C++ Constructor for first instance only?

Question

So I have a class with static variables, the reason they are static is for (while it may seem insignificant) efficiency (only being required to load once, reduce redundancy of storage in memory of the same file).

Anyway what I'd like to know, is there a way to check if a variable has been loaded?

Or is there a way to have a specific constructor called the first time an instance of this class is created and another used while other instances exist?

If neither of these are appropriate what is the solution?

Static member variables are constructed and initialized before your `main` function is called. If `main` has been called, then the static member variables have been "loaded". — Some programmer dude, Sep 26 '15 at 13:34
There is a SingleTon Design Pattern for Efficient Code Design,please go through this:: https://sourcemaking.com/design_patterns/singleton/cpp/1 — Nishant, Sep 26 '15 at 13:46
@Nishant_b9: ...and then discard it as a terrible idea and forget you ever read about it. — Lightness Races in Orbit, Sep 26 '15 at 14:00
Thanks for the responses, @Joachim Pileborg I didn't know static member variables had to be set before `main` was called, I was initalizing them in my .cpp without values. @Nishant I know the singelton pattern but I didn't feel it was applicable here as I still wanted the resources to be alterable between different instances if required. — CS Dude, Sep 26 '15 at 21:47

score 3 · Accepted Answer · edited May 23 '17 at 12:29

If your static members are private, and initialized in the same translation unit as all of your class's member functions, then the standard guarantees that the static members will be initialized before they are used. See: When are static C++ class members initialized?

There are other situations where this guarantee does not help you (e.g. accessing non-private static members from another translation unit, or from inline member functions).

You can play games with isInitialized flags, but be aware that without further work this is not thread-safe.

The C++ FAQ recommends to wrap static class instances in functions, this ensures that they are initialized on first use. e.g.:

Fred& x()
{
    static Fred* ans = new Fred();
    return *ans;
}

Source: https://isocpp.org/wiki/faq/ctors#static-init-order-on-first-use

score 0 · Answer 2 · answered Sep 26 '15 at 13:38

0

Here you go:

class Test {
  static bool isInitialized;
public:
  Test() {
    if (!isInitialized) {
      // do whatever you need here
      // ...
      isInitialized = true;
    }
  }
};

bool Test::isInitialized = false;

answered Sep 26 '15 at 13:38

John Zwinck

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@MarcusMüller: come again? – John Zwinck Sep 26 '15 at 13:39
John, sorry about that :) I was just expecting to see that pattern (checking for a static set in the class decl); I also misread your code, so I'm going to retract my vote. – Marcus Müller Sep 26 '15 at 13:40

score 0 · Answer 3 · answered Sep 26 '15 at 13:48

0

You may do something like:

struct StaticData
{
  // some variables
};

class YourClass
{
public:
    YourClass(/*..*/) {
        if (staticData == nullptr) {
            staticData = std::make_unique<StaticData>(/*..*/)
        }
    }
private:
    static std::unique_ptr<StaticData> staticData;
};

static std::unique_ptr<StaticData> YourClass::staticData;

answered Sep 26 '15 at 13:48

Jarod42

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Or just instantiate it normally without any dynamic allocation complicating your code. – Lightness Races in Orbit Sep 26 '15 at 14:00
@LightnessRacesinOrbit: I suspect than OP initializes it with some provided arguments. But else you're right. – Jarod42 Sep 26 '15 at 14:04

C++ Constructor for first instance only?

3 Answers3