为什么它是非法的:将指针的向量复制到指向常量的指针向量中

Question

问题

以下代码不能在C++ 11(也不是C++ 14)中编译.我理解编译器输出的错误,但为什么标准不允许这样做？

//main.cpp

#include <vector>

int main(void)
{
    double a = 3.0;
    double b = 3.0;

    //It works with mere pointers
    const double* ptrToConst = &a;
    /***/ double* ptrToObj   = &a;
//  ptrToObj = ptrToConst; //Illegal : that's understandable…
    ptrToConst = ptrToObj;   //Works

    //But the same doesn't work with vectors to pointers
    std::vector<const double*> ptrsToConst = {&a, &b};
    std::vector</***/ double*> ptrsToObj   = {&a, &b};
//  ptrsToObj = ptrsToConst; //Illegal : that's understandable
    ptrsToConst = ptrsToObj; //Illegal : but why?!
}

该错误来自该行ptrsToConst = ptrsToObj.实际上,似乎不可能将指针向量复制std::vector<T*>到指向常量的指针向量中std::vector<const T*>.请注意,在这两种情况下,指针本身是不恒定的.

为什么这项行动是非法的？什么是最优雅的工作？

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更多详情

如果我通过调用编译clang++ --std=c++11 main.cpp,将显示以下错误消息:

main.cpp:19:17: error: no viable overloaded '='
    ptrsToConst = ptrsToObj; //Illegal : but why?!
    ~~~~~~~~~~~ ^ ~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/stl_vector.h:436:7: note: candidate
      function not viable: no known conversion from 'vector<double *, allocator<double *>>' to 'const
      vector<const double *, allocator<const double *>>' for 1st argument
      operator=(const vector& __x);
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/stl_vector.h:448:7: note: candidate
      function not viable: no known conversion from 'vector<double *, allocator<double *>>' to 'vector<const
      double *, allocator<const double *>>' for 1st argument
      operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/stl_vector.h:470:7: note: candidate
      function not viable: no known conversion from 'std::vector<double *>' to 'initializer_list<value_type>' (aka
      'initializer_list<const double *>') for 1st argument
      operator=(initializer_list<value_type> __l)
      ^
1 error generated.

尝试使用gcc(g ++)会产生类似的错误消息.

显然,实现向量的方式不允许我尝试执行的操作.然而,这是关于const正确性的安全操作,对吧？

Answer 1

你可以做到这一点,而不是operator=.您需要assign成员函数,该函数对每个元素执行转换.

ptrsToConst.assign(ptrsToObj.begin(), ptrsToObj.end());