C++ 中的 const 向量引用参数

Question

我认为自己对 C++ 的更基本方面相当了解，但有时我会感到困惑。我正在尝试实现一对作用于std::vector 实例的函数，并对它们的值进行一些操作，而不是就地更改它们。自然地，我认为通过const& 是合理的做法。但是，下面的MWE编译失败，我想了解一下原因：

#include<vector>

void func1(const std::vector<const char* const>& v){
     // do stuff
}

void func2(const std::vector<const std::vector<const char* const> >& v){
  for(int i=0;i<v.size();++i){
    func1(v[i]);
  }
}

int main(){
    return 0;
}

错误信息看起来有点像这样：

In file included from...include/c++/4.9.3/x86_64-unknown-linux-gnu/bits/c++allocator.h:33:0,
                 from...include/c++/4.9.3/bits/allocator.h:46,
                 from...include/c++/4.9.3/vector:61,
                 from test.cxx:2:
.../ext/new_allocator.h: In instantiation of ‘class __gnu_cxx::new_allocator<const std::vector<const char* const> >’:
.../bits/allocator.h:92:11:   required from ‘class std::allocator<const std::vector<const char* const> >’
.../ext/alloc_traits.h:172:53:   required from ‘struct __gnu_cxx::__alloc_traits<std::allocator<const std::vector<const char* const> > >’
.../bits/stl_vector.h:75:28:   required from ‘struct std::_Vector_base<const std::vector<const char* const>, std::allocator<const std::vector<const char* const> > >’
.../bits/stl_vector.h:214:11:   required from ‘class std::vector<const std::vector<const char* const> >’
test.cxx:9:18:   required from here
.../ext/new_allocator.h:93:7: error: ‘const _Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::const_reference) const [with _Tp = const std::vector<const char* const>; __gnu_cxx::new_allocator<_Tp>::const_pointer = const std::vector<const char* const>*; __gnu_cxx::new_allocator<_Tp>::const_reference = const std::vector<const char* const>&]’ cannot be overloaded
       address(const_reference __x) const _GLIBCXX_NOEXCEPT
       ^
.../ext/new_allocator.h:89:7: error: with ‘_Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::reference) const [with _Tp = const std::vector<const char* const>; __gnu_cxx::new_allocator<_Tp>::pointer = const std::vector<const char* const>*; __gnu_cxx::new_allocator<_Tp>::reference = const std::vector<const char* const>&]’
       address(reference __x) const _GLIBCXX_NOEXCEPT
       ^
.../ext/new_allocator.h: In instantiation of ‘class __gnu_cxx::new_allocator<const char* const>’:
.../bits/allocator.h:92:11:   required from ‘class std::allocator<const char* const>’
.../ext/alloc_traits.h:172:53:   required from ‘struct __gnu_cxx::__alloc_traits<std::allocator<const char* const> >’
.../bits/stl_vector.h:75:28:   required from ‘struct std::_Vector_base<const char* const, std::allocator<const char* const> >’
.../bits/stl_vector.h:214:11:   required from ‘class std::vector<const char* const>’
test.cxx:10:14:   required from here
.../ext/new_allocator.h:93:7: error: ‘const _Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::const_reference) const [with _Tp = const char* const; __gnu_cxx::new_allocator<_Tp>::const_pointer = const char* const*; __gnu_cxx::new_allocator<_Tp>::const_reference = const char* const&]’ cannot be overloaded
       address(const_reference __x) const _GLIBCXX_NOEXCEPT
       ^
.../ext/new_allocator.h:89:7: error: with ‘_Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::reference) const [with _Tp = const char* const; __gnu_cxx::new_allocator<_Tp>::pointer = const char* const*; __gnu_cxx::new_allocator<_Tp>::reference = const char* const&]’
       address(reference __x) const _GLIBCXX_NOEXCEPT

我对错误消息声称不能重载的事实感到困惑，指的是在func2 中调用func1。解决 const 对向量条目的引用似乎存在问题，但我真的不明白为什么这段代码无法编译。对于此类行为的任何解释、建议或指向文档的指针，我都很高兴。

Answer 1

std::vector<const char* const>

您正在尝试使用const 类型实例化一个向量。该类型至少需要可移动，但事实并非如此。改为：

std::vector<const char*>

改为

Answer 2

如果您事先知道这意味着什么，编译器会正确报告其问题。

std::allocator 对address 有两个重载。

address(reference __x)
address(const_reference __x)

编译器的问题是，对于模板类型const T，T& 和const T& 这两个类型将是相同的类型。然后它认为同一个重载有两个副本。