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我正在尝试初始化一个span<const T*>- 即指向 const 数据的指针列表。但是，const指针和span<>可用构造函数之间的转换规则阻碍了我。

我确定有办法做到这一点，但我找不到正确的转换/调用。

旁白：我实际上没有使用 C++20，而是tcb::span，它使用较旧的P0122R7构造函数集，带有 (pointer,size) 而不是迭代器。但我怀疑在 C++20 上工作会让我朝着正确的方向前进。

下面的示例演示了我正在尝试做的事情，以及一些修复它的失败尝试：

Godbolt上的实时链接

#include<span>
#include<vector>
#include<iostream>
using std::cout;
using std::endl;

int main() {

    int a = 0;
    int b = 1;
    int c = 2;

    std::vector<int*> pointers = { &a, &b, &c };
    
    // This declaration does not work; no span<> constructor matches it.
    std::span<const int*> cspan(&pointers[0], pointers.size());

    // This declaration also does not work; the cast is fine, but still no span<> constructor matches.
    //std::span<const int*> cspan(static_cast<const int *const*>(&pointers[0]), pointers.size());

    // This declaration works, but then "cspan" cannot be sorted, since
    // the pointers themselves are const, and cannot be overwritten.
    //std::span<const int* const> cspan(&pointers[0], pointers.size());
    
    // Sort the span (by address)
    // This is the code I want to work. I just need some way to declare 'cspan' properly.
    std::sort(cspan.begin(), cspan.end());
    
    return 0;
}

有任何想法吗？

4

1 回答 1

1

有什么问题：

#include<span>
#include<vector>
#include<iostream>
using std::cout;
using std::endl;

int main() {

    int a = 0;
    int b = 1;
    int c = 2;

    std::vector<int*> pointers = { &a, &b, &c };

    std::span<const int*> cspan(const_cast<const int**>(pointers.data()), pointers.size());

    std::sort(cspan.begin(), cspan.end());
    
    return 0;
}
于 2022-01-20T00:37:43.297 回答