Setting a global variabel and reading it - c++

Question

I'm creating a international program, so I need multiple languages in it. On the first screen I have a button with a flag on it, when clicking that flag, it knows the language and what file he needs to read.
For example: if you click the dutch flag it will read a file with Dutch sentences. It will fill the vector with the sentences, later on I use the vector to set the labels/buttons etc to the right language.
The thing is, im using multiple forms and I want to tell my other forms, the language = DUTCH, set the text in the labels/buttons into Dutch. But I need a global variable to work with that.
So my question is: How can I set a global variable?
I need a string thats just saying: "Dutch" or "English".
And I need a global vector for the lines of the file.
MainScreen.h:
extern std::string Global_Language;
extern std::vector<std::string> Global_VectorLanguage;
MainScreen.cpp:
std::string Global_Language = "English"; // since it's by default English
void __fastcall TMainForm::btnLanguageDutchClick(TObject *Sender)
{
TMainForm::SetLanguage("Dutch", "Nederlands.txt");
}
void __fastcall TMainForm::btnLanguageEnglishClick(TObject *Sender)
{
TMainForm::SetLanguage("English", "English.txt");
}
void TMainForm::SetLanguage(string a_Language, string a_fileName)
{
string line;
ifstream myfile;
Global_VectorLanguage.resize(100);
int Index = 0;
myfile.open(a_fileName.c_str());
if (myfile.is_open())
{
while(getline(myfile, line))
{
Global_VectorLanguage[Index] = line;
Index++;
}
myfile.close();
Global_Language = a_Language;
MainForm->Caption = Global_VectorLanguage[0].c_str();
lblHPI->Caption = Global_VectorLanguage[1].c_str();
lblPhoneNo->Caption = Global_VectorLanguage[2].c_str();
lblEmail->Caption = Global_VectorLanguage[3].c_str();
btnStart->Caption = Global_VectorLanguage[4].c_str();
btnStop->Caption = Global_VectorLanguage[5].c_str();
btnResetEmergency->Caption = Global_VectorLanguage[6].c_str();
gboxLicenseInfo->Caption = Global_VectorLanguage[7].c_str();
lblName->Caption = Global_VectorLanguage[8].c_str();
lblVersion->Caption = Global_VectorLanguage[9].c_str();
}
}
ManualScreen.cpp:
__fastcall TManualForm::TManualForm(TComponent* Owner)
: TForm(Owner)
{
if(Global_Language == "Dutch")
{
//set labels/buttons to Dutch
}
}

Answer 1

(What you are caring about is internationalization & localization and your operating system might have support for that, see e.g. locale(7), setlocale(3), gettext(3) etc... on Linux and with C++11 <locale> standard header)
You might declare a few global variables as extern in some header file and defile it in one source file.
E.g. in your header.h file a declaration:
extern std::string my_global_lang;
and in your localization.cc file a definition:
std::string my_global_lang{"English"};
However you often should prefer declaring a static variable inside some class (so called a class variable).
class MyLocalization {
static std::string global_lang;
/// etc...
};
Then use MyLocalization::global_lang
Having many global variables or static member variables is considered poor style. Generally, a program should have few global variables (less than you have fingers on one hand). You might prefer packing state into explicit instances of some class, perhaps declaring some singleton class ProgramState; and have one instance of it created early in your main; the QApplication class of Qt could inspire you.
I strongly recommend enabling all warnings & debug info when compiling (e.g. g++ -Wall -Wextra -g if using GCC) and reading some good C++ programming book and sites. We cannot teach you the basics of C++ programming (which is difficult) here.

Answer 2

You handle global variables just like you do local variables. The difference is mainly where the variable is declared. Just try something like:
#include <string>
std::string language = "English";
int main(void)
{
...
if (/*user changes language to Dutch*/)
{
if (language == "English")
language = "Dutch";
else
language = "English";
}
...
}