将FILE *转换为ifstream C ++，Android NDK

Question

I'm coding an Android project with an NDK C++ component and have a file that needs a lot of parsing. The NDK only allows me to get a FILE* pointer to the file that I'm dealing with, not an ifstream which has a few more convenience functions associated with it. Is there anyway to convert an FILE* (cstdio) to ifstream (iostream)?

Answer 1

In general, you can not convert a FILE* to an std::ifstream . However, this isn't really necessary anyway: It is reasonable straight forward to create a custom stream buffer which can be used to initialize an std::istream . Using an std::istream should be sufficient because the extra functionality provided by std::ifstream doesn't really help with parsing anyway. As long as you don't need to use seeking, creating a stream buffer for reading from a FILE* is really simple. All it takes is overriding the the std::streambuf::underflow() function:

class stdiobuf
    : std::streambuf
{
private:
    FILE* d_file;
    char  d_buffer[8192];
public:
    stdiobuf(FILE* file): d_file(file) {}
    ~stdiobuf() { if (this->d_file) fclose(this->d_file); }
    int underflow() {
        if (this->gptr() == this->egptr() && this->d_file) {
            size_t size = fread(this->d_file, 8192);
            this->setg(this->d_buffer, this->d_buffer, this->d_buffer + size);
        }
        return this->gptr() == this->egptr()
            ? traits_type::eof()
            : traits_type::to_int_type(*this->gptr());
    }
};

All what remains is to initialize an std::istream to use a stdiobuf :

stdiobuf     sbuf(fopen(...));
std::istream in(&sbuf);

I just typed in the above code and currently I can't try it out. However, the basic should be correct although there may be types and possibly even a little flaw.

Answer 2

Note there isn't a way to retrieve an std::ifstream , but you can get std::istream .

Though not a part of standard C++, sometimes std::basic_filebuf exposes an extension via a constructor which takes std::FILE * .

 basic_filebuf(FILE *fp, char_type *buf = 0, streamsize n = /* default size */); 

Constructs an object of class basic_filebuf , initializing the base class with basic_streambuf<charT,traits>() . It then calls open(fp, buf, n) .

The way you should use this is as follows...

FILE *pf = ...; /* e.g. fopen("/etc/passwd", "r") */
std::filebuf buf(pf);
std::istream stream(&buf);

Now, if this extension is not available, then I'm afraid there's not much you can do other than try to implement your own std::streambuf which implements the required work.