使用智能指针时出现“没有运算符“[]”匹配这些操作数”如何解决

Question

Here is my code:

class Mystring
{
private:
    unsigned int size;
    shared_ptr<char>message=make_shared<char>();
public:
    Mystring(const char* input):size(strlen(input)),message(make_shared<char>(size+1))
    {cout<<"Created"<<endl;memcpy(this->message,input,size+1);}
    Mystring(const Mystring& other):size(other.size),message(make_shared<char>(size+1))
    {cout<<"Created"<<endl;memcpy(this->message,other.message,size+1);}
    ~Mystring(){cout<<"Deleted"<<endl;}
    friend ostream& operator<<(ostream& Cout,const Mystring& mystring)
    {Cout<<mystring.message<<endl;return Cout;}
    char& operator[](const unsigned int  index)
    {return this->message[index];}
};

I want to use smart pointers and operator [] properly. How can I solve these problems:

 no suitable conversion function from "std::shared_ptr<char>" to "void *" exists no operator "[]" matches these operands

Answer 1

So lots of problems with this code

1. You have a shared char array, so the type is shared_ptr<char[]> not shared_ptr<char> . This also means you cannot use make_shared , since it doesn't work with arrays.

2. If you want to access the pointer to the array use get() .

3. Your copy constructor allocates a new array, which is a bit weird since why are you using shared_ptr , if you don't want to share the array? On the other hand the default assignment operator will share the array, so you have a strange situation where sometimes when you copy Mystring you will share the array, and sometimes you won't.

4. Various other minor issues

Here's a version that works

class Mystring
{
private:
    size_t size;
    shared_ptr<char[]> message;
public:
    Mystring(const char* input) : size(strlen(input)), message(new char[size+1])
    {
        cout<<"Created"<<endl;
        memcpy(message.get(), input, size+1);
    }
    Mystring(const Mystring& other): size(other.size), message(other.message)
    {
        cout<<"Created"<<endl;
    }
    ~Mystring()
    {
        cout<<"Deleted"<<endl;
    }
    friend ostream& operator<<(ostream& Cout,const Mystring& mystring)
    {
        Cout<<mystring.message.get()<<endl;return Cout;
    }
    char& operator[](size_t index)
    {
        return message.get()[index];
    }
};

I changed the copy constructor so that it does share the underlying array. You can change it back if you wish. However is you want a Mystring which does not share it's array with other Mystring objects then it would make more sense to use unique_ptr<char[]> instead of shared_ptr .