Question

我想通过将常见参数（包括open ofstream）绑定到多次调用的函数来减少代码大小，但clang和gcc都拒绝编译该程序：

#include "functional"
#include "fstream"
using namespace std;
void writer(ofstream& outfile, int a) {
    outfile << "asdf " << a;
}
int main() {
    ofstream outfile("test");
    writer(outfile, 3);
    auto writer2 = bind(writer, outfile, placeholders::_1);
    writer2(1);
    writer2(2);
    writer2(3);
    return 0;
}

Clang错误看起来没有用，但是gcc给出了：

/opt/local/gcc-4.9.1/include/c++/4.9.1/tuple:140:42: error: use of deleted function ‘std::basic_ofstream<char>::basic_ofstream(const std::basic_ofstream<char>&)’
  : _M_head_impl(std::forward<_UHead>(__h)) { }
                                      ^
In file included from testi.cpp:2:0:
/opt/local/gcc-4.9.1/include/c++/4.9.1/fstream:602:11: note: ‘std::basic_ofstream<char>::basic_ofstream(const std::basic_ofstream<char>&)’ is implicitly deleted because the default definition would be ill-formed:
     class basic_ofstream : public basic_ostream<_CharT,_Traits>
           ^

我做错了什么或绑定了一个不可能的流（为什么不）？

Answer 1

您收到的错误消息非常清楚：

error: use of deleted function 
  ‘std::basic_ofstream::basic_ofstream(const std::basic_ofstream&)’
    : _M_head_impl(std::forward(__h)) { }

复制构造函数已删除，因此无法复制std::ofstream。如果要在参考中将参数包装到std::bind，请使用std::ref。

auto writer2 = bind(writer, ref(outfile), placeholders::_1);

Answer 2

如果你将流转移到绑定中，那么你的代码是合法的C ++ 11：

auto writer2 = bind(writer, std::move(outfile), placeholders::_1);

如果这仍然不适合你，那是因为gcc尚未实现可移动流。我知道这项工作正在进行中，但我不知道它落入哪个gcc版本。

为什么不能绑定绑定？

2 个答案: