C ++ std： T * const> [英] C++ std:vector< T* const>

问题描述

我必须继续一个程序。我之前的程序员使用了很多结构：

i have to continue a programm. The programmer before me used the structure a lot:

std:vector< T* const>

他在Visual Studio C ++ 2010中写了ist，我使用g ++，它遇到一些编译错误。

He wrote ist in Visual Studio C++ 2010 and was able to compile this. I am using g++ and it thwrows some compilation errors.

    g++ -g -Wall -c -std=c++11 -pedantic -I/usr/include/SuperLU/ src/Cell.cpp -o obj/Cell.o
In file included from src/Cell.cpp:13:0:
src/Cell.h:81:2: warning: extra ';' [-pedantic]
In file included from /usr/include/x86_64-linux-gnu/c++/4.7/./bits/c++allocator.h:34:0,
                 from /usr/include/c++/4.7/bits/allocator.h:48,
                 from /usr/include/c++/4.7/string:43,
                 from /usr/include/c++/4.7/bits/locale_classes.h:42,
                 from /usr/include/c++/4.7/bits/ios_base.h:43,
                 from /usr/include/c++/4.7/ios:43,
                 from /usr/include/c++/4.7/ostream:40,
                 from /usr/include/c++/4.7/iostream:40,
                 from src/Cell.cpp:2:
/usr/include/c++/4.7/ext/new_allocator.h: In instantiation of 'struct __gnu_cxx::new_allocator<BattPackage::Leg* const>':
/usr/include/c++/4.7/bits/allocator.h:89:11:   required from 'class std::allocator<BattPackage::Leg* const>'
/usr/include/c++/4.7/bits/alloc_traits.h:92:43:   required from 'struct std::allocator_traits<std::allocator<BattPackage::Leg* const> >'
/usr/include/c++/4.7/ext/alloc_traits.h:110:10:   required from 'struct __gnu_cxx::__alloc_traits<std::allocator<BattPackage::Leg* const> >'
/usr/include/c++/4.7/bits/stl_vector.h:76:28:   required from 'struct std::_Vector_base<BattPackage::Leg* const, std::allocator<BattPackage::Leg* const> >'
/usr/include/c++/4.7/bits/stl_vector.h:208:11:   required from 'class std::vector<BattPackage::Leg* const>'
src/Cell.h:283:39:   required from here
/usr/include/c++/4.7/ext/new_allocator.h:83:7: error: 'const _Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::const_reference) const [with _Tp = BattPackage::Leg* const; __gnu_cxx::new_allocator<_Tp>::const_pointer = BattPackage::Leg* const*; __gnu_cxx::new_allocator<_Tp>::const_reference = BattPackage::Leg* const&]' cannot be overloaded
/usr/include/c++/4.7/ext/new_allocator.h:79:7: error: with '_Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::reference) const [with _Tp = BattPackage::Leg* const; __gnu_cxx::new_allocator<_Tp>::pointer = BattPackage::Leg* const*; __gnu_cxx::new_allocator<_Tp>::reference = BattPackage::Leg* const&]

想要这个结构是一个指针的向量，在那里他可以添加和删除指针和操作指针的目标，但不改变指针指向的对象。

What he wanted with this structure was a vector of pointers, where he can add and remove the pointers and manipulate the target of the pointers but not change on which object the pointer points.

根据我的理解，它不应该编译，因为T * const是不可分配的。

As far as I understand it should not compile because T* const is not assignable.

有人知道为什么它在Visual Studio编译？
我可以在声明中替换，而不需要更改完整的代码？

Does anyone know why it compiles in Visual Studio ? And can I replace that in the declarations without the need to change the complete code?

推荐答案

std :: vector 使用分配器为其成员分配，重新分配和释放内存。 Allocator需求仅为 T 类型的分配器 X 定义，其中 T 是任何非const，非引用对象类型（C ++ 11表27）。所以使用 std :: vector< T * const> 会给你未定义的行为，因为它使用的分配器没有为const元素类型定义。

std::vector uses an Allocator to allocate, reallocate, and deallocate memory for its members. The Allocator requirements are only defined for an Allocator X for type T, where T is "any non-const, non-reference object type" (C++11 Table 27). So using a std::vector<T* const> gives you undefined behaviour, because the allocator it uses is not defined for a const element type.

你最好尽快修复这个问题。将声明更改为 std :: vector< T *> 不太可能有很多问题。

You're better off fixing this as soon as possible. There are unlikely to be many issues with changing the declarations to std::vector<T*>.

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