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

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?

Answer 1

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.