Writing a c-string directly into a std::string

Question

I want to get a log message from a C library. The library provides two functions :

int get_log_length(); // Returns the length of the string, including the terminating null character
void get_log(char *buffer); // Writes the log into buffer

I want to write that log into a std::string and return it. The normal way (I think) would be :

std::string get_log() {
    int length = get_log_length();
    char *buffer = new char[length];
    get_log(buffer);
    std::string str(buffer, length - 1);
    delete[] buffer;
    return str;
}

But I would like to skip using a buffer, so I thought of doing this :

std::string get_log() {
    std::string str;
    str.reserve(get_log_length());
    get_log(str.data()); // Bad: undefined behaviour
    return str;
}

But the size() of the std::string would remain unchanged, and would result in undefined behaviour (cf. cppreference).

Is there a way to only write the log once ?

AndyG · Accepted Answer · 2019-12-08 18:18:35Z

4

You'll want to resize the string instead of reserve:

std::string get_log() {
   std::string str;
   str.resize(get_log_length());
   get_log(str.data());
   return str;
}

This will update the string's underlying buffer to the appropriate size AND fill the string out with '\0' characters. Afterwards, those characters are overwritten by get_log

One thing to be wary of, though, is that get_log is likely going to try to write a \0 terminator to the string, which is not what you want in a std::string, so you may need to lop the last character off.

OR, the other way around; get_log_length() might return the size of the log message minus the terminating character (like strlen would), but get_log might try to write a terminating '\0' anyway, meaning that you'd need to size you string as get_log_length() + 1, and then afterwards str.pop_back to remove the terminator. Just double check this behavior.

edited Dec 8, 2019 at 18:18

answered Dec 8, 2019 at 17:46

AndyG

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7 Comments

HolyBlackCat Over a year ago

The easiest way to avoid the extra \0 is to do .resize(get_log_length()-1), rather than resizing the string later. Then get_log is going to overwrite the existing null-terminator instead of adding a new one.

AndyG Over a year ago

@HolyBlackCat yeah that's a possibility. There's always chance for these kinds of off-by-one errors when going from C strings to C++ std::strings

AndyG Over a year ago

@EuroMicelli: data() has a non-const overload in C++17

AndyG Over a year ago

@EuroMicelli: And it was admittedly a little late in coming; vector had a non-const data function since C++11. Writing to the buffer returned by the non-const overload is not UB

Remy Lebeau Over a year ago

@EuroMicelli yes, there is: &str[0] instead of str.data(). From C++11 onward, this is guaranteed to work ok. In practice, it will work in most implementations of earlier versions, too

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