可以 static_const<size_t> ()、std::as_const() 或 static_cast<const size_t> () 用于声明数组？

Question

I tried the three successive lines (each one of them alone) but no one of them worked. Why??

int main()
{
    size_t j{8};


    char arr[static_cast<const size_t>(j)]={'t'};
    char arr[static_const<size_t>(j)]={'t'};
    char arr[std::as_const(j)]={'t'};


    arr[7]='\0';
    std::cout<<arr;


    return 0;
}

Answer 1

None of your 3 examples are constants at compile-time , because j is not assigned a value until runtime . No amount of casting j will change that.

To assign j at compile-time, you need to declare it as const (or constexpr in C++11 and later) and then you can use j as-is for declaring the array:

int main()
{
    const size_t j{8};
    // or: constexpr size_t j{8};

    char arr[j]={'t'};

    arr[7]='\0';
    std::cout<<arr;

    return 0;
}

Answer 2

Ok, let's look at an updated bit of code - that actually compiles:

#include <iostream>

int main()
{
    constexpr size_t j{8};

    char arr[j]={'t'};

    arr[7]='\0';
    std::cout<<arr;

    return 0;
}

j is now a compile-time constant, suitable for using to size a built-in array.