C++ - 在类上使用 &String[0] 访问 char*

Question

我只是在试验并制作自己的字符串类。 (主要是因为我想在自定义方法中构建一些东西，例如“toBase64”等。无论如何，我想知道在使用 &String[0] 时如何访问 char* 的私有(private)成员。

我认为您可以使用运算符重载，但我目前只有它，因为 String[0] 返回 char*。 (我知道 & 是指针运算符)。

字符串.h

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

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

        LPSTR operator[](int);

        ~Coffee_String();
    };
}

字符串.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

    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

您的 operator[] 返回了错误的值。为了让&String[index]访问正确的内存地址，operator[]需要返回指定索引处字符的引用，而不是返回字符串指针本身，正如您目前所做的那样。

如果您查看 std::string::operator[] 的实际声明，您会看到它返回一个 std::string::reference (又名 char &) 或 std::string::const_reference(又名 const char &)(取决于它是否在非-const 或 const std::string 对象)。

尝试更像这样的东西:

字符串.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);
    };
};

字符串.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

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;
}