Calling functions

Hi,

I'm trying to make a function wich calls other functions based on user input.
Example:
I type help and the program executes the function help.

Can this be done, if so how?

any help will be greatly apreciated

To some extent, yes, this can be done. One way is to map the "command" names to the corresponding function pointers, then after reading user input, search for the name, get the corresponding key, and then call the function. If the functions may have different signatures, you probably would need a hierarchy of function objects instead if you want to make use of std::map for the mapping.

You would have to write some code that "translates" the string help to a call to the function help. In the simplest form, that would look somewhat like this:

Code:

---

    char input[maxsize];
    // some code to read input from the user.
    if (strcmp(input, "help"))
    {
      help();
    }

---

There are other solutions that make it easier to write the code and expand the number of functions if there are many functions. But the principle is still a string comparison and a call to the relevant function.

It is IMPOSSIBLE to make C or C++ directly call a function from a string representing the name, because the name is only there in the source code. Once the compiler has processed the source-code, there is no way to know what the name of a function is.

I got sidetracked for a bit, hence the rather long time to reply.

--
Mats

thanks laserlight and matsp
I'll play with both and see what suits my purpose best.

I think i'll get a long if then else statement if I go with matsp though.

Quote:

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Originally Posted by Kreathyr

thanks laserlight and matsp
I'll play with both and see what suits my purpose best.

I think i'll get a long if then else statement if I go with matsp though.

---

Laserlights suggestion is a much more C++ solution, and I'd say that's the "proper" way to do it. Mine is the most basic (and actually not C++ at all - but it does describe the most simple solution possible). And I did say that there are better solutions if you have LOTS of functions to call.

--
Mats

Still, that was a C solution. The equivalent C++ solution would be:

Code:

---

    std::string input;
    // some code to read input from the user.
    if (input == "help") help();

---

No need to do it the hard way.
A sample of laserlight's solution:

Code:

---

// Create and initialize map
typedef void (fnc_ptr)();
typedef std::map<std::string, fnc_ptr*> CommandMap_t;
CommandMap_t CommandMap;
CommandMap["help"] = &help;

// Get input from user
std::string input;
// ...

// Find function and call it
CommandMap_t::iterator it = CommandMap.find(input);
if (it)
    it->second(); // Call function

---

Not tested for errors.

As I said in the previous post, my explanation was more to show the BASIC principle. It is indeed a good idea to use a map. However, the basic problem is still that SOMEHOW you need to compare a string with a constant string, and if it is a match, call the relevant function. Now, we can ACHIEVE that in many different ways.

--
Mats

Quote:

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Originally Posted by matsp

As I said in the previous post, my explanation was more to show the BASIC principle.

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Yes, I do agree with you, but you used chars and strcmp while you can use std::string and operator == instead (C++ version).

Quote:

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Originally Posted by Elysia

Yes, I do agree with you, but you used chars and strcmp while you can use std::string and operator == instead (C++ version).

---

My old C habits tend to stick out now and again, I do admit.

--
Mats

Hi again,

I tried figuring this map thing out myself, not a big succes...
Then I tried the example Elysia provided and get the following error when compiling.

Code:

---

B:\help>g++ help.cpp -o help.exe
help.cpp: In function `int main()':
help.cpp:23: error: could not convert `it' to `bool'

---

I don't know much about C++ yet, but line 23 says:

Code:

---

if (it)

---

thanks for all the help so far

That is because Elysia was just giving you a rough sketch, not working code. If you want a working example:

Code:

---

#include <iostream>
#include <map>
#include <string>

// The dummy functions to call.
void foo();
void bar();

typedef void (*Function)();
typedef std::map<std::string, Function> FunctionMap;

// Print the list of functions.
void listFunctions(std::ostream& out, const FunctionMap& function_map);

int main()
{
    using namespace std;
    // Create and populate the function map.
    FunctionMap function_map;
    function_map["foo"] = foo;
    function_map["bar"] = bar;

    cout << "The functions are:\n";
    listFunctions(cout, function_map);

    string command;
    cout << "Which function would you like to call?\n>";
    cin >> command;

    // Find function and call it if it exists.
    FunctionMap::iterator found = function_map.find(command);
    if (found != function_map.end())
    {
        (*found->second)();
    }
    else
    {
        cout << "Sorry, no function with that name exists." << endl;
    }
}

void foo()
{
    std::cout << "You have called foo()!" << std::endl;
}

void bar()
{
    std::cout << "You have called bar()!" << std::endl;
}

// Print the list of functions.
void listFunctions(std::ostream& out, const FunctionMap& function_map)
{
    for (FunctionMap::const_iterator iter = function_map.begin(), end = function_map.end();
        iter != end; ++iter)
    {
        out << iter->first << '\n';
    }
}

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I suggest that you read some material on how to use a std::map, e.g., cppreference.com's entry on map.

Thanks, this one works.

I'm currently reading trough the reference you mentioned.
Can't make much op from it, but maybe your example will help.

Again, thank you very very much.

Quote:

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Originally Posted by Elysia

Code:

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typedef void (fnc_ptr)();
typedef std::map<std::string, fnc_ptr*> CommandMap_t;

---

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I understand why you want a function typedef and then declare a pointer to it, instead of the more common function pointer typedef.
But then, don't call the function type 'fnc_ptr'; it's misleading.

Quote:

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Originally Posted by Kreathyr

Can this be done?

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Naah, it can't be done. Computers can't do much of anything, really. It's better to use them for target practice.

Quote:

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Originally Posted by CornedBee

I understand why you want a function typedef and then declare a pointer to it, instead of the more common function pointer typedef.
But then, don't call the function type 'fnc_ptr'; it's misleading.

---

Because I don't like to typedef pointers.
If I do it this way, I have to add the * to it, making it more obvious that it is indeed a pointer.
But you do have a point on the misleading name... I'll think of it naming it function or something.

Quote:

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Because I don't like to typedef pointers.
If I do it this way, I have to add the * to it, making it more obvious that it is indeed a pointer.

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As I said, I understand this part.

Right, so you did. My bad.