在 consteval 时提供 constexpr 安全的简化异常消息，否则提供 stringstream 详细信息

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];
}

你可以这样使用它：

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;
}

太棒了！臀部重量！

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

这在运行时间抛出“在抛出 'ArrayIndexOutOfRangeException' 的实例后调用终止”和“what()：你做错了。”
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];
}

但这是不允许的：“不能在 constexpr 函数中定义非文字类型 'std::stringstream'（又名 'basic_stringstream'）的变量”。
对于编译时版本，我可以接受更简单的错误消息，并且只在运行-time 版本中进行复杂的字符串操作。

所以...如何？

请注意，这是 C++17（一些 GCC 风格），它可能没有 C++20 所具有的一些与 constexpr 相关的特性。

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

Answer 1

您可以将错误消息创建移动到另一个函数中：

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

在 consteval 时提供 constexpr 安全的简化异常消息，否则提供 stringstream 详细信息

Provide constexpr-safe simplified exception message when consteval'd, otherwise stringstream verbose info

c++

templates

constexpr

c++17