Fill and return data from read in C++

Question

I would like to know the best way to make the following in c++, please. I have a Parser and a Gather classes; the gather consume the parser.

I have a method in the Parser class to parse my file with three parameters: process ID (int), path (string, char, char*...???) and string format (also I dont know which is best here - string, const string, etc...).

Its better to create a data[] in the Parse.method and return it, or to pass a reference to method, and keep the data variable in the gather class?

char* ProcInfoParser::parseStatm(const int _processPid, std::string _path, std::string _strFormat) { char path[32]; char* data[2042]; int tps = sysconf(_SC_CLK_TCK); int fd = open(path, O_RDONLY); if (fd < 0) { perror("open"); return "-1"; //wrong... } if (read(fd, data, 2048) == -1) { perror("read"); return "-1"; //wrong... } close(fd); char name[1024]; long unsigned int utime, virt; long int rss; sscanf(data, "%*d %s %*c %*d %*d %*d %*d %*d %*u %*u %*u %*u %*u %lu" " %*u %*d %*d %*d %*d %*d %*d %*u %lu %ld", name, &utime, &virt, &rss); //Iwish to use the _stringFormat variable here... return data; }

I really dont know which one to choose between char (char*...) or string; I also want to make a path with the process ID like string.Format("/proc/(0)/statm",processID); does the sprintf function work like that? Maybe I should take a look at boost...

Thanks in advance

paul23 · Accepted Answer · 2011-10-31 09:40:55Z

(also I dont know which is best here - string, const string, etc...).

With the "const" qualifier you specify the argument to be "constant": so you won't change the argument. When passing arguments by value, this serves little use: A temporary copy for the duration of the function is made anyways.

On the other hand, passing an argument by reference, it becomes important to consider the const-keyword correctly:

Is the argument small/fast to copy (pointers, numbers) - Use pass-by-value
Is the argument used as "output" (the function is expected to change the argument) - use pass-by-reference
Do you only look at the argument, not changin it - Use "const" & pass-by-reference
If you wish to change the argument, yet keep changes to the local scope - Use pass-by-value.

Also basically: std::string is better than character arrays, unless you have a specific reason for using a character array. Though that normally falls under the micro optimizations.

When returning a character array, you are responsible to make a garbage collection scheme, you have to declare the array on the heap & clean it externally.
In the code you posted you violated this, by returning a pointer-to-local variable.

I'd let the function signature look like, do yourself a favour and prevent the problems with pointers:

std::string ProcInfoParser::parseStatm(int _processPid, const std::string& _path, const std::string& _strFormat);

oh whatever · Accepted Answer · 2011-10-31 09:44:26Z

You have several problems here:

char* data[2042];

Is an array of pointers. I don't think that that's what you wanted, you wanted an array of chars, which would be:

char data[2042];

Also, you pass std::strings and return char*, which is not really consistent. If you return std::string - you won't have an issue. You can pass it around by value, and keep it in the class that owns it.

Last but not least - you're returning a pointer to a local variable, which doesn't exist when you exit the function, that's a recipe for trouble. Returning std::string will solve it too.

Bottom line - you're mixing C and C++ code here. Although C++ allows that, it is usually a common source for bugs and mistakes. Why not using C++ streams and strings?

Because I really dont know then exist right know... :( Im gonna take a look at then, thanks for your reply!

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