2("..");

    cout << (string1 + string2);


    system("pause");
    return 1;
}

Answer 1

在

const MyString operator+(MyString& string1, MyString& string2)
{
    MyString concatString;
    concatString.setLength(string1.length + string2.length);
    strcat(string1.content, string2.content);
    concatString.setContent(string1.content);

    return concatString;
}

concatString創建空的，setLength只設置長度不（再）分配的內容 ，讓你在strcpy的一個setContent空指針

您還需要在concatString中而不是string1中復制並concat

因此，舉例來說：

void MyString::setLength(int length) 
{
    if (length > this->length) {
      char * b = new char[length + 1];

      if (this->content != NULL) {
        strcpy(b, this->content);
        delete [] this->content;
      }
      this->content = b;
    }
    this->length = length;
}

const MyString operator+(const MyString& string1, const MyString& string2)
{
    MyString concatString;
    concatString.setLength(string1.length + string2.length);
    strcpy(concatString.content, string1.content);
    strcat(concatString.content, string2.content);

    return concatString;
}

setContent不能僅僅做一個strcpy ，例如做得更好

void MyString::setContent(char* content) 
{
  if (content == NULL) {
    if (this->content != NULL) 
      delete [] this->content;
    this->content = NULL;
    this->length = 0;
  }
  else {
    setLength(strlen(content));
    strcpy(this->content, content);
  }
}

在完成這兩個變化，編譯和執行：

pi@raspberrypi:/tmp $ g++ -pedantic -Wextra -g MyString.cpp Main.cpp 
In file included from MyString.cpp:2:0:
MyString.h:22:25: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
     const int getLength();
                         ^
MyString.cpp:41:31: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
 const int MyString::getLength()
                               ^
In file included from Main.cpp:2:0:
MyString.h:22:25: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
     const int getLength();

pi@raspberrypi:/tmp $ ./a.out
Content:
---..
Length:
5
sh: 1: pause: not found

在valgrind下

pi@raspberrypi:/tmp $ valgrind ./a.out
==6134== Memcheck, a memory error detector
==6134== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==6134== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==6134== Command: ./a.out
==6134== 
Content:
---..
Length:
5
sh: 1: pause: not found
==6134== 
==6134== HEAP SUMMARY:
==6134==     in use at exit: 0 bytes in 0 blocks
==6134==   total heap usage: 5 allocs, 5 frees, 21,261 bytes allocated
==6134== 
==6134== All heap blocks were freed -- no leaks are possible
==6134== 
==6134== For counts of detected and suppressed errors, rerun with: -v
==6134== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)

要在編譯期間沒有警告，請不要返回const int，而只是返回int

最好使長度為size_t而不是int

的getLength和的getContent可以是常量 （ int getLength() const和const char* getContent() const ）

---

作為克里斯托弗的言論說operator+返回字符串的副本，你沒有定義拷貝構造函數，也不是operator= 。 當一個類包含指向它需要去界定，並與近期C ++還舉

Answer 2

這段代碼有兩個問題。

首先，您需要實現3規則 ，因此還需要提供復制構造函數和賦值運算符。

然后setLength()調整字符串的最大長度，但是它無法分配任何內容，因此您可能會創建緩沖區溢出，或者由於nullptr而使用默認構造函數的情況下使用UB。 這就是在operator+()發生的情況。

實施3規則后，對operator +的快速修復可能是：

const MyString operator+(MyString& string1, MyString& string2)
{
    MyString concatString(string1.length + string2.length, string1.content);
    strcat(concatString.content, string2.content);

    return concatString;   // but this requires copy constructor to work
}

對於基於長度的構造函數，應確保該長度大於要復制的字符串。 因此，您可以斷言這一點，或者使用strncpy()