A header only "csv" parser which is fast and versatile with modern C++ api. Requires compiler with C++17 support. [Can also be used to convert strings to specific types.](#The-converter)
The library can be used as a single header file as **`ssp.hpp`**, but it sufferes a slight performance loss when converting floating point values since the **`fast_float`** library is not present within the file.
The parser can be told to use only certain columns by parsing the header. This can be done by using the **`use_fields`** method. It accepts any number of string-like arguments or even an **`std::vector<std::string>`** with the field names. If any of the fields are not found within the header or if any fields are defined multiple times it will result in an error.
The header can be ignored using the **`ss::ignore_header`** [setup](#Setup) option or by calling the **`ignore_next`** metod after the parser has been constructed.
The fields with which the parser works with can be modified at any given time. The praser can also check if a field is present within the header by using the **`field_exists`** method.
The alternate example will be used to show some of the features of the library. The **`get_next`** method returns a tuple of objects specified inside the template type list.
If a conversion could not be applied, the method would return a tuple of default constructed objects, and the **`valid`** method would return **`false`**, for example if the third (grade) column in our csv could not be converted to a float the conversion would fail.
If **`get_next`** is called with a **`tuple`** as template parameter it would behave identically to passing the same tuple parameters to **`get_next`**:
*Note, it does not always return a student tuple since the returned tuples parameters may be altered as explained below (no void, no restrictions, ...)*
Whole objects can be returned using the **`get_object`** function which takes the tuple, created in a similar way as **`get_next`** does it, and creates an object out of it:
And finally, using something I personally like to do, a struct (class) with a **`tied`** method which returns a tuple of references to to the members of the struct.
The method can be used to compare the object, serialize it, deserialize it, etc. Now **`get_next`** can accept such a struct and deduce the types to which to convert the csv.
By default, many of the features supported by the parser are disabled. They can be enabled within the template parameters of the parser. For example, to enable quoting and escaping the parser would look like:
Quoting can be enabled by defining **`ss::quote`** within the setup parameters. A single character can be defined as the quoting character, for example to use **`"`** as a quoting character:
Escaping can be enabled by defining **`ss::escape`** within the setup parameters. Multiple character can be defined as escaping characters.It simply removes any special meaning of the character behind the escaped character, anything can be escaped. For example to use ``\`` as an escaping character:
Spacing can be enabled by defining **`ss::trim`** , **`ss::trim_left`** or **`ss::trim_right`** within the setup parameters. Multiple character can be defined as spacing characters, for example to use ``' '`` as an spacing character **`ss::trim<' '>`** needs to be defined. It removes any space from both sides of the row. To trim only the right side **`ss::trim_right`** can be used, and intuitively **`ss::trim_left`** to trim only the left side. If **`ss::trim`** is enabled, those lines would have an equivalent output:
Multiline can be enabled by defining **`ss::multilne`** within the setup parameters. It enables the possibility to have the new line characters within rows. The new line character needs to be either escaped or within quotes so either **`ss::escape`** or **`ss::quote`** need to be enabled. There is a specific problem when using multiline, for example, if a row had an unterminated quote, the parser would assume it to be a new line within the row, so until another quote is found, it will treat it as one line which is fine usually, but it can cause the whole csv file to be treated as a single line by mistake. To prevent this **`ss::multiline_restricted`** can be used which accepts an unsigned number representing the maximum number of lines which can be allowed as a single multiline. Examples:
Passing **`void`** makes the parser ignore a column. In the given example **`void`** could be given as the second template parameter to ignore the second (age) column in the csv, a tuple of only 2 parameters would be retuned:
Values can also be converted to **`std::string_view`**. It is more efficient then converting values to **`std::string`** but one must be careful with the lifetime of it.
**`std::optional`** could be passed if we wanted the conversion to proceed in the case of a failure returning **`std::nullopt`** for the specified column:
Similar to **`std::optional`**, **`std::variant`** could be used to try other conversions if the previous failed _(Note, conversion to std::string will always pass)_:
Custom **`restrictions`** can be used to narrow down the conversions of unwanted values. **`ss::ir`** (in range) and **`ss::ne`** (none empty) are one of those:
If the restrictions are not met, the conversion will fail. Other predefined restrictions are **`ss::ax`** (all except), **`ss::nx`** (none except) and **`ss::oor`** (out of range), **`ss::lt`** (less than), ...(see *restrictions.hpp*):
To define a restriction, a class/struct needs to be made which has a **`ss_valid`** method which returns a **`bool`** and accepts one object. The type of the conversion will be the same as the type of the passed object within **`ss_valid`** and not the restriction itself. Optionally, an **`error`** method can be made to describe the invalid conversion.
