Solved

How to declare a global variable inside a function?

Posted on 2008-09-30
8
2,341 Views
Last Modified: 2013-11-17
How do I declare a global variable inside a function? (so that it can be used by other functions)
0
Comment
Question by:Chrysaor
8 Comments
 
LVL 31

Expert Comment

by:Zoppo
ID: 22605830
Hi Chrysaor,

that's not possible (at least not in C/C++). You have to declare that variable at the global (or a namespace) scope.

ZOPPO
0
 
LVL 10

Expert Comment

by:peetm
ID: 22605846
>>How do I declare a global variable inside a function? (so that it can be used by other functions)

Only by having the function return a reference or pointer to the variable.
0
 
LVL 14

Expert Comment

by:wayside
ID: 22605932
Do you really need it to be global, or just be able to use it in other functions?

You can't define a truly global variable a function, although by declaring it as static you can make it persist past the life of the function. Then you can return a pointer or reference to it as peetm suggested.

int *my_func() {

  static int foo;
  .
  .
  .

  return &foo;
}

Because foo is declared as static, it lives on after the function returns.
0
Live: Real-Time Solutions, Start Here

Receive instant 1:1 support from technology experts, using our real-time conversation and whiteboard interface. Your first 5 minutes are always free.

 
LVL 31

Expert Comment

by:Zoppo
ID: 22605982
You could even do something like this to restrict usage of such a global variable to a given set of functions, i.e.:

> class A
> {
>      static bool Flag; // declare the global flag
>      friend void foo(); // allow 'foo' to access the flag
>};

>bool A::Flag = true; // instantiate the flag

void foo()
{
      A::Flag = true; // OK
}

void bar()
{
      A::Flag = true; // Error: no access to private member
}

ZOPPO
0
 

Author Comment

by:Chrysaor
ID: 22606092
Basically, I have a function which it returns an int (let's say int number(); ) I have another one function, let's call it void body(); . When the body() is called, it creates a vector like this: vector<int> P(number()), and assign values to the vectors e.g P[0] =1; P[1] = 7; etc etc. Then , I want from another function to recall the values of e.g P[0],P[1] etc..

That's why I asked if it was possible to declare vector<int> P(number) a global variable. Any suggestions on how to solve my problem?
Thanks a lot, sorry for your time.
0
 
LVL 31

Accepted Solution

by:
Zoppo earned 500 total points
ID: 22606162
Well, you can simply return the vector from body(), i.e.:

> std::vector <int> body()
> {
>  std::vector <int> P( number() );
>  // initialize P ...
>  return P;
> }
> ...
> std::vector <int> P = body();

or pass it by reference

> void body( std::vector <int> &P )
> {
>  P.resize( number() );
>  // initialize P ...
> }
> ...
> std::vector <int> P;
> body( P );

ZOPPO
0
 
LVL 31

Expert Comment

by:Zoppo
ID: 22606175
Addition: the second possibility is more efficient since there's no need to create a copy of the vector ...
0
 
LVL 40

Expert Comment

by:evilrix
ID: 23246887
>> the second possibility is more efficient since there's no need to create a copy of the vector
Modern compilers support Named Return Value Optimization (NRVO), so 1 and 2 are likely to be optimal solutions.
http://msdn.microsoft.com/en-us/library/ms364057.aspx
0

Featured Post

Live: Real-Time Solutions, Start Here

Receive instant 1:1 support from technology experts, using our real-time conversation and whiteboard interface. Your first 5 minutes are always free.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
returning a dereferenced pts in C++ 10 149
Reference external folders in Visual Studio 4 81
VS2015 compilation and missing DLLs 9 147
PHPStorm debugging issues 1 44
What is C++ STL?: STL stands for Standard Template Library and is a part of standard C++ libraries. It contains many useful data structures (containers) and algorithms, which can spare you a lot of the time. Today we will look at the STL Vector. …
Go is an acronym of golang, is a programming language developed Google in 2007. Go is a new language that is mostly in the C family, with significant input from Pascal/Modula/Oberon family. Hence Go arisen as low-level language with fast compilation…
The viewer will learn how to use the return statement in functions in C++. The video will also teach the user how to pass data to a function and have the function return data back for further processing.
The viewer will learn how to user default arguments when defining functions. This method of defining functions will be contrasted with the non-default-argument of defining functions.

785 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question