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I'm trying to make strftime accept a std::string.
I want to know if it's possible or not to do this conversion!
OBS I want to alter only strftime
In the example I did below, he accepts, but is returning an empty value
Code:#include <iostream>
#include <string>
int main() {
time_t Capture = time(0);
std::string Data_Hora;
strftime((char*)Data_Hora.c_str(), 20, "%d/%m/%Y %T", localtime(&Capture)); // 25/05/2020 12:51:00
std::cout << Data_Hora;
}
Do you realize that std::string.c_str(), returns a const C-string, which means that you can't modify that C-string?Quote:
In the example I did below, he accepts, but is returning an empty value
Code:#include "bits/stdc++.h"
using namespace std;
int main() {
time_t Capture = time(0);
char d [20];
strftime(d, 20, "%c", localtime(&Capture)); // 25/05/2020 12:51:00
std::string Data_Hora = d;
std::cout << Data_Hora;
return 0;
}
There are two problems with your code:
- The std::string is empty.
- You're calling c_str(), which returns a const char*, and then casting away the constness, which could be problematic if the original std::string was const to begin with.
Now, the latter is more of a theoretical problem, but the former is a practical problem because you immediately run into the issue of how to get the std::string to have the space to store the result of strftime. If you try and prefill it with 20 chars, great, but then strftime writes a null terminated string so the operative length would actually be less than 20, whereas the std::string will report its length as 20.
Consequently, I would suggest that just using a temporary array of char is simplest:
If you really want to directly use a std::string, then you would likely need to construct it with 20 null characters, pass &result[0] instead of c_str() to strftime, then erase the remaining null characters after calling strftime so that the length of the string and the length() member function will align.Code:std::string format_time(std::time_t time_value)
{
char result[20];
std::strftime(result, sizeof(result), "%d/%m/%Y %T", std::localtime(&time_value));
return std::string{result};
}
// ...
std::string Data_Hora{format_time(time(nullptr))};
std::cout << Data_Hora;
// ...
I didn't understand what you meant.Quote:
Originally Posted by jimblumberg
If I put inside strftime a variable char, the strftime passes to my variable the date/time formatted.
But I'm not getting this when I use std::string
And when I tried to convert in various ways being the only one that allowed to compile was from above, the variable is not getting any value.
That way I know how to do, what I wish is to know or to be able to convert the std::string into the strftimeQuote:
Originally Posted by Zeus_
Read laserlight's comment. The c_str() method in your usage is not the way it works or is supposed to work.
I'm trying to understand your explanationQuote:
Originally Posted by laserlight
So far I just understood that you inform me that I would be deleting the content that String received on strftime
And that I wouldn't have space in the string to receive the value.
How could I do this that you inform above? Without creating a char variable, just using std::string imagining that I understand that this is possible by your answer.Quote:
If you really want to directly use a std::string, then you would likely need to construct it with 20 null characters, pass &result[0] instead of c_str() to strftime, then erase the remaining null characters after calling strftime so that the length of the string and the length() member function will align.
Yes, I am trying to understand, and it seems that the answer is that it is not possible to use std::string, the way I would like. I just really want to be sure.Quote:
Originally Posted by Zeus_
What's your answer to that? Is it possible or not?
I know it's out of the focus of my question, but I'd like to understand from the example you gave above why you used bits/stdc++.hQuote:
Originally Posted by Zeus_
Notice that in my example I used
#include <iostream>
#include <string>
So I'd like to know if using as you did is the most ideal, most optimized, with fewer libraries.
Because it seems that the way I did it is wrong or unnecessary, or exaggerated.
Yes, like this:Quote:
Originally Posted by marcelo.br
But as you can see using a temporary char array is simpler.Code:std::string format_time(std::time_t time_value)
{
std::string result(20, '\0');
std::strftime(&result[0], result.size(), "%d/%m/%Y %T", std::localtime(&time_value));
result.erase(result.find_first_of('\0'));
return result;
}
(I should also add a caveat that this approach should only be used if you're compiling with respect to C++11 or later -- which should be the case in 2020 -- as that was when changes to std::string indicated that using &result[0] to directly access the internals of the string object should work... but even then it is still up for some debate and I don't remember the details. To be absolutely safe and compatible with C++03, you would use the old approach of a temporary std::vector<char>, but if you were going to do that, you might as well use a char array here.)
Zeus_ probably just copied the header inclusion from a test program and forgot to remove it. It is a non-standard convenience header.Quote:
Originally Posted by marcelo.br