【发布时间】：2013-09-06 07:27:03
【问题描述】：
有人告诉我，在插入大量元素之前调用vector.reserve() 是一种很好的做法。我刚遇到一种情况，我想将大量 const 元素放入 vector 中。但是，当调用reserve() 时，会引发编译器错误。
考虑以下重现问题的代码：
#include <vector>
int main()
{
std::vector<const int> vec;
vec.reserve(2);
}
这会导致以下巨大的编译器错误：
在 /usr/include/x86_64-linux-gnu/c++/4.8/bits/c++allocator.h:33:0 包含的文件中，
来自 /usr/include/c++/4.8/bits/allocator.h:46，
来自 /usr/include/c++/4.8/vector:61，
来自vecreserve.cpp:1：
/usr/include/c++/4.8/ext/new_allocator.h：在“struct __gnu_cxx::new_allocator”的实例化中：
/usr/include/c++/4.8/bits/allocator.h:92:11：来自‘class std::allocator’所需
/usr/include/c++/4.8/bits/alloc_traits.h:90:43: 来自‘struct std::allocator_traits >’
/usr/include/c++/4.8/ext/alloc_traits.h:121:10: 来自‘struct __gnu_cxx::__alloc_traits >’
/usr/include/c++/4.8/bits/stl_vector.h:75:28: 来自‘struct std::_Vector_base >’
/usr/include/c++/4.8/bits/stl_vector.h:210:11：从‘class std::vector’需要
vecreserve.cpp:5:26: 从这里需要
/usr/include/c++/4.8/ext/new_allocator.h:93:7: 错误：'const _Tp* __gnu_cxx::new_allocator::address(__gnu_cxx::new_allocator::const_reference) const [with _Tp = const int; __gnu_cxx::new_allocator::const_pointer = const int*; __gnu_cxx::new_allocator::const_reference = const int&]' 不能被重载
地址(const_reference __x) const _GLIBCXX_NOEXCEPT
^
/usr/include/c++/4.8/ext/new_allocator.h:89:7: error: with ‘_Tp* __gnu_cxx::new_allocator::address(__gnu_cxx::new_allocator::reference) const [with _Tp = const int; __gnu_cxx::new_allocator::pointer = const int*; __gnu_cxx::new_allocator::reference = const int&]’
地址（参考 __x） const _GLIBCXX_NOEXCEPT
^
/usr/include/c++/4.8/ext/new_allocator.h：在 'void __gnu_cxx::new_allocator::deallocate(__gnu_cxx::new_allocator::pointer, __gnu_cxx::new_allocator::size_type) 的实例化中 [with _Tp = const int ; __gnu_cxx::new_allocator::pointer = const int*; __gnu_cxx::new_allocator::size_type = long unsigned int]’：
/usr/include/c++/4.8/bits/stl_vector.h:174:4: 需要来自 'void std::_Vector_base::_M_deallocate(std::_Vector_base::pointer, std::size_t) [with _Tp = const int ; _Alloc = std::allocator; std::_Vector_base::pointer = const int*; std::size_t = long unsigned int]’
/usr/include/c++/4.8/bits/vector.tcc:80:28: 需要来自'void std::vector::reserve(std::vector::size_type) [with _Tp = const int; _Alloc = std::allocator; std::vector::size_type = long unsigned int]’
vecreserve.cpp:6:16: 从这里需要
/usr/include/c++/4.8/ext/new_allocator.h:110:30：错误：从‘const void*’到‘void*’的无效转换[-fpermissive]
{ ::运算符删除（__p）； }
^
在 /usr/include/c++/4.8/ext/new_allocator.h:33:0 包含的文件中，
来自 /usr/include/x86_64-linux-gnu/c++/4.8/bits/c++allocator.h:33，
来自 /usr/include/c++/4.8/bits/allocator.h:46，
来自 /usr/include/c++/4.8/vector:61，
来自vecreserve.cpp:1：
/usr/include/c++/4.8/new:95:6: 错误：初始化‘void operator delete(void*)’的参数 1 [-fpermissive]
无效运算符删除（无效*）_GLIBCXX_USE_NOEXCEPT
^
在 /usr/include/c++/4.8/vector:60:0 包含的文件中，
来自vecreserve.cpp:1：
/usr/include/c++/4.8/bits/stl_algobase.h：在'_OI std::__copy_move_a(_II, _II, _OI) 的实例化中 [with bool _IsMove = true; _II = 常量 int*; _OI = const int*]':
/usr/include/c++/4.8/bits/stl_algobase.h:428:38: 需要从'_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = true; _II = 常量 int*; _OI = const int*]’
/usr/include/c++/4.8/bits/stl_algobase.h:460:17: 需要从'_OI std::copy(_II, _II, _OI) [with _II = std::move_iterator; _OI = const int*]’
/usr/include/c++/4.8/bits/stl_uninitialized.h:93:53: 来自'static _ForwardIterator std::__uninitialized_copy::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator; _ForwardIterator = const int*]’
/usr/include/c++/4.8/bits/stl_uninitialized.h:117:41: 来自‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator; _ForwardIterator = const int*]’
/usr/include/c++/4.8/bits/stl_uninitialized.h:258:63: 来自‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator&) [with _InputIterator = std::move_iterator; _ForwardIterator = 常量 int*; _Tp = const int]’
/usr/include/c++/4.8/bits/stl_vector.h:1142:29: 来自 'std::vector::pointer std::vector::_M_allocate_and_copy(std::vector::size_type, _ForwardIterator, _ForwardIterator) [与 _ForwardIterator = std::move_iterator; _Tp = 常量 int; _Alloc = std::allocator; std::vector::pointer = const int*; std::vector::size_type = long unsigned int]’
/usr/include/c++/4.8/bits/vector.tcc:75:70: 需要来自'void std::vector::reserve(std::vector::size_type) [with _Tp = const int; _Alloc = std::allocator; std::vector::size_type = long unsigned int]’
vecreserve.cpp:6:16: 从这里需要
/usr/include/c++/4.8/bits/stl_algobase.h:390:70: 错误：没有匹配函数调用'std::__copy_move::__copy_m(const int*&, const int*&, const int*& )'
_Category>::__copy_m(__first, __last, __result);
^
/usr/include/c++/4.8/bits/stl_algobase.h:390:70：注意：候选人是：
/usr/include/c++/4.8/bits/stl_algobase.h:368:9: 注意：模板静态 _Tp* std::__copy_move::__copy_m(const _Tp*, const _Tp*, _Tp*) [with _Tp = _Tp; bool _IsMove = true]
__copy_m(const _Tp* __first, const _Tp* __last, _Tp* __result)
^
/usr/include/c++/4.8/bits/stl_algobase.h:368:9：注意：模板参数推导/替换失败：
/usr/include/c++/4.8/bits/stl_algobase.h:390:70：注意：推断参数“_Tp”（“int”和“const int”）的冲突类型
_Category>::__copy_m(__first, __last, __result);
^
为什么会这样？我不应该在vector 的const 元素上调用reserve 吗？如果是，为什么不呢？
【问题讨论】：
-
因为你的
const int -
@P0W 我知道，你能说得更具体点吗？为什么我不能在
const int的vector上调用reserve？ -
因为当您调用 reserve() 时，它会为它们分配空间，但是您以后将无法修改它们（因为它们是 const ......）所以它们会 始终具有由 reserve() 分配的默认值。也就是说，对于这个非常特殊的问题，我会抛弃 const（因为它只会影响编译器检查，而不影响生成的代码）。