Custom types can be used when converting values. A specialization of the **`ss::extract`** function needs to be made and you are good to go. A custom conversion for an enum would look like this:
The shape enum will be used in an example below. The **`inline`** is there just to prevent multiple definition errors. The function returns **`true`** if the conversion was a success, and **`false`** otherwise. The function uses **`const char*`** begin and end for performance reasons.
Detailed error messages can be accessed via the **`error_msg`** method, and to enable them **`ss::string_error`** needs to be included in the setup. If **`ss::string_error`** is not defined, the **`error_msg`** method will not be defined either.
An error can be detected using the **`valid`** method which would return **`false`** if the file could not be opened, or if the conversion could not be made (invalid types, invalid number of columns, ...). The **`eof`** method can be used to detect if the end of the file was reached.
The parser can also be used to effectively parse files whose rows are not always in the same format (not a classical csv but still csv-like). A more complicated example would be the best way to demonstrate such a scenario.
The delimiter is " ", and the number of columns varies depending on which shape it is. We are required to read the file and to store information (shape and area) of the shapes into a data structure in the same order as they are in the file.
The **`try_next`** method works in a similar way as **`get_next`** but returns a **`composite`** which holds a **`tuple`** with an **`optional`** to the **`tuple`** returned by **`get_next`**. This **`composite`** has an **`or_else`** method (looks a bit like **`tl::expected`**) which is able to try additional conversions if the previous failed. **`or_else`** also returns a **`composite`**, but in its tuple is the **`optional`** to the **`tuple`** of the previous conversions and an **`optional`** to the **`tuple`** of the new conversion. (sounds more complicated than it is.
To fetch the **`tuple`** from the **`composite`** the **`values`** method is used. The value of the above used conversion would look something like this:
Similar to the way that **`get_next`** has a **`get_object`** alternative, **`try_next`** has a **`try_object`** alternative, and **`or_else`** has a **`or_object`** alternative. Also all rules applied to **`get_next`** also work with **`try_next`** , **`or_else`**, and all the other **`composite`** conversions.
Each of those **`composite`** conversions can accept a lambda (or anything callable) as an argument and invoke it in case of a valid conversion. That lambda itself need not have any arguments, but if it does, it must either accept the whole **`tuple`**/object as one argument or all the elements of the tuple separately. If the lambda returns something that can be interpreted as **`false`** the conversion will fail, and the next conversion will try to apply. Rewriting the whole while loop using lambdas would look like this:
It is a bit less readable, but it removes the need to check which conversion was invoked. The **`composite`** also has an **`on_error`** method which accepts a lambda which will be invoked if no previous conversions were successful. The lambda can take no arguments or just one argument, an **`std::string`**, in which the error message is stored if **`string_error`** is enabled:
First of all, *type_traits.hpp* and *function_traits.hpp* contain many handy traits used in the parser. Most of them are operating on tuples of elements and can be utilized in projects.
To convert a string the **`convert`** method can be used. It accepts a c-string as input and a delimiter, as **`std::string`**, and retruns a **`tuple`** of objects in the same way **`get_next`** does it for the parser. A whole object can be returned too using the **`convert_object`** method, again in an identical way **`get_object`** doest it for the parser.
The converter has also the ability to just split the line, ~~tho it does not change it (kinda statically), hence the name of the library~~ and depending if either quoting or escaping are enabled it may change the line, rather than creating a copy, for performance reasons (the name of the library does not apply anymore, I may change it). It returns an **`std::vector`** of **`std::pair`**s of pointers, begin and end, each pair representing a split segment (column) of the whole string. The vector can then be used in a overloaded **`convert`** method. This allows the reuse of the same line without splitting it on every conversion.
The same setup parameters also apply for the converter, tho multiline has not impact on it. Since escaping and quoting potentially modify the content of the given line, a converter which has those setup parameters defined does not have the same convert method, **`the input line cannot be const`**.
