C++ - 用字符串[0]在類

Question

上訪問char *

我只是試驗並創建自己的字符串類。 （主要是因爲有一些東西我想用諸如「toBase64」等自定義方法構建等等。無論如何，我想知道如何在使用& String [0]時訪問char *的私有成員。

我認爲你可以使用操作符重載，但我目前只把它當作字符串[0]返回char *。（我知道&是指針運算符）。

STRING.H

namespace CoffeeBeans 
{ 
    class _declspec(dllexport) Coffee_String 
    { 
     char* String; 
     int StringLength; 

    public: 
     Coffee_String(); 
     Coffee_String(LPCSTR CString); 

     LPSTR operator[](int); 

     ~Coffee_String(); 
    }; 
} 

String.cpp

#include "stdafx.h" 
#include "String.h" 
#include <Windows.h> 

CoffeeBeans::Coffee_String::Coffee_String() { 
    this->String = nullptr; 
    this->StringLength = 0; 
} 

CoffeeBeans::Coffee_String::~Coffee_String() { 

    if (String != nullptr) { 
     delete[] this->String; 
     this->String = nullptr; 
     this->StringLength = 0; 
    } 
} 

CoffeeBeans::Coffee_String::Coffee_String(LPCSTR CString) { 

    int StringLength = strlen(CString) + 1; 

    this->String = new char[StringLength](); 
    this->StringLength = StringLength - 1; 
    memcpy_s(this->String, StringLength, CString, StringLength); 
} 

LPSTR CoffeeBeans::Coffee_String::operator[](int) 
{ 
    return this->String; 
} 

Main.cpp的

case WM_CREATE:{ 
     CoffeeBeans::Coffee_String String("Test"); 

     //I want to be able to do 
     //strcpy_s(&String[0], 3, "hi"); //Copy "hi" into the private variable char*String. 
     //I know this isn't a practical use, I wanted quick example (I would really pass it to recv (WinSock2)) 

     MessageBeep(0); 

    break; 
} 

Answer 1

你operator[]將返回錯誤的值。爲了讓&String[index]訪問正確的內存地址，operator[]需要返回對指定索引處字符的引用，而不是像當前那樣返回字符串指針本身。

如果你看看std::string::operator[]實際申報，你會看到它返回一個std::string::reference（又名char &）或std::string::const_reference（又名const char &）（取決於它被稱爲在非const或常量std::string對象） 。

嘗試更多的東西是這樣的：

STRING.H

namespace CoffeeBeans 
{ 
    class _declspec(dllexport) Coffee_String 
    { 
     char* String; 
     int StringLength; 

    public: 
     Coffee_String(); 
     Coffee_String(const Coffee_String &src); 
     Coffee_String(const char *src); 
     ~Coffee_String(); 

     char& operator[](int index); 
     const char& operator[](int index) const; 

     Coffee_String& operator=(const Coffee_String &rhs); 
    }; 
}; 

String.cpp

#include "stdafx.h" 
#include "String.h" 
#include <algorithm> 
#include <cstring> 

CoffeeBeans::Coffee_String::Coffee_String() { 
    String = nullptr; 
    StringLength = 0; 
} 

CoffeeBeans::Coffee_String::Coffee_String(const CoffeeBeans::Coffee_String &src) { 
    StringLength = src.StringLength; 
    String = new char[StringLength+1]; 
    std::copy(src.String, src.String+StringLength, String); 
    String[StringLength] = 0; 
} 

CoffeeBeans::Coffee_String::Coffee_String(const char *src) { 
    StringLength = std::strlen(str); 
    String = new char[StringLength+1]; 
    std::copy(src, src+StringLength, String); 
    String[StringLength] = 0; 
} 

CoffeeBeans::Coffee_String::~Coffee_String() { 
    delete[] String; 
    String = nullptr; 
    StringLength = 0; 
} 

char& CoffeeBeans::Coffee_String::operator[](int index) 
{ 
    return String[index]; 
} 

const char& CoffeeBeans::Coffee_String::operator[](int index) const 
{ 
    return String[index]; 
} 

CoffeeBeans::Coffee_String& CoffeeBeans::Coffee_String::operator=(const CoffeeBeans::Coffee_String &rhs); 
{ 
    Coffee_String temp(rhs); 
    std::swap(String, temp.String); 
    std::swap(StringLength, temp.String); 
    return *this; 
} 

Main.cpp的

case WM_CREATE: { 
    CoffeeBeans::Coffee_String String("Test"); 

    strcpy_s(&String[0], 3, "hi"); //Copy "hi" into the private variable char *String... 
    // note that the content of String will become "hi\0t\0", not "hi\0" 
    // and StringLength will still be 4... 

    MessageBeep(0); 

    break; 
}