Looking for a simple wrapper for a dynamic array of string pointers in C++

Question

I need a simple class to wrap a c style array of pointer strings, namely char **
I need minimal operations on my wrapper including adding elements( const or non const ), iterating, random access and querying size. I started using a std::vector like below but I don't like it for reasons described below
This is my start

struct CstrVector
{
   CstrVector(){}
    char *convertToC(const std::string & str)
    {
        char *c = new char[str.size()+1];
        std::strcpy(c, str.c_str());
        return c; 
    }
    void add( const char* c)
    {
       //??
    }
    void add(const std::string& str )
    {
        data.push_back(convertToC(str));
    }
    void add(const std::string& s, unsigned int pos )
    {
        if( pos < data.size())
        {
            data[pos] = convertToC(s);
        }
    } 
   ~CstrVector()
   {
     for ( size_t i = 0 ; i < data.size() ; i++ )
     {
        delete [] data[i];
     }
   }
   char ** vdata;
  std::vector<char*> data; // I want to replace this with char ** vdata
};

Why not use
std::vector<char*>
I don't see a benefit it since I still have to allocate and free the memory outside the container ( std::vector ) anyway
Why not use
std::vector<std:string>
because I want to be able to easily pass the array to a C function that will manipulate / change it contents.

You don't see a benefit in using std::vector? I can see one: you don't have to write it. — R. Martinho Fernandes
– R. Martinho Fernandes, Commented Aug 24, 2011 at 15:49
You said adding elements( const or non const ) but why would you add on a const ? — Neel Basu
– Neel Basu, Commented Aug 24, 2011 at 15:53
You don't mention std::vector< std::vector <char > > in your list of choices: this is probably cleaner and more efficient than std::vector<std:string> — SSJ_GZ
– SSJ_GZ, Commented Aug 24, 2011 at 15:54
@R. Martinho Fernandes I use std::vector all the time but I guess I wonder if it is worth it in this case. The vector does not even manage the memory for me. All I get is iteration and random access. — mohan
– mohan, Commented Aug 24, 2011 at 15:55
@MrLunchtime The nested vector will not have contiguous memory I think — mohan
– mohan, Commented Aug 24, 2011 at 15:56

Alok Save · Accepted Answer · 2011-08-24 15:52:03Z

2

You should be using

std::vector<std:string>

If you want to pass char string to c functions you can simply do so using string::c_str

answered Aug 24, 2011 at 15:52

Alok Save

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7 Comments

R. Martinho Fernandes Over a year ago

Unfortunately string::c_str is const.

qid Over a year ago

He's not trying to pass a C-string, he's trying to pass an array of C-strings, which is not as easily done. It could be encapsulated in a method that dynamically allocates an array of pointers and sets each of them to the c_str result.

Alok Save Over a year ago

@R. Martinho Fernandes: Yup, that's right, that would add a little overhead of copying it in a temp buffer before passing it on ahead.

mohan Over a year ago

@Als C functions cannot write to std::string. May be they can I just don't know how

Alok Save Over a year ago

@mohan: Ofcourse, they can't, they know nothing about string being a type. I am a little confused about your comment here and what you quote in the question.

|

Constantinius · Accepted Answer · 2011-08-24 15:55:00Z

2

You could still use the std::vector<char *>. The vector guarantees to store its contents in a consecutive manner, more precisely in an array. If you want to pass the char ** array to a C function you just have to pass the address of the first element in the vector:

void c_function(char ** data);

std::vector<char *> myvec;
...

c_function(&myvec[0]);

Notes:

You still have to do the memory management for the single char*s
this only works for std::vector<char*> and not for std::vector<std::string>!

answered Aug 24, 2011 at 15:55

Constantinius

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Comments

Nemanja Trifunovic · Accepted Answer · 2011-08-24 15:52:46Z

1

Would Boost ptr_vector work for you?

answered Aug 24, 2011 at 15:52

Nemanja Trifunovic

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2 Comments

mohan Over a year ago

I am not sure. Is boost::ptr_vector<char> same as std::vector<char*> ?

Nemanja Trifunovic Over a year ago

@mohan: it is similar, but deletes the objects that the contained pointers point to when it is destructed. More info: boost.org/doc/libs/1_47_0/libs/ptr_container/doc/…

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