在Consteval d时提供ConstexPr-Saf的简化异常消息

Question

想象一下这个简单的constexpr函数：

// Whatever, the exact values don't matter for this example
constexpr float items[100] = { 1.23f, 4.56f };
constexpr int length = 12;

constexpr float getItem(int index)
{
    if (index < 0 || index >= length)
    {
        // ArrayIndexOutOfRangeException has a constructor that takes a const char* and one that takes a std::string.
        throw ArrayIndexOutOfRangeException("You did a bad.");
    }
    return items[index];
}

您可以这样使用它：

int main()
{
    constexpr float f1 = getItem( 0); std::cout << f1 << std::endl; // Works fine
    constexpr float f2 = getItem( 1); std::cout << f2 << std::endl; // Works fine
    constexpr float f3 = getItem(-1); std::cout << f3 << std::endl; // Does not compile: "constexpr variable 'f3' must be initialized by a constant expression", "subexpression not valid in a constant expression"
    constexpr float f4 = getItem(20); std::cout << f4 << std::endl; // Does not compile: "constexpr variable 'f4' must be initialized by a constant expression", "subexpression not valid in a constant expression"
    return 0;
}

太好了！屁股的重量！

    volatile int i;
    i = 123; // As a placeholder for something like this: std::cin >> i;
    float f5 = getItem(i);
    std::cout << f5 << std::endl;

在投掷'arrayindexoutofrangeException'的实例'和“ what what（）：您做了不好的实例之后，这将在运行时投入运行时。
好的，这不是很有帮助，我想创建一个更好的错误消息：

constexpr float getItem(int index)
{
    if (index < 0 || index >= length)
    {
        std::stringstream stream;
        stream << "You did a bad. getItem was called with an invalid index (" << index << "), but it should have been non-negative and less than the total number of items (" << length << ").";
        throw ArrayIndexOutOfRangeException(stream.str());
    }
    return items[index];
}

但是不允许这样做：“非文字类型'std :: stringstream'（aka'basic_stringstream'的变量）无法在constexpr函数中定义。<<<<<<<<<<<<<<<<<<<<<<<< br> 我可以为编译时版提供更简单的错误消息，并且只有在运行时版本中进行复杂的字符串操作。

那...怎么样？

请注意，这是C ++ 17（一些GCC风味），可能没有C ++ 20具有的某些与ConstexPR相关的功能。

其他代码要求该函数保持constexpr。
如果可能的话，我想避免重复该功能。

Answer 1

您可以在另一个函数中移动错误消息创建：

std::string get_error_message(int index, int length = 10)
{
    std::stringstream stream;
    stream << "You did a bad. getItem was called with an invalid index ("
           << index
           << "), but it should have been non-negative "
           << "and less than the total number of items ("
           << length << ").";
    return stream.str();
}

constexpr float getItem(int index)
{
    constexpr int length = 10;
    constexpr std::array<float, length> items{};

    if (index < 0 || index >= length)
    {
        throw std::runtime_error(get_error_message(index, length));
    }
    return items[index];
}

demo