Question

I am trying to write a function to convert a std::string to char* .
The first one I have written was this:

char* $ (string str)
{
    char* cstr;
    const unsigned int length=str.size();
    cstr=new char[1000];
    for(int i=0;i<length;i++)
        cstr[i]=str[i];
    cstr[length]=0;
    return cstr;
}

But the problem was the memory leak: let’s suppose that I do this:

char* cstr;
string str1("hello"),str2("hello2");
cstr=$(str1);
cstr=$(str2);

There is a memory leak in this case.The first allocated string is not reachable but it’s reference is lost.
So I made the same using static:

char* $ (string str)
{
    static char cstr[1000];
    const unsigned int length=str.size();
    for(int i=0;i<length;i++)
        cstr[i]=str[i];
    cstr[length]=0;
    return cstr;
}

But the problem now is that the static char fields are accessible:

char* cstr;
string str("hello");
cstr=$(str);
$(str)[5]='!';

This is possibile, the 6th character is modified and so also the C-style string pointed by cstr is modified.

Using const:

const char* $ (string str)
{
    static char cstr[1000];
    const unsigned int length=str.size();
    for(int i=0;i<length;i++)
        cstr[i]=str[i];
    cstr[length]=0;
    return cstr;
}

The problem is that a char pointer is not compatible with a const char pointer, so I can’t do this:

string str("hello");
char* cstr;
cstr=$(str);

But I can only use a const char pointer.
What I would do is to have a function which the return value could be placed only as right operand, but not as left operand of an assignment.How could this be done?

I tried to do this:

char* toCharArray(string& str)
{
    std::unique_ptr<char>p(new char[1000]);
    char* temp=p.get();
    for(unsigned int i=0;i<str.size();i++)
    {
        *temp=str[i];
        temp++;
    }
    return p.get();
}

But the problem is still there, I don’t see the difference between this and the other solution I posted using static.Since a code like this:

char* cstr;
string str("hello");
cstr=toCharArray(str);
toCharArray(str)[0]='o';
cout << cstr;

Modifies the string (prints “oello”).
Problem still not solved.

Answer 1

http://www.cplusplus.com/reference/string/string/c_str/

Look at the example, specifically:

char * cstr, *p;

string str ("Please split this phrase into tokens");

cstr = new char [str.size()+1];
strcpy (cstr, str.c_str());