如何在默認構造函數中將char數組初始化爲null？

Question

我想在默認構造函數中設置一個等於null（'\ 0'）的char數組。我試圖設置一個已經初始化的字符指針（該類的受保護成員）的值爲空，當沒有參數傳遞。如果有正確的參數傳遞，該程序完美工作，但是當我將它們設置爲默認構造函數時，出現分段錯誤：11錯誤。

這是頭文件：

#ifndef _DVD_H_ 
#define _DVD_H_ 

class DVD { 
protected: 
    unsigned int id; 
    char* title; 
    char* director; 
    char* makecopy(const char*); 
public: 
    DVD(unsigned int i, const char* t, const char* dir); 
    void display(); 
    DVD() 
    { 
     const char temp[] = {'\0'}; 
     id = 0; 
     title = makecopy(temp); 
     director = makecopy(temp); 

    } 
}; 

#endif /* _DVD_H_ */ 

，這是.cpp文件：

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

DVD::DVD (unsigned int i, const char* t, const char* dir) 
{ 
    //Constructing id 
    id = i; 
    //Constructing title 
    title = makecopy(t); 
    //Constructing director 
    director = makecopy(dir); 
} 

void DVD::display() 
{ 
    //Creating variables to track the length of the title/director and the location of the space. 
    int len_t,len_dir,space_dir; 
    for (len_t = 0; title[len_t] != '\0'; len_t++) 
     ; 
    for (len_dir = 0; director[len_dir] != '\0'; len_dir++) 
     ; 
    for (space_dir = 0; director[space_dir] != ' '; space_dir++) 
     ; 
    //Display the information in the desired format. 
    cout << '[' << id << ". " << title << '/'; 
    for (int p = 0; p < space_dir; p++) 
    cout << director[p]; 
    for (int j = space_dir; j < len_dir; j++) 
     cout << director[j]; 
    cout << ']' << endl; 
} 

char* DVD::makecopy(const char* str) 
{ 
    int len; 
    char* copy; 
    for (len = 0; str[len] != '\0'; len++) 
     ; 
    copy = new char[len+1]; 
    for (int i = 0; i < len+1; i++) 
     copy[i] = str[i]; 
    return copy; 
} 

，這是其他.cpp文件

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

int main() 
{ 
    char str[] = "Gandhi"; 
    DVD d1(4, str, "Richard Attenborough"); 
    DVD d2; 
    cout << "First step: " << endl; 

    cout << "After constructors:" << endl; 
    d1.display(); cout << endl; // [4. Gandhi/Richard Attenborough] 
    d2.display(); cout << endl; // [0. /] 


    cout << "Test for dynamically allocated copies" << endl; 
    d1.display(); cout << endl; // [4. Gandhi/Richard Attenborough] 
    d2.display(); cout << endl; // [0. /] 
} 

Answer 1

你是不是實施Rule of Three，你的makecopy()是可疑的。

嘗試一些更喜歡這個：

#ifndef _DVD_H_ 
#define _DVD_H_ 

class DVD 
{ 
protected: 
    unsigned int id; 
    char* title; 
    char* director; 

    static char* makecopy(const char* str); 

public: 
    DVD(); 
    DVD(unsigned int i, const char* t, const char* dir); 
    DVD(const DVD &src); 
    ~DVD(); 

    DVD& operator=(const DVD &rhs); 

    void display() const; 
}; 

#include "DVD.h" 
#include <iostream> 

DVD::DVD() 
    : id(0), title(NULL), director(NULL) 
{ 
} 

DVD::DVD(unsigned int i, const char* t, const char* dir) 
    id(i), title(makecopy(t)), director(makecopy(dir)) 
{ 
} 

DVD::DVD(const DVD& src) 
    id(src.id), title(makecopy(src.title)), director(makecopy(src.director)) 
{ 
} 

DVD::~DVD() 
{ 
    delete[] title; 
    delete[] director; 
} 

DVD& DVD::operator=(const DVD& rhs) 
{ 
    if (this != &rhs) 
    { 
     DVD copy(rhs); 

     id = copy.id; 

     char *c_temp = copy.title; 
     copy.title = title; 
     title = c_temp; 

     c_temp = copy.director; 
     copy.director = director; 
     director = c_temp; 
    } 

    return *this; 
} 

void DVD::display() const 
{ 
    //Display the information in the desired format. 
    std::cout << '[' << id << ". " << title << '/' << director << ']' << std::endl; 
} 

char* DVD::makecopy(const char* str) 
{ 
    int len = 0; 
    if (str) 
    { 
     while (str[len] != '\0') 
      ++len; 
    } 

    char* copy = new char[len+1]; 
    for (int i = 0; i < len; ++i) 
     copy[i] = str[i]; 
    copy[len] = '\0'; 

    return copy; 
} 

話雖這麼說，我強烈建議你使用std::string代替。讓它和編譯器，處理所有的內存管理爲您提供：

#ifndef _DVD_H_ 
#define _DVD_H_ 

#include <string> 

class DVD 
{ 
protected: 
    unsigned int id; 
    std::string title; 
    std::string director; 

public: 
    DVD(); 
    DVD(unsigned int i, const std::string& t, const std::string& dir); 

    void display() const; 
}; 

#endif /* _DVD_H_ */ 

#include "DVD.h" 
#include <iostream> 

DVD::DVD() 
    : id(0) 
{ 
} 

DVD::DVD(unsigned int i, const std::string& t, const std::string& dir) 
    : id(i), title(t), director(dir) 
{ 
} 

void DVD::display() const 
{ 
    //Display the information in the desired format. 
    std::cout << '[' << id << ". " << title << '/' << director << ']' << std::endl; 
}