std :: back_inserter需要const_reference在旧的GCC。为什么？ [英] std::back_inserter needs const_reference on older GCC. Why?

问题描述

我目前正在查看一些可以在较新版本的GCC上编译的代码，但不是在较旧的版本上。在我的case我使用 std :: back_inserter 到 std :: copy 一些数据从一个数据结构到自定义数据结构。如果我忘记了 typedef value_type& const_reference typedef在这个自定义数据结构中，但是，这不会在GCC 4.4上编译。相同的代码在GCC 4.5上编译和运行就好了。

I am currently looking at some code that can be compiled on newer versions of GCC but not on older ones. In my case I am using a std::back_inserter to std::copy some data from one data structure to a custom data structure. If I forget the typedef value_type & const_reference typedef in this custom data structure however, this will not compile on GCC 4.4. The same code compiles and runs just fine on GCC 4.5.

这两个编译器版本之间有什么区别，使得代码在一个版本上编译，但不在其他。我猜想它与C ++ 11的实现有关，这在GCC 4.4中不太完整。可能是 decltype 或另一个新的C ++ 11关键字的东西，我猜猜。

What is the difference in between these two compiler versions, that makes the code compile on one version but not on the other. I would guess it has something to do with the implementation of C++11 which was much less complete in GCC 4.4. Probably something with decltype or another new C++11 keyword, I would guess.

，如果我使用 std :: back_inserter 而不定义 const_reference 类型？我通常认为，必须实现完整的typedef（ value_type ，引用， const_reference 等），以便与STL算法库兼容？或者我可以安全地假设，如果我的代码编译在这种情况下，我不会调用任何危险的（例如移动语义，这将毁灭我的其他数据结构）。

Also is this code correct, if I use the std::back_inserter without defining the const_reference type? I usually thought that one has to implement the full set of typedefs (value_type, reference, const_reference etc) in order to be compatible with the STL-algorithms library? Or can I safely assume that if my code compiles in this case I am not invoking anything dangerous (e.g. move semantics, which would destroy my other datastructure).

推荐答案

标准（1998）说 std :: back_insert_iterator 需要 Container :: const_reference 。在24.4.2.1模板类back_insert_iterator，[lib.back.insert.iterator]中，它表示：

The standard (1998) says that std::back_insert_iterator needs Container::const_reference. In "24.4.2.1 Template class back_insert_iterator", [lib.back.insert.iterator], it says:

back_insert_iterator<Container>&
operator=(typename Container::const_reference value);

2011标准只需要 Container :: value_type ，

The 2011 standard only wants Container::value_type,

back_insert_iterator<Container>&
operator=(const typename Container::value_type& value);
back_insert_iterator<Container>&
operator=(typename Container::value_type&& value);

因此，为了兼容两个版本的C ++标准，定义 value_type 和 const_reference_type 。

So, to be compatible with both versions of the C++ standard, define both value_type and const_reference_type.

在GCC 4.4.6和4.5.1中， operator = 的定义是相同的（ libstdc ++ - v3 / include / bits / stl_iterator.h ）：

In both GCC 4.4.6 and 4.5.1, the definition of operator= is identical (libstdc++-v3/include/bits/stl_iterator.h):

  back_insert_iterator&
  operator=(typename _Container::const_reference __value)
  {
    container->push_back(__value);
    return *this;
  }

我得到两个编译器相同的错误，也许你需要请仔细检查是否使用正确的编译器版本。

and I get the same error with both compilers, perhaps you'll need to double check if you're using the correct compiler versions.

这篇关于std :: back_inserter需要const_reference在旧的GCC。为什么？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！