Question

I have a class that has a vector of another class objects as a member. In many functions of this class I have to do same operation on all the objects in the vector:

class Small {   public:     void foo();      void bar(int x);     // and many more functions };  class Big {   public:     void foo()     {         for (size_t i = 0; i <  VectorOfSmalls.size(); i++)             VectorOfSmalls[i]->foo();     }     void bar(int x)     {         for (size_t i = 0; i <  VectorOfSmalls.size(); i++)             VectorOfSmalls[i]->bar(x);     }     // and many more functions   private:     vector<Small*> VectorOfSmalls; }; 

I want to simplify the code, and find a way not to duplicate going other the vector in every function.

I’ve considered creating a function that receives a pointer to function, and calls the pointed function on every member of a vector. But I am not sure that using pointers to functions in C++ is a good idea.

I have also been thinking about functors and functionoids, but it will force me to create a class per each function and it sounds like an overkill.

Another possible solution is creating a function that receives a string, and calls the command according to the string:

void Big::call_command(const string & command) {     for (size_t i = 0; i <  VectorOfSmalls.size(); i++)     {        if (command == 'foo')            VectorOfSmalls[i]->foo();        else if (command == 'bar')            VectorOfSmalls[i]->bar();     } } void Big::foo() {     call_command('foo'); } 

But it might work slow (unneeded creation of a string instead of just a function call), and also creates a problem if functions have different signature.

So what would you recommend? Should I leave everything the same as it is now?

EDIT: I can use only STL and not boost (old compilers).

Answer 1

Well you can rewrite the for loops to use iterators and more of the STL like this:

void foo() {     std::for_each(VectorOfSmalls.begin(), VectorOfSmalls.end(), std::mem_fun(&Small::foo)); }  void bar() {     std::for_each(VectorOfSmalls.begin(), VectorOfSmalls.end(), std::mem_fun(&Small::bar)); } 

beyond that, you could use some macros to avoid retyping that a lot, but I’m not a huge fan of that. Personally, I like the multiple functions over the single one which takes a command string. As it gives you more versatility over how the decision is made.

If you do go with a single function taking a param to decide which to do, I would use an enum and a switch like this, it would be more efficient than strings and a cascading if. Also, in your example you have the if to decide which to do inside the loop. It is more efficient to check outside the loop and have redundant copies of the loop since ‘which command’ only needs to be decided once per call. (NOTE: you can make the command a template parameter if it is known at compile time, which it sounds like it is).

class Big { public:     enum Command {         DO_FOO,         DO_BAR     };  void doit(Command cmd) {     switch(cmd) {     case DO_FOO:         std::for_each(VectorOfSmalls.begin(), VectorOfSmalls.end(), std::mem_fun(&Small::foo));         break;     case DO_BAR:         std::for_each(VectorOfSmalls.begin(), VectorOfSmalls.end(), std::mem_fun(&Small::bar));         break;     } }; 

Also, as you mentioned, it is fairly trivial to replace the &Small::whatever, what a member function pointer and just pass that as a parameter. You can even make it a template too.

class Big { public:     template<void (Small::*fn)()>     void doit() {         std::for_each(VectorOfSmalls.begin(), VectorOfSmalls.end(), std::mem_fun(fn));     } }; 

Then you can do:

Big b; b.doit<&Small::foo>(); b.doit<&Small::bar>(); 

The nice thing about both this and the regular parameter methods is that Big doesn’t need to be altered if you change small to have more routines! I think this is the preferred method.

If you want to be able to handle a single parameter, you’ll need to add a bind2nd too, here’s a complete example:

#include <algorithm> #include <functional> #include <iostream> #include <vector>  class Small { public:     void foo() { std::cout << 'foo' << std::endl; }     void bar(int x) { std::cout << 'bar' << std::endl; } };   class Big { public:     template<void (Small::*fn)()>     void doit() {         std::for_each(VectorOfSmalls.begin(), VectorOfSmalls.end(), std::mem_fun(fn));     }      template<class T, void (Small::*fn)(T)>     void doit(T x) {         std::for_each(VectorOfSmalls.begin(), VectorOfSmalls.end(), std::bind2nd(std::mem_fun(fn), x));     } public:     std::vector<Small *> VectorOfSmalls; };  int main() {     Big b;     b.VectorOfSmalls.push_back(new Small);     b.VectorOfSmalls.push_back(new Small);      b.doit<&Small::foo>();     b.doit<int, &Small::bar>(5); }