Converting file to char* Issue

Question

I am a beginner at C++ programming, and I encountered an issue. I want to be able to convert the contents of a file to a char*, and I used file and string streams. However, it's not working.

This is my function that does the work:

char* fileToChar(std::string const& file){

    std::ifstream in(file);
    if (!in){
        std::cout << "Error: file does not exist\n"; 
        exit(EXIT_FAILURE);
    }

    std::stringstream buffer;
    buffer << in.rdbuf() << std::flush;
    in.close();

    return const_cast<char *>(buffer.str().c_str());
}

However, when I test the method out by outputting its contents into another file like this:

std::ofstream file("test.txt");
file << fileToChar("fileTest.txt");

I just get tons of strange characters like this:

îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþ[...etc]

What exactly is going on here? Is there anything I missed? And if there's a better way to do this, I would be glad to know!

Answer 1

return const_cast<char *>(buffer.str().c_str());

returns a pointer to the internal char buffer of a temporary copy of the internal buffer of the local stringstream. Long story short: As soon as you exit the function, this pointer points to garbage.

Btw, even if that was not a problem, the const_cast would be dangerous nonsense, you are not allowed to write through the pointer std::string::c_str returns. Legitimate uses of const_cast are extremely rare.

And for the better way: The best and easiest way would be returning std::string. Only if this is not allowed, a std::vector<char> (preferred) or new char[somelength] (frowned on) would be viable solutions.

Answer 2

char* fileToChar(std::string const& file){

This line already shows that something is going into the wrong direction. You return a pointer to some string, and it's completely unclear to the user of the function who is responsible for releasing the allocated memory, if it has to be released at all, if nullptr can be returned, and so on.

If you want a string, then by all means use std::string!

std::string fileToChar(std::string const& file){

return const_cast<char *>(buffer.str().c_str());

Another line that should make all alarms go off. const_cast is always a workaround to some underlying problem (or some problem with external code).

There is usually a good reason why something is const. By forcing the compiler to turn off the security check and allowing it to attempt modifications of unmodifiable data, you typically turn compilation errors into hard-to-diagnose run-time errors.

Even if this function worked correctly, any attempt to modify the result would be undefined behaviour:

char* file_contents = fileToChar("foo.txt");
file_contents[0] = 'x'; // undefined behaviour

But it does not work correctly anyway. buffer.str() returns a temporary std::string object. c_str() returns a pointer to that temporary object's internally managed memory. The object's lifetime ends when the full expression return const_cast<char *>(buffer.str().c_str()) has been evaluated. Using the resulting pointer is therefore undefined behaviour, too.

---

The problems sound complicated, but the fix is easy. Make the function return std::string and turn the last statement into return buffer.str();.

Answer 3

If your question is, how to read the content of a file into an buffer, consider my following suggestion. But take care that buffer is big enough for the file content. A file size check and preallocation of the memory is advised before calling fileToChar().

bool fileToChar(std::string const& file, char* buffer, unsigned int &buffer_size )
{
    FILE *f = fopen( file.c_str(), "rb" );

    if( f == nullptr )
    {
        return false;
    }

    fseek(f , 0, SEEK_END );

    const int size = ftell( f );

    rewind( f );
    fread( buffer, 1, size, f );
    fclose( f );

    return true;
}