Value parsing improvement

[rsore]

Description

In this development branch, the function from_string has been renamed to parse_value to better convey the purpose of the function. This is especially important as it is this function that users of the library can use to support parsing their own value types from the command-line.

Additionally, I have reworked the implementation of parsing integers and floating-point numbers and added support for parsing string_views and time units. Below are some more details for each type:

Parsing integers

Previosuly, integers were parsed using functions like std::stoi and std::stol. Though this is fine, it gets annoying using all the different versions of the function based on the integer type (int, long, unsigned short...). It is also generally less performant than the alternative we are using now: std::from_chars. Instead of handling all integer types separately, I have specialized parse_value for all types that satisfy the concept std::integral and use std::from_chars to handle the actual parsing after some input validation. Another benefit is getting support for parsing values in hexadecimal and binary format basically for free.

Parsing floating-point numbers

For floating-point types, we were previously using the floating-point equivalent of the integer ones; std::stof, std::stod and std::stold
. As with integer types, I have in this branch utilized std::from_chars, specializing parse_value for all types that satisfy the concept std::floating_point.

String views

I decided to add support for parsing std::string_view. All it does it basically just return the input as the output, but it is an essential specialization as it allows users to specify string as the value type of an option without requiring to allocate an entire std::string. The once concern with this is that if users decide to manipulate the elements of argv after parsing it could lead to issues, as the string views point to the actual memory in the argv array. It is essential to document this behaviour to users and enfore they do not mess with argv after parsing if they choose to use string view as a value type.

Time units

Passing time points and durations is quite common in command-line interfaces so it is useful for the library to be able to parse this for the user automatically. In this branch, I have specialized parse_value for all std::chrono::duration type, meaning they can specify the value type to be for example std::chrono::seconds, or more specialized types like std::chrono::duration<double, std::milli>.

Error handling in parse_value

I decided to implement an exception class ParseValueException that is templated on the type that is parsed. This enables some centralizing some common error message logic, and printing the type of the type that the error occurs for. For example, trying to parse "asd" as an int may produce the following error message:

Unable to parse "asd" as type "32-bit signed integer": Invalid format

Similarly, trying to parse "asd" as the type std::chrono::milliseconds may produce the following error message:

Unable to parse "asd" as type "milliseconds": Unable to parse "asd" as type "64-bit signed integer": Invalid format

Since part of parsing a std::chrono::milliseconds involved parsing the underlying type, we end up with an error message that explains quite a lot of what went wrong: We tried to parse "asd" as type "milliseconds", this involved parsing it as a 64-bit signed integer, and that failed because the format was invalid.

How Has This Been Tested?

The file tests/parse_value_tests.cpp contains a comprehensive test of tests (868 assertions in 117 test cases) for all specializations of parse_value.

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