Using Preprocessor DATE dynamically - c++

Question

I'm trying to use __DATE__ in C++Builder, and I need to use it dynamically. For example, if a user click on a button, the label containing __DATE__ will update following the system date.
I did that for now:
label1->Text = "Data: " __DATE__;
This is in a function to set the date, called DateTime(). For the button I already did the click event, but I need the DateTime() to run dynamically.

Answer 1

__DATE__ is a compile-time constant. It is the date on which the .cpp file is compiled. This is not what you want in this situation.
C++Builder's RTL has a Sysutils::Date() function that you can use instead, eg:
#include <System.SysUtils.hpp>
label1->Text = _T("Data: ") + Date().DateString();
Date() returns a TDateTime representing the current system date (if you want to include time as well, use Sysutils::Now() instead). Its DateString() method formats the TDateTime into a String using the user's locale settings. If you want to format the TDateTime yourself, you can use its FormatString() method for that, eg:
#include <System.SysUtils.hpp>
label1->Text = _T("Data: ") + Date().FormatString(_T("yyyy-mm-dd"));
or:
label1->Text = Date().FormatString(_T("'Data: 'yyyy-mm-dd"));
If you want a more standard C++ solution, look at the std::time() and std:::strftime() functions, eg:
#include <ctime>
std::time_t now_c = std::time(nullptr);
std::tm now_tm = *std::localtime(&now_c);
char buffer[11] = {};
std::strftime(buffer, sizeof(buffer), "%Y-%m-%d", &now_tm);
label1->Text = _T("Data: ") + String(buffer);
or:
label1->Text = String().sprintf(_T("Data: %s"), buffer);
Or, look at the <chrono> library introduced in C++11, such as std::chrono::system_clock, eg:
#include <chrono>
#include <ctime>
std::chrono::system_clock::time_point now = std::chrono::system_clock::now();
std::time_t now_c = std::chrono::system_clock::to_time_t(now);
std::tm now_tm = *std::localtime(&now_c);
char buffer[11] = {};
std::strftime(buffer, sizeof(buffer), "%Y-%m-%d", &now_tm);
label1->Text = _T("Data: ") + String(buffer);
or:
label1->Text = String().sprintf(_T("Data: %s"), buffer);
Alternatively:
#include <chrono>
#include <ctime>
#include <sstream>
#include <iomanip>
std::chrono::system_clock::time_point now = std::chrono::system_clock::now();
std::time_t now_c = std::chrono::system_clock::to_time_t(now);
std::tm now_tm = *std::localtime(&now_c);
std::wostringstream oss;
oss << L"Data: " << std::put_time(&now_tm, L"%Y-%m-%d");
label1->Text = oss.str().c_str();

Answer 2

The __DATE__ symbol is a pre-processor macro, hence it is only defined at compile time.
That means that the actual value of __DATE__ will not change during the execution of your program. It will be the time of your compilation. Forever the same value in the executable that you have just compiled.
If you need date string that reflects the current system clock/date, then you will need to use some function that queries the system for the current, as time(). Definitely not a compile-time-pre-processor macro string as __DATE__. See other ctime functions to assist you in formatting the desired string.
Finally, about building the string: you will need to compose strings at runtime much as sprintf does. Your construction "Data: " __DATE__ is just valid because you are again concatenating string as compilation time (it is the compiler pre-processor the one doing the concatenation, no your program). Here you can read about this capability of the C pre-processor. Also,here is described that this is a C99 standard behaviour, but cannot tell if it was not defined in an earlier C standard.