How do I use an std::fstream from within a block?

Question

I'm having trouble trying to use an ifstream from within a block. (This is part of a larger, complex project, so I whipped up a quick little source file with just the relevant parts.)

// foo.cpp, in its entirety:

#include <iostream>
#include <fstream>
#include <Block.h>

int main() {
    __block std::ifstream file("/tmp/bar") ;
    // ^ tried this with and without the __block
    void (^block)() = ^{
        file.rdbuf() ;
        file.close() ;
        file.open("/tmp/bar") ;
    } ;
    block() ;
}

If I declare the ifstream with __block, I get:

foo.cpp:6:24: error: call to implicitly-deleted copy constructor of
      'std::ifstream' (aka 'basic_ifstream<char>')
        __block std::ifstream file("/tmp/bar") ;
                              ^~~~

If I declare it without __block, I get:

foo.cpp:8:3: error: call to implicitly-deleted copy constructor of
      'const std::ifstream' (aka 'const basic_ifstream<char>')
                file.rdbuf() ;
                ^~~~
                // rdbuf() and (presumably) other const functions

foo.cpp:9:3: error: member function 'close' not viable: 'this' argument has
      type 'const std::ifstream' (aka 'const basic_ifstream<char>'), but
      function is not marked const
                file.close() ;
                ^~~~
                // open(), close(), and (presumably) other non-const functions

What's the proper way to use fstreams inside of blocks?

I don't know what an objective-c block is, but could you use lambdas instead? — Pubby, Feb 26 '13 at 02:26
@Pubby I don't know what a lambda is, so I'm using Objective-C-style blocks instead. XD — Blacklight Shining, Feb 26 '13 at 02:27
It's, ummm…a command-line program written in `C++`? So, yes, `C++`, but also `C++/CLI`? Unless the `./CLI` is something else. — Blacklight Shining, Feb 26 '13 at 02:35
It's not standard, no. But I can still use it, can't I? After all, `third-party library name here` isn't part of standard C++. — Blacklight Shining, Feb 26 '13 at 02:38
`` is straight C, I believe, to be compatible with Objective-C and C++. (Feel free to correct me if I'm wrong; I'm really just guessing.) — Blacklight Shining, Feb 26 '13 at 02:39
@us2012 no, it uses [Blocks (C language extension)](http://en.wikipedia.org/wiki/Blocks_(C_language_extension)) — Karthik T, Feb 26 '13 at 02:45
@KarthikT Nonstandard language _extension_, okay. Is it bad to use it or something, then? o.o — Blacklight Shining, Feb 26 '13 at 02:46
@BlacklightShining you are welcome to use if you are ok with non portable code. The confusion here is that you do not explain that in your question, so we are left guessing what it is, until we look at the tags.. — Karthik T, Feb 26 '13 at 02:49
@BlacklightShining but yes, as pubby says, lamdas are the new standard way to do what the extension tries to do. — Karthik T, Feb 26 '13 at 02:51
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/25110/discussion-between-blacklight-shining-and-karthik-t) — Blacklight Shining, Feb 26 '13 at 02:52

score 3 · Accepted Answer · edited May 23 '17 at 10:24

3

From Block Implementation Specification

It is an error if a stack based C++ object is used within a block if it does not have a copy constructor.

Which is the first error - ifstream blocks copy. __block requires copy.

As the quote says, one option is to declare ifstream on heap(new/delete).. but that is messy.

The rest of the errors are simple const correctness errors. Not declaring __block imports the objects as a const copy, which is the first of the two errors, and it cannot be used to call non const functions like close.

Try to switch to lamda expressions from C++11 and see if they alleviate these issues.

edited May 23 '17 at 10:24

Community

1
1

answered Feb 26 '13 at 03:08

Karthik T

31,456
5
68
87

not declaring `__block` copies the variable when the block is created – newacct Feb 26 '13 at 08:31
@newacct fixed it to a const copy – Karthik T Feb 26 '13 at 08:34

How do I use an std::fstream from within a block?

1 Answers1