std::basic_ifstream throws std::bad_cast

Question

I am trying to read data from a file using below code. (Note that you need to enable C++11 features on GCC to make this compile.)

#include <fstream>

typedef unsigned char byte;

int main()
{
    std::string filename = "test.cpp";
    std::basic_ifstream<byte> in(filename, std::basic_ifstream<byte>::in | std::basic_ifstream<byte>::binary);
    in.exceptions(std::ios::failbit | std::ios::badbit);
    byte buf[5];
    in.read(buf, 5);
    return 0;
}

However, when reading data I get an exception:

terminate called after throwing an instance of 'std::bad_cast'
  what():  std::bad_cast

This happens when the in.read(buf, 5) command is invoked.

I know that I can suppress this exception by not setting the exception mask I set but this does not fix the problem, it only masks it. Without an exception mask, the code keeps working but 0 characters are read.

Does anyone know why this exception is thrown? And how do I make it go away?

For me, it doesn't even compile: `no matching constructor for initialization of 'std::basic_ifstream` — , Jul 13 '13 at 08:00
@H2CO3 As stated in the introduction, you need to enable C++11. Otherwise the constructor for the `basic_ifstream` does not exist. Alternative fix: use `filename.c_str()` instead of `filename`. — Chris, Jul 13 '13 at 08:08
This problem has been solved here: http://stackoverflow.com/q/19205531/331024 It has a full implementation of char_traits and codecvt — x-x, Oct 10 '13 at 22:27

score 5 · Accepted Answer · edited May 23 '17 at 11:49

5

c++ STL only contains two specializations of char_traits:

   struct char_traits < char >;
   struct char_traits <wchar_t >;

For the code posted to work a definition of char_traits<byte> is required.

More details in this SO question

edited May 23 '17 at 11:49

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answered Jul 13 '13 at 08:49

suspectus

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Mike Kinghan · Answer 2 · 2013-07-13T08:55:55.347

3

If you redefine byte as char the bad_cast exception will no longer occur.

I presume the basic_ifstream template is not fully debugged for unsigned char specialization. According to the Standard § 27.3, char_traits<CharType> need only be instantiated by the library for CharType = {char|char16_t|char32_t|wchar_t}

edited Jul 13 '13 at 08:55

answered Jul 13 '13 at 08:48

Mike Kinghan

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std::basic_ifstream throws std::bad_cast

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