C++:<=有符号和无符号之间的冲突

Question

我已经围绕 .substr 函数创建了一个包装器:

wstring MidEx(wstring u, long uStartBased1, long uLenBased1)
{
    //Extracts a substring. It is fail-safe. In case we read beyond the string, it will just read as much as it has
    // For example when we read from the word HELLO , and we read from position 4, len 5000, it will just return LO
    if (uStartBased1 > 0)
    {
       if (uStartBased1 <= u.size())
        {
            return u.substr(uStartBased1-1, uLenBased1);
        }
    }
    return wstring(L"");
}

它工作正常，但是编译器给我警告“<= 有符号和无符号之间的冲突”。

谁能告诉我如何正确地做到这一点？

非常感谢!

Answer 1

您应该使用wstring::size_type(或size_t)而不是long:

wstring MidEx(wstring u, wstring::size_type uStartBased1, wstring::size_type uLenBased1)
{
    //Extracts a substring. It is fail-safe. In case we read beyond the string, it will just read as much as it has
    // For example when we read from the word HELLO , and we read from position 4, len 5000, it will just return LO
    if (uStartBased1 > 0)
    {
       if (uStartBased1 <= u.size())
        {
            return u.substr(uStartBased1-1, uLenBased1);
        }
    }
    return wstring(L"");
}

这是 u.size() 的确切返回类型。这样，您可以确保比较给出预期的结果。

如果您正在使用 std::wstring 或其他标准库容器(如 std::vector 等)，则 x::size_type 将被定义为 size_t。所以使用它会更加一致。