std codecvt in and std codecvt do in in C++
Information interchange is essential to the computer industry, but it's not always easy. Depending on the encoding system, characters are the fundamental units of text that can be represented in a variety of ways. It is where character code conversion comes into play, and the std::codecvt aspect in C++ offers strong tools to handle these conversions.
This article explores the inner workings of two crucial C++ functions for converting character sequences between internal and external representations: std::codecvt::in and std::codecvt::do_in.
Character Encodings:
Consider keeping text files on a computer. For processing purposes, every character needs to have a numerical representation whether it be an alphabet, integer, or symbol. We refer to this numerical system as character encoding. Typical encodings consist of:
- Basic English characters can be encoded using the commonly used 7-bit ASCII (American Standard Code for Information Interchange) encoding.
- Unicode Transformation Format - 8-bit, or UTF-8: A variable-length encoding that may convey a wide variety of characters from many languages.
- Broad character encodings (UTF-16, UTF-32): fixed-length encodings that express a greater character set by using more bits per character.
- A trade-off between comprehensiveness (the capacity to represent more characters) and economy (fewer bits) must be considered when selecting an encoding.
What is the std::codecvt?
The locale library in C++ offers internationalization and localization tools through std::codecvt (the Character Conversion Facet). It is made possible by the std::codecvt facet that allows character conversion between the program's internal character representation, which is typically wide characters, and the external character representation. It is typically single-byte characters like ASCII or multi-byte encodings like UTF-8 and is used for file I/O or interfacing with external systems.
The std::codecvt is a class template that defines an interface for character conversion. It takes three template parameters:
- InternT: The internal character type (typically wchar_t).
- ExternT: The external character type (typically char for single-byte or a custom type for multi-byte encodings).
- StateT: It is the state type that is used to track conversion state (often std::mbstate_t).
std::codecvt::in and std::codecvt::do_in
The std::codecvt class provides several methods for character conversion. Two important ones are:
std::codecvt::in: It is a virtual function declared as:
virtual int in(state_type* to, const extern_type* from, const extern_type* from_end,
intern_type* to_next, const conversion_result& result);
The in function is responsible for updating the state data (to_next) and translating a series of external characters (from) into internal characters (to).
std::codecvt::do_in: It is a protected member function declared as:
protected: virtual int do_in(state_type* to, const extern_type* from, const extern_type* from_end,
intern_type* to_next, const conversion_result& result);
This function is similar to in but is protected, which means that it can only be called by member functions of the std::codecvt class or its derived classes. It performs the actual conversion logic for a single external character, considering the current conversion state (to).
The Relationship Between in and do_in:
- The in function uses a loop to cycle through the external character sequence.
- It calls do_in function to transform one external character for each iteration.
- The number of external characters used or an error code that is returned by do_in, which also modifies the conversion state.
- It decides when to end the iteration and update the pointers and result object based on the conversion status and return value.
Implementing a Custom std::codecvt Facet:
Although there are several pre-built std::codecvt facets for common encodings in the standard library, we may also develop custom facets for unique conversion requirements. Here's a summary of the actions needed:
Derive from std::codecvt:
Create a class that inherits from std::codecvt<InternT, ExternT, StateT>, specifying the internal, external, and state types for the conversion.
Override do_in:
Implement the do_in function to manage the conversion logic for a single external character. This feature ought to:
- Take the external character (from), the pointer to the buffers (to_next and from_end), and the current conversion state (to).
- Apply the appropriate conversion logic in accordance with the encoding schemes.
- Depending on the processed character, update the conversion status.
- In the event that the conversion is unsuccessful, return an error code or the amount of external characters used (typically 1).
Handle Multi-Byte Encodings:
Handle Multi-Byte Encodings: When converting between multi-byte encodings, such as UTF-8, several external characters may be necessary to represent a single internal character. Use the conversion state (to) to track the current byte location and handle incomplete character sequences correctly.
Error Handling: Implement error checking in do_in to identify erroneous sequences or unsupported characters in the external data. Apply the relevant error code to the result object returned by in.
Example:
Let us take an example to illustrate the std::codecvt function in C++.
// codecvt::in example
#include <iostream> // std::wcout, std::wcout
#include <locale> // std::locale, std::codecvt, std::use_facet
#include <string> // std::wstring
#include <cwchar> // std::mbstate_t
int main ()
{
typedef std::codecvt<wchar_t,char,std::mbstate_t> facet_type;
std::locale mylocale;
const facet_type& myfacet = std::use_facet<facet_type>(mylocale);
const char mystr[] = "Test string";
// prepare objects to be filled by codecvt::in :
wchar_t pwstr[sizeof(mystr)]; // the destination buffer (might be too short)
std::mbstate_t mystate = std::mbstate_t(); // the shift state object
const char* pc; // from_next
wchar_t* pwc; // to_next
// translate characters:
facet_type::result myresult = myfacet.in (mystate,
mystr, mystr+sizeof(mystr), pc,
pwstr, pwstr+sizeof(mystr), pwc);
if ( myresult == facet_type::ok )
{
std::wcout << L"Translation successful: ";
std::wcout << pwstr << std::endl;
}
return 0;
}
Output:
![std::codecvt::in and std::codecvt::do_in in C++](https://static.tutorialandexample.com/cpp/std-codecvt-in-and-std-codecvt-do-in-in-cpp1.png)