c++, truncate a char array

Question

Hello, I'm working on a project where I have a Time class and I need to format the time.

void Time::FormatTime(char *string, unsigned int max_string_len) {
    ostrstream fd;
    ft << hour << ":" << minutes;
    cout << ft.str() << endl;    
}

The user passes in a pointer to their string and the max length of the string and I need to check that the time string isn't longer than max_string_len and if it is, then truncate it. I'm not sure how to do the truncation as it's been awhile since I've written any C++.

I'd like to not use the STL if possible.

Thanks

Answer 1

If the first parameter can be modified then you can just place a 0 in the correct position to be a string terminator, however this is a fairly nasty (but fast) solution. Otherwise, you'll want to create a new array of characters and only copy over the correct number of characters, remembering to allocate an extra character for the zero terminator.

Answer 2

This does what you want:

 void Time::FormatTime(char *string, unsigned int max_string_len) {
      ostrstream fd;
      ft << hour << ":" << minutes;
      std::string str = ft.str();
      str.resize(max_string_len);
      strcpy(string, str.c_str());
 }

EDIT: I've updated my code thanks to the helpful feedback of potatoswatter and quanmrana.

Answer 3

if( str.length() > max_string_len ) 
{ 
    str = str.substr( 0, max_string_len );
}

strncpy( string, str.c_str(), max_string_len );

Answer 4

From your requirements, you seem intent on writing C instead of C++, but I guess that's your business. For your specification, however, it appears that the right answer is strftime().

Answer 5

I'm half guessing at what you want, but I think it's something like the following, where I'm assuming or changing:

• ft really should be fd
• I used an ostringstream object instead of an ostrstream
• max_string_len is the size of the destination buffer in characters (including the character that will hold the '\0' terminator)
• you're not really interested in the formatted string being sent to std::cout, but want it in the caller's provided buffer
• I changed the name of parameter string to s so it wouldn't be confused with the std:string type.

The code:

void Time::FormatTime(char *s, unsigned int max_string_len) {
    if (max_string_len == 0) {
        return;  // no buffer, bail out
    }

    std::ostringstream fd;

    fd << hour << ":" << minutes;

    size_t len = fd.str().copy( s, max_string_len - 1);  // leave room for the null terminator

    s[len] = '\0';
}

If you really don't want any part of the STL, then the following C-styled code should do the trick:

void Time::FormatTime(char *s, unsigned int max_string_len) {

    snprintf( s, max_string_len, "%02u:%02u", hour, minutes);
}

It's simpler, but may C++ developers don't like using the printf() family because it's not typesafe.

Answer 6

Always use strncpy() to safely truncate a string into a char[N].

void Time::FormatTime(char *str, unsigned int max_string_len) {
    if ( len == 0 ) return;
    ostringstream ft; // strstream is obsolete, use stringstream
    ft << hour << ":" << minutes;
    strncpy( str, ft.str().c_str(), max_string_len );
    str[ max_string_len - 1 ] = 0;
}