Is it safe to return a string object which points to a local char variable

Question

I believe that the following code is safe to return the string, even though inelegant, and works as expected, but I'd like confirmation of that, and I'd also like a test case to wrap around it that proves it is safe; ie that the contents of the string are available to the caller via .c_str() method, after the stack is re-used: ie, how can I force the stack on which the local variable resides to be freed/re-used (I'm using Visual Studio 2019 and PVS-Studio static code analysis, neither of which generates a warning). I don't like replacing the local szLocalTime with a global, nor do I like the idea of a malloc in the function as that places the onus on the caller to free. I've added some examples of cases that fill in the char[], eg C lib or Win32 functions that don't know about strings.

string FormatTimestamp(SYSTEMTIME st)
{
    char szLocalTime[1000];

    szLocalTime[0] = 0;

    // *** Some code that depends on a char buffer fills szLocalTime...
    // eg. strftime(szLocalTime, sizeof(szLocalTime)...);
    // eg. GetDlgItemText(hDlg, ID, szLocalTime, sizeof(szLocalTime));



    string s = szLocalTime; // <-- the question relates to this
    return s;
}

Anything named an iterator, view, range or ptr will need to keep the original source around. However, most standard containers make copies and don't need the original. — Zan Lynx, Sep 03 '19 at 15:37
Unrelated: Possibly `*** Some code that fills szLocalTime in Here ***` could take advantage of a `std::stringstream` or [Howard Hinnant's Date library](https://github.com/HowardHinnant/date) to simplify things. — user4581301, Sep 03 '19 at 15:41
Are you sure you meant your code like that? Maybe you were actually wondering about [returning a reference to a local var](https://stackoverflow.com/questions/4643713/c-returning-reference-to-local-variable)? Which in your case would mean changing the function signature to returning a string&. — nada, Sep 03 '19 at 15:42

score 4 · Answer 1 · answered Sep 03 '19 at 15:34

4

std::string always copies its initializer. So it is safe.

answered Sep 03 '19 at 15:34

Maxim Egorushkin

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Alecto Irene Perez · Answer 2 · 2019-09-03T16:08:20.927

-1

As stated by Maxim, std::string always makes a copy of whatever it's given, so this is safe to do. If you want to avoid copying, you could also just do this:

string FormatTimestamp(SYSTEMTIME st)
{
    std::string szLocalTime(1000, '\0'); 

    // String automatically initialized to 0
    // *** Some code that fills szLocalTime in Here ***

    szLocalTime.resize(strlen(szLocalTime.data())); 
    return szLocalTime; // <-- the question relates to this 
}

edited Sep 03 '19 at 16:08

answered Sep 03 '19 at 15:38

Alecto Irene Perez

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`std::string szLocalTime(1000);` won't compile. You need `std::string szLocalTime(1000, 0);` which means `szLocalTime[0] = 0;` is not needed. – NathanOliver Sep 03 '19 at 15:39
1

Moreover, with *move semantic* the string is not copied, just moved. – BiagioF Sep 03 '19 at 15:40
2

@BiagioFesta The copying of the characters is avoided by this solution. Move semantics are irrelevant. – Lightness Races in Orbit Sep 03 '19 at 15:51

Is it safe to return a string object which points to a local char variable

2 Answers2