Don't understand static boolean behavior

Question

I have a header file that has some static variables for all of my files to use. I have a boolean variable in there initialized to 0 -

//in utility.h
static bool read_mess = false;

that I want to change to true if --view-read-messages is in the command line arguments so that I can do something like this when I get a message from a client -

//code from a different file
if(UTILITY_H::read_mess)
    std::cout<<"\nMessage successfully received from Client 2: "<<in2;

In main, I check for the command line argument and set the variable, read_mess, to true -

//this is in a for, where temp is the command line arg[i]
else if(strcmp(temp.c_str(), "--view-read-messages") == 0) {
    UTILITY_H::read_mess = true;
}

I can print the value of read_mess after this line in main and it says that it is true. But when I'm checking if its true in the if statement I posted above, read_mess is back to false. Why does this happen? I'm sure its just something simple, but I can't seem to make it work. Are all of the variables in utility.h reinitialized each time I do UTILITY_H::? And if so, why?

Also, I moved the variable to a main static, but that had the same problem. — Sterling, Sep 19 '11 at 18:30
This might be stupid, but just to clarify this - are you sure that you are NOT doing 'if(UTILITY_H::read_mess = false) {}'? — Desu_Never_Lies, Sep 19 '11 at 18:30
This creates a static variable in *every* .cpp file that includes the header. That's not what you want. Use the *extern* keyword in the header and define the variable in one of the .cpp files. — Hans Passant, Sep 19 '11 at 18:31

Flexo · Accepted Answer · 2011-09-19T18:37:26.990

7

static in this context means "local" (to the translation unit). There will be multiple copies of read_mess in your program, one per translation unit which is not the same thing as a header file. (In your case you can most likely approximate "translation unit" as .cpp or .c or .cc file).

Probably what you meant to do was to declare an extern variable, or static class member and define it in just one translation unit.

In practice using extern means in your header file you want to write:

extern bool read_mess;

But in one and only one other place that isn't a header:

bool read_mess = false;

edited Sep 19 '11 at 18:37

answered Sep 19 '11 at 18:31

Flexo

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Thanks a lot for the info. One question though - when I define the extern in a .cpp for a class, it works fine. But if I do that in main, it doesn't even compile (undefined references). Why is that? – Sterling Sep 19 '11 at 18:56
I think you have another problem too - `UTILITY_H::` is *very* odd. I would usually assume that `UTILITY_H` is a guard macro. If it is then by chance what you have will work, but I would strongly recommend consulting a good book or possibly asking another question. See http://sscce.org/ for advice on how to write a good question though, good questions usually post *minimal* complete code that demonstrates the problem in question and some text that explains what the unexpected element was too. – Flexo Sep 19 '11 at 18:59

score 1 · Answer 2 · edited May 23 '17 at 12:16

When you declare a static variable in a header file an copy of the staic variable gets created in each Translation unit(headers+source file) in which the file is included.

You are checking the value of the static variable which is a copy defined for that translation unit, it is not the same as the one you initialized in another translation unit.

You are better off using a extern if you want to access the variable across different files.

utility.h - Include this in all files where you want to access read_mess

extern bool read_mess;

File1.cpp - Define read_mess in one of the source files

#include"utility.h"

bool read_mess = false;

File2.cpp - Access read_mess in any of the source files

#include "utility.h"

if(read_mess)
{
    //do what interests you
}

Also,

C++03 standard: 7.3.1.1/2 says:

The use of the static keyword is deprecated when declaring objects in a namespace scope, the unnamed-namespace provides a superior alternative.

re your flag: There is nothing a mod can do about it. keep an eye out, if they don't revert, you'll have to email team @ StackOverflow. Only a dev can help you at that point. — , Sep 22 '11 at 17:22

score 1 · Answer 3 · answered Sep 19 '11 at 18:31

static global variables are privates to each .c or .cpp file (or translation unit). If you print out the address of read_mess (e.g., printf("%x", &read_mess);), you will see different addresses, which means two separate copies of the boolean variable exist.

A solution would be remove static keyword, or replace with extern. And, place the definition of that variable only once in any .c or .cpp file.

Don't understand static boolean behavior

3 Answers3