How to pre-allocate empty class object which is then created within curly braces of if statement?

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How do I avoid the error of the object not being allocated in this scope when using it outside the if statement?

I have tried to declare an empty class but struggled to see how exactly.

class mvn {
public:
    int hello;
    mvn(int hello_) {
        hello = hello_;
    }
    ~mvn();
    int MyMethod() {
        return hello;
    }
}


int main() {
    bool firstrun = true;
    for(int i = 0; i < 100; i++)
        {

            if (firstrun == true)
            {
                mvn my_instance(100);
                firstrun = false;
            }


            // do more things with the instance which in reality is a wrapper
            // to random number generation and needs to be created on first run

        }


    return my_instance.MyMethod();
}

This would not compile saying that my_instance is declared out of scope, because it is local to the curly braces.

Is there a way to pre-allocate an empty instance so that it can be overwritten if firstrun is true but not thereafter, just so the compiler knows 'it's there'? My real implementation is much more complex than this of course, but this is a MWE.

EDIT: I added another element that i probably should not have abstracted away from. It's the fact that I simply cannot (or should not) create the instance more than once as each recreation is the re-seeding of a random number generator, which should be seeded/created from within (because this happens on multiple threads).

Now one thing I got was that everyone else here knows a lot more than I do. I am not sure which way is the best way to swing this and/or if the 'singleton pattern' is applicable here?

What you have is a static variable: "just so the compiler knows 'it's there'":

int myMethod()
{
    static mvn my_instance(100);
    return my_instance.MyMethod();
}

The second time, the compiler will reuse the existing variable.

Still, not a great design (see "singleton" pattern online).

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std::optional is a way to allow future initialization

std::optional<mvn> my_instance;

if (some_condition) {
   my_instance.emplace(100);
}

// do more things with the instance

if (my_instance) {
    my_instance->MyMethod();
}

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It may sound a bit old school, but it looks like a classic case of dynamic allocation to me, so a pointer would do the trick. Use std::unique_ptr<mvn> or some of its brethren.

edit:

Ok, to clarify this a bit:

#include<memory>

class mvn {
public:
    int hello;
    mvn(int hello_) {
        hello = hello_;
    }
    ~mvn() {};
    int MyMethod() {
        return hello;
    }
};


int main() {
    bool firstrun = true;
    std::unique_ptr<mvn> mvn_ptr(nullptr);

    for(int i = 0; i < 100; i++)
        {

            if (firstrun == true)
            {
                mvn_ptr.reset(new mvn(100));
                firstrun = false;
            }

        }

    return mvn_ptr->MyMethod();
}

This compiles and runs.

~/tmp $ g++ fun.cpp -std=c++11
~/tmp $ ./a.out 
~/tmp $ echo "$?"
100

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A much simpler approach is to simply make your calculation logic a member function of another class. Then you can make your object a member of that class, and it will be created there as a necessity for the first run.

struct mvn {
    int hello;
    mvn(int hello) : hello(hello) { }
    int MyMethod() { return hello; }
}

struct wrapper {
    mvn m;
    wrapper(int hello) : m(hello) { }
    int go() {
        for (int i = 0; i < 100; ...
        return m.MyMethod();
    }
}

int main() {
    wrapper w(42);
    return w.go();
}

That way you don't need to allocate anything dynamically, and you avoid the need for an if altogether.

If you have multiple go() functions or simply want to parametrize, write it as such instead:

template<typename F>
int wrapper::go(F&& f) {
    std::forward<F>(f)(m);
    return m.MyMethod();
}

Where F is any callable taking mvn&, e.g. [](mnv& m) { for (...) m.foo(); }.

And of course every call to go() will use the pre-existing m inside the wrapper object; you just need to share this object around. Calling it from different threads is another matter, and might require sufficient precautions to avoid breaking the state of m.

Code Conventions for the Java Programming Language: 7 , Sign-In · Create an Account The last revision to this document was made on April 20, 1999 The opening brace should be at the end of the line that begins the compound The if-else class of statements should have the following form: Copy An empty for statement (one in which all the work is done in the initialization,  That syntax requires MATLAB R2018a. In most situations there is no need to pre-allocate a table. Just construct it from efficiently created columns of data.

Move the if into a function that returns a mvn instance.

mvn make_mvn(bool firstrun) {
    if(firstrun == true)
    {
        mvn my_instance(100);
        firstrun = false;
    }
    return mvn(0); // substitute as necessary
}

int main() {
    bool firstrun = true;
    mvn my_instance = make_mvn(firstrun);
    // do more things with the instance
    return my_instance.MyMethod();
}

You can change make_mvn as necessary.

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Comments
  • @MatthieuBrucher how do you mean? SHould I declare anything as static first? Sorry I don't really understand.
  • While your compiler may not care about whitespace, it makes a big difference in readability. Please make sure code you share here is formated in a consistent way that makes it easy to read. This is not only a show of respect for your fellow users but will improve the odds that users will take the time to read your question making it more likely that you will get useful feedback and answers.
  • main can only be run once though.
  • @eerorika agreed, I suppose this function is probably not called main in OP original code.
  • From your example, you might directly have mvn my_instance(100); outside of the loop, in main scope.
  • thanks for your answer! But hang on a sec, would that not mean that a method declares its own instance? I will look up singleton pattern for sure.
  • That just means that this function will reuse an object for all the calls to it. Here, it's not a method, it's just a simple function.
  • @Hirek Beware that singleton is often regarded as an anti-pattern. It's often used in inappropriate ways as a quick fix that leads to trouble down the line. There exists legitimate uses for the pattern but it's a significantly overused pattern. I assume this is why Matthieu says "Still, not a great design (see "singleton" pattern online).". This design is essentially an implementation of the singleton pattern and is likely a design decision that merits being reconsidered.
  • Yes, exactly. It's a dangerous pattern.
  • Static variables, global variables and singletons are all equally bad.
  • thanks so much @Jarod42 just wondering does it work with arguments as well? Will read up on std::optional!
  • The issue is persistence of the object across calls, not optionality.
  • @Hirek: I use 100 as argument as you.
  • @MatthieuBrucher: persistence needed for OP was unclear, but with his last Edit (usage in a loop), optional might be used (even if it is unclear why OP uses local scope for initialization :/)