How can I find the type of the value present in vector of type string? - c++

Question

I have a config file:
#X,Name,hostid,processid,executecommand,Max Runs, Max failure time
X,Main_XYZ_service,1,1,/opt/bada/bin,3,6,300
I parsed the above config file and stored each of the values in a vector of type string.
This is stored in vector<string> scanned:
//scanned[0]=X
//scanned[1]=Main_XYZ_service.........
long H_hostid = atoi(scanned[5].c_str());
how can I detect the type of the elements present in the vector?
If I call atoi() with a string that doesn't have a number, atoi() returns 0, but it also returns 0 if the string contains the number 0. How can I correctly assign a value to H_hostid?

Answer 1

In an absolute sense, you can't. If you encounter the string "0", you
can't know whether the user intended a string, an integer or a floating
point value. On the other hand, if you know what you need, you can try
to convert (say by using boost::lexical_cast), and generate an error
if it doesn't match. Alternatively, you can use regular expressions for
pattern matching, and decide what type you want as a result of what
pattern matches.
For a configuration file, I'd suggest the former. Just keep everything
as a string until you know what you need, then try the conversion (using
something reasonable for the conversion, which will report an error, and
not atoi).

Answer 2

Don't use atoi() - as you say, there is no way to detect errors. Use std::istringstream in C++, or strtol() in C.
long H_hostid;
std::istringstream stream(scanned[5]);
if (!(stream >> H_hostid)) {
// handle error
}
You could also use boost::lexical_cast, which does the same thing as that example, and throws an exception if the conversion fails.

Answer 3

If this is the data stored as a single string:
X,Main_XYZ_service,1,1,/opt/bada/bin,3,6,300
then the solution is, split this string using , as separator, and store each token in an array of size 8, then you can interpret each token based on the index as shown below:
char,string, int, int, string, int, int, int
0 1 2 3 4 5 6 7
Code would look like this:
std::string s = "X,Main_XYZ_service,1,1,/opt/bada/bin,3,6,300";
std::vector<std::string> tokens = split(s); //write the function yourself!
char c = tokens[0]; //token with index 0 is char
std::string service = tokens[1]; //token with index 1 is string
std::string path = tokens[4]; //token with index 4 is string
int int_indices[] = {2,3,5,6,7}; //rest of the index has int : total 5
for(int i = 0 ; i < 5 ; i++ )
{
try
{
int value = boost::lexical_cast<int>(tokens[int_indices[i]]);
//store value
}
catch(const bad_lexical_cast &)
{
std::cout << tokens[int_indices[i]] <<" is not an int" << std::endl;
}
}

Answer 4

Whenever you write a config file to be used by your application, you know in advance in what order the values will appear in that file. Otherwise, an xml or key value coding will be a better option to write a config file for general case. Personally, I would never create a config file as you have shown in your example.