C++ -- how to initialize the static variable inside a member function?

Question

void ClassA::Func()
{
    static map<int, string> mapIntStr;

    mapIntStr[0] = m_memberVariable0; <= just want to do once & m_memberVariable* are not static
    mapIntStr[1] = m_memberVariable1; <= just want to do once
    ...
}

I want to ONLY initialize the variable mapIntStr once. As we know, we can put the initialize code inside a static function and call that static function and store the return value into mapIntStr.

Here I would like to get a quick & dirty solution. As I remember, there is something called static scope. I would like to initialize mapIntStr ONCE without calling a static function. how can I do it?

Thank you

Is there any specific reason for this not to be a static member variable? — oh whatever
– oh whatever, Commented Mar 23, 2011 at 3:50

Xeo · Accepted Answer · 2011-03-23 03:58:42Z

6

void ClassA::Func()
{
    static map<int, string> mapIntStr;

    if(mapIntStr.empty()){
      mapIntStr[0] = m_memberVariable0;
      mapIntStr[1] = m_memberVariable1;
      // ...
    }
}

How about that? :)
Edit
Well, the best solution would be to take the mapIntStr out of the function and into the class. And then you won't get around the static function call.

//in ClassA.h
class ClassA{
public:
  void Func();

  static map<int,string> InitStatic();
  static map<int,string> mapIntStr;
};

//in ClassA.cpp
#include "ClassA.h"
void ClassA::Func(){
  // use mapIntStr
}

map<int,string> ClassA::InitStatic(){
  map<int,string> ret;
  // init ret
  return ret;
}

map<int,string> ClassA::mapIntStr = ClassA::InitStatic();

That're about the only options that I know of. So you've got the choice. Initialize the map once in the func or even in a contructor the first time a ClassA object is created (better version of the two) and live with the overhead of a nearly noop call to empty(), which will most likely be a single instruction only after proper inlining and will impose no overhead whatsoever, or use a static initialization function.

edited Mar 23, 2011 at 3:58

answered Mar 23, 2011 at 3:51

Xeo

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4 Comments

q0987 Over a year ago

This is not the best solution b/c CPU will waste time to check the if condition -- thank you

Xeo Over a year ago

Well of course.. the best solution would be something else, brb, editing it in.

q0987 Over a year ago

Thank you very much for your proposal. As I mentioned in my question, I am looking for something different from calling a static function to initialize the mapIntStr. -- thank you

Bastien Léonard Over a year ago

@q0987: you don't seem to realize how small a few CPU cycles are.

Shuo · Accepted Answer · 2011-03-23 05:12:49Z

1

Create a inside struct, then initialize everything in the constructor of that struct, and then you declare a static variable of that struct.

void fun()
{
    struct setter
    {
         setter(){}
    };

    static setter setup;
}

For your case:

void ClassA::Func()
{
     struct Map : public std::map<int, string>
     {
          Map()
          {
              (*this)[0] = something;
              (*this)[1] = somethingElse;
          }
     }
     static Map map;
}

answered Mar 23, 2011 at 5:12

Shuo

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