Question

Ok, this may be really noobish question, but i am wishing there is something i dont know yet.

I go through a file, and check which string each line has, depending on the string value i execute a different function for it (or functions).

This is how i do it now:

Edit: I need to use variables outside the if-else-if range inside the if’s, updated code:

string s1 = "used";
string s2 = "in";
string s3 = "functions";

if(str == "something"){
    something = process(s1, s2);
}else if(str == "something else"){
    something = process(s2, s3);
}else if(str == "something more"){
    something = process(s1, s3);
    something = process(s1, s2);
}else if(str == "something again"){
    // do more stuff
}else if(str == "something different"){
    // do more stuff
}else if(str == "something really different"){
    // do more stuff
}

I am afraid this will become “slow” after i have to repeat those else if lines a lot…

I tried to use switch() statement, but obviously it doesnt work here, is there something similar to switch() to use here?

Answer 1

If you just want to execute different functions, you can use a map from strings to function pointers or functors like boost::function / tr1::function:

void f1() { /* ... */ }
void f2() { /* ... */ }

// ... creating the map:
typedef void (*FuncPtr)();
typedef std::map<std::string, FuncPtr> FuncMap;
FuncMap fnMap;
fnMap["something"] = &f1;
fnMap["something else"] = &f2;

// ... using the map:
FuncMap::const_iterator it = fnMap.find(str);
if (it != fnMap.end()) { // is there an entry for str?
    it->second(); // call the function
}

---

As for passing parameters, with the details given so far i’d probably go for passing the unparsed remainder of the line or a list of string tokens to the functions and let them handle those as they see fit:

void f1(const std::vector<std::string>& tokens) { /* ... */ }
// ...

typedef void (*FuncPtr)(const std::vector<std::string>&);
typedef std::map<std::string, FuncPtr> FuncMap;
// ...

std::vector<std::string> tokens = /* ...? */; 
FuncMap::const_iterator it = fnMap.find(str);
if (it != fnMap.end()) { 
    it->second(tokens); 
}

You could also take a look at the interpreter example from Boost.FunctionTypes (header, source file) wether that is similar to your scenario.