运算符重载（+ =，=）自己的字符串

Question

I'm trying to create my own class string. I've got some issues with operator overloading.

My_string.h

  #include <cstring>
  #include <iostream>
    class My_string
    {
    private:
        char *value;
    public:
        My_string();
        My_string(char *);
        ~My_string();
        My_string operator +=(const My_string&);
        My_string operator =(const My_string&);
        void show()const;
    };

My_string.cpp

#include "stdafx.h"
#include "My_string.h"


My_string::My_string()
{
    value = new char[1];
    strcpy(value, "");
}

My_string::My_string(char * r_argument)
{

    value = new char[strlen(r_argument) + 1];
    strcpy(value, r_argument);
}

My_string::~My_string()
{
    delete[]value;
}

My_string My_string::operator+=(const My_string &r_argument)
{
    char * temp_value = new char[strlen(value) + strlen(r_argument.value) + 1];
    strcpy(temp_value, value);
    strcat(temp_value,r_argument.value);
    delete[]value;
    value = new char[strlen(value) + strlen(r_argument.value) + 1];
    strcpy(value, temp_value);
    delete[]temp_value;
    return *this;
}

void My_string::show() const
{
    std::cout << value << std::endl;
}

My_string My_string::operator =(const My_string & r_argument)
{
    delete[] value;
    value = new char[strlen(r_argument.value)+1];
    strcpy(value, r_argument.value);
    return *this;
}

How to overload += and = operators ?They both causes runtime errors. I need all to be in dynamically allocated memory.

Debug Assertion Failed! ... Expression: _CrtisValidHeapPointer(block).

Answer 1

operator+= and operator= normally return references to this .

Currently you are returning by value, which is using the compiler-generated copy constructor. That constructor grabs the data buffer pointer value which is the root cause of your crash: multiple delete[] s on a pointer will not end well!

Start by researching the "Rule Of 3", build your copy constructors and assignment operators, fix the overloaded operator return types, and continue from there.

Answer 2

At least the copy assignment operator contains a serious bug when an object is assigned to itself because at first it is deleted. Also the operators should return reference to themselves.

Well, the operators can be defined the following way

My_string & My_string::operator +=( const My_string &r_argument )
{
    if ( r_argument.value[0] )
    { 
        char *temp_value = new char[strlen(value) + strlen(r_argument.value) + 1];
        strcpy(temp_value, value);
        strcat(temp_value,r_argument.value);

        delete [] value;
        value = temp_value;
    }

    return *this;
}

and

My_string & My_string::operator =( const My_string &r_argument )
{
    if ( this != &r_argument )
    {
        char *temp_value = value;

        size_t n = strlen( r_argument.value );

        if ( n != strlen( value ) )
        {
            temp_value = new char[ n + 1 ];
        }
        else
        {
            value = nullptr;
        }

        strcpy( temp_value, r_argument.value );

        delete [] value;

        value = temp_value;
    }

    return *this;
}

Take into account that you need to define explicitly also the copy constructor.