Logging is an important feature of any serious program. In general, one should use established libraries for logging. However, for the sake of showcasing several new features from C++20, now available in Visual Studio 2019 16.10, I will show, in this post, how one can quickly write a simple logging function.<\/p>\n\n\n\n
What I am interested in logging is the following:<\/p>\n\n\n\n
To get this done, I’ll use the following C++ 20 featurse:<\/p>\n\n\n\n
The end result is should be looking like the following:<\/p>\n\n\n\n
[I] 2021-07-03 09:07:15.5349107 GMT+3 | cpp_test.cpp:main:51 | Logging from main!\n[E] 2021-07-03 09:07:15.5477301 GMT+3 | cpp_test.cpp:execute:46 | Error in execute!<\/pre>\n\n\n\nLet’s take all these features mentioned previously one at a time. We’ll start with the type of the message. For this, I’ll simply define a scoped enum as follows:<\/p>\n\n\n\n
enum class log_level : char\n{\n Info = 'I',\n Warning = 'W',\n Error = 'E'\n};<\/pre>\n\n\n\nNext, is the time when the event that’s being logged occurred. The current time can be retried with
std::chrono::system_clock::now()<\/code>. The value returned by this call is astd::chrono::system_clock::time_point<\/code>. However, this is a Coordinated Universal Time (UTC) value, and I specified that I want to display the local time. This can be done easily, with two C++20 features:<\/p>\n\n\n\n
- std::chrono::zoned_time<\/a>, a class that represents a pairing between a time point and a time zone, enabling us to display the time within a specified time zone<\/li>
- std::chrono::current_zone()<\/a>, which is a function that returns the current time zone (from the time zone database).<\/li><\/ul>\n\n\n\n
We can use these to convert from UTC to the local time zone as follows:<\/p>\n\n\n\n
auto as_local(std::chrono::system_clock::time_point const tp)\n{\n return std::chrono::zoned_time{ std::chrono::current_zone(), tp };\n}<\/pre>\n\n\n\nThe
zoned_time<\/code> value can be converted to a string using the new text formatting library and the std::format()<\/a> function.<\/p>\n\n\n\nstd::string to_string(auto tp)\n{\n return std::format(\"{:%F %T %Z}\", tp);\n}<\/pre>\n\n\n\nIn this snippet,
%F<\/code>,%T<\/code>, and%Z<\/code> are specifiers that indicate how the time point will be formatted. The full list of standard specifiers for chrono is available here<\/a>. For basic and string types, the standard specifiers are available here<\/a>.<\/p>\n\n\n\nNext on the list is the source code location. The C++20 class
std::source_location<\/code> contains information about the file name (thefile_name()<\/code> function), line (theline()<\/code> function), column (thecolumn()<\/code> function), and function name (thefunction_name()<\/code> function) . The staticcurrent()<\/code> function returns an object of this type initialized with information about the location of the call site.<\/p>\n\n\n\n