C++ c_str doesn't return entire string

Question

I've tried the following code with both normal ifstreams and the current boost:iostream I'm using, both have the same result.

It is intended to load a file from physfs into memory then pass it to a handler to process (eg Image, audio or data). Currently when c_str is called it only returns a small part of the file.

        PhysFS::FileStream file("Resources/test.png" , PhysFS::OM_READ);

    if(file.is_open()) {

        String* theFile;

        theFile = new String((std::istreambuf_iterator<char>(file)), 
        std::istreambuf_iterator<char>());

        String::iterator it;
        for ( it=theFile->begin() ; it < theFile->end(); it++ ) {
            std::cout << *it; 
        } // Outputs the entire file

        std::cout << theFile->c_str(); // Outputs only the first few lines

    }

The iterator loop outputs the entire png file as expected, but the c_str call only returns the first few characters (\211PNG).

I've been trying variations of this code for quite some time with no success. Any ideas?

Answer 1

I imagine that the next character is a null (ASCII 0) byte. c_str() simply gives you a *char, therefore your write to stdout is interpreted as a class C string which ends at the first null byte.

If you really need a C-like interface to this string, the main thing is that theFile->c_str() points to your data and theFile.length gives you the number of characters in the string. So you might want to do something like this:

char *c_value = theFile->c_str()
for (int i = 0; i < theFile.length; i++)
{
   cout << c_value[i];
}

The real solution depends on why you are converting to a char * in the first place. If you are calling a legacy function that only accepts char *, there is likely also a length argument to that legacy function.

Answer 2

One of the bytes is probably 0. That means end of string to cout when passing a char* (which c_str is)

Answer 3

I would consider using std::vector<unsigned char> instead of std::string for this. It is a lot easier to deal with binary data in a vector. You can reference the underlying pointer using &vec[0] if you need access to a C-style array. I would also make sure that your file abstraction use std::ios_base::binary for the file mode under the hood as well.