How to convert std::string_view to double?

Question

I'm writing a c++ parser for a custom option file for an application. I have a loop that reads lines in the form of option=value from a text file where value must be converted to double. In pseudocode it does the following:

while(not EOF)
    statement <- read_from_file
    useful_statement <- remove whitespaces, comments, etc from statement
    equal_position <- find '=' in useful_statement
    option_str <- useful_statement[0:equal_position)
    value_str <- useful_statement[equal_position:end)
    find_option(option_str) <- double(value_str)

To handle the string splitting and passing around to functions, I use std::string_view because it avoids excessive copying and clearly states the intent of viewing segments of a pre-existing std::string. I've done everything to the point where std::string_view value_str points to the exact part of useful_statement that contains the value I want to extract, but I can't figure out the way to read a double from an std::string_view.

I know of std::stod which doesn't work with std::string_view. It allows me to write

double value = std::stod(std::string(value_str));

However, this is ugly because it converts to a string which is not actually needed, and even though it will presumably not make a noticeable difference in my case, it could be too slow if one had to read a huge amount of numbers from a text file.

On the other hand, atof won't work because I can't guarantee a null terminator. I could hack it by adding \0 to useful_statement when constructing it, but that will make the code confusing to a reader and make it too easy to break if the code is altered/refactored.

So, what would be a clean, intuitive and reasonably efficient way to do this?

Answer 1

Since you marked your question with C++1z, then that (theoretically) means you have access to from_chars. It can handle your string-to-number conversion without needing anything more than a pair of const char*s:

double dbl;
auto result = from_chars(value_str.data(), value_str.data() + value_str.size(), dbl);

Of course, this requires that your standard library provide an implementation of from_chars.