如何将char *转换为字符串？

Question

When I convert char* to an string it gives an bad memory allocation error in 'new.cpp' . I used following method to convert char* called 'strData' and 'strOrg' to string.

   const char* strData = dt.data();
   int length2 = dt.length();
   string s1(strData);

First time it work without any problem. But in the second convertion it gives above error. When I swap the two conversion in the order, it give the error always in the second conversion regardless of the char* I am converting. Whole code is shown in the following.

    mysqlpp::Query query = conn.query("SELECT data,origin from image where id =2");
    mysqlpp::UseQueryResult res = query.use();
    mysqlpp::Row eee= res.fetch_row();
    mysqlpp::Row::reference dt = eee.at(0);
    mysqlpp::Row::reference org = eee.at(1);

    const char* strData = dt.data();
    int length2 = dt.length();
    string s1(strData);
    istringstream is1(s1);  
    char * imgData =  new char;
    is1.read(reinterpret_cast<char *> (imgData), length2);
    delete [] strData;

    const char* strOrg = org.data();
    int length3 = org.length();
    string s2(strOrg);
    istringstream is2(s2);  
    char * imgOrg =  new char;
    is2.read(reinterpret_cast<char *> (imgOrg), length3);
    delete [] strOrg;

This where the error comes from

    void *__CRTDECL operator new(size_t size) _THROW1(_STD bad_alloc)
    {       
    void *p;
    while ((p = malloc(size)) == 0)
            if (_callnewh(size) == 0)
            {       // report no memory
            static const std::bad_alloc nomem;
            _RAISE(nomem);
            }

    return (p);
    }

How can I solve this?

Answer 1

Instead of

char * imgData = new char;
is1.read(reinterpret_cast<char *> (imgData), length2);

try

char * imgData = new char[length2];
is1.read(reinterpret_cast<char *> (imgData), length2);

When you read data from an istringstream using read , the buffer you provide must have enough space to hold the results!

If you call new char; you get space for one char . Use new char[n]; to get space for n.

Answer 2

    delete [] strData;

This is bad. The line above it probably is also but I know this one is.

You're deleting dt.data(). If I recall correctly this is guaranteed to be the internal buffer of the string.

This may or may not be your underlying problem, like I said, I suspect the line above it is bad also since you pass in a pointer to a single character to what would seem to expect a buffer of some length.

Answer 3

I believe the problem (or at least part of the problem) lies with your allocation:

char * imgData =  new char;

This only allocates 1 char, and then istream.read will assume that imgData is a buffer of chars (notice plural) and place whatever it reads into the single char you allocated, and then beyond that into the memory used by who knows what.

The result is typically called "undefined behaviour" - sometimes you'll get away with it as in the first instance, other times you won't, as in the second conversion.