流的向量在C++ 11中

Question

以下代码

vector<ofstream> v;
v.emplace_back("file1.txt");
v.emplace_back("file2.txt");
for (int i = 0, ilen = v.size(); i < ilen; ++i)
    v[i] << "Test" << i << endl;

在VS2013中编译正常,但在GCC中失败并显示不可读的消息.似乎VS2013的行为是正确的.

• 我不复制流,但是就地创建它;
• 当vector变得足够大时,内容应该移动到新的内存区域.

虽然我在标准中找不到合适的地方,但却说明了这一点.请问有人引用它吗？

Answer 1

如果你向下滚动到clang抛出的错误的结尾,你会看到这个:

/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/basic_ios.h:66:23: note: copy constructor of 'basic_ios<char, std::char_traits<char> >' is implicitly deleted because base class 'std::ios_base' has an inaccessible copy constructor

这是gcc长错误列表中的对应行:

/usr/include/c++/4.8/bits/basic_ios.h:66:11: note: 'std::basic_ios<char>::basic_ios(const std::basic_ios<char>&)' is implicitly deleted because the default definition would be ill-formed:

     class basic_ios : public ios_base

这是因为的libstdc ++缺少移动构造函数basic_ios,由于上市这里的状态页面上.

27.5 | Iostreams基类| 部分| 缺少移动和交换操作basic_ios.

而且这里是相关的Bugzilla.如果使用libc ++,您的代码将使用clang进行编译.

从错误报告中复制的一个更简单的例子也无法编译:

#include <sstream>
#include <utility>
std::stringstream getss(){
   std::stringstream q;
   return std::move(q);
}

Answer 2

要解决您的问题:

template<typename T, typename...Args>
std::unique_ptr<T> make_unique(Args&&...args) {
  return std::unique_ptr<T>( new T(std::forward<Args>(args)...) );
}

如果你讨厌打字ofstream:

 template<typename...Args>
 std::unique_ptr<std::ofstream> make_up_ofstream(Args&&...args) {
   return make_unique<std::ofstream>(std::forward<Args>(args)...);
 }

给你:

std::vector<std::unique_ptr<std::ofstream>> v;
v.emplace_back(make_up_ofstream("file1.txt"));
v.emplace_back(make_up_ofstream("file2.txt"));
for (int i = 0, ilen = v.size(); i < ilen; ++i)
  *(v[i]) << "Test" << i << endl;

这很近,不是吗？

这让我想写make_up一下,推断出unique_ptr你分配给它的类型:

// dense boilerplate, obsolete in C++1y:
template<unsigned...>struct indexes{typedef indexes type;};
template<unsigned Max,unsigned...Is>struct make_indexes:make_indexes<Max-1,Max-1,Is...>{};
template<unsigned...Is>struct make_indexes<0,Is...>:indexes<Is...>{};
template<unsigned Max>using make_indexes_t=typename make_indexes<Max>::type;

template<typename T>using type=T;

template<typename... Args>
struct up_maker {
  std::tuple<Args...> args;
  template<class T, class...Ds, unsigned...Is>
  std::unique_ptr<T,Ds...> helper( indexes<Is...> ) && {
    return std::unique_ptr<T,Ds...>( new T(std::forward<Args>( std::get<Is>(args) )...) );
  }
  template<class T, class...Ds>
  operator type<std::unique_ptr<T,Ds...>>() && {
    return std::move(*this).helper<T,Ds...>( make_indexes_t< sizeof...(Args) >{} );
  }
  explicit up_maker( Args&&... args_in ):args( std::forward<Args>(args_in)... ) {}
  up_maker( up_maker const& ) = delete;
  up_maker( up_maker && ) = default;
  up_maker& operator=( up_maker const& ) = delete;
  up_maker& operator=( up_maker && ) = default;
};

template<typename...Args>
up_maker<Args...> make_up( Args&&... args ) {
  return up_maker<Args...>( std::forward<Args>(args)... );
}

如果我写的正确,在你的代码中摆脱更多的样板:

std::vector<std::unique_ptr<std::ofstream>> v;
v.emplace_back(make_up("file1.txt"));
v.emplace_back(make_up("file2.txt"));
for (int i = 0, ilen = v.size(); i < ilen; ++i)
  (*v[i]) << "Test" << i << std::endl;

...很多代码只是为了摆脱两个_ofstream,但它很有趣.

实例