C++ for Scientists - Technische Universität Dresden

2.2. VARIABLES 23
2.2.3 Scope of variables
Global definition: Every variable that we intend to use in a program must have been declared
with its type specifier at an earlier point in the code. A variable can be either of global or local
scope. A global variable is a variable that has been declared in the main body of the source
code, outside all functions. After declaration, global variables can be referred from anywhere in
the code, even inside functions. This sounds very handy because it is easily available but when
your software grows it becomes more difficult and painful to keep track of the global variables’
modifications. At some point, every code change bears the potential of triggering an avalanche
of errors. Just do not use global variables. Sooner or later you will regret this. Believe us.
Global constants like
const double pi= 3.14159265358979323846264338327950288419716939;
are fine because they cannot cause side effects.
Local definition: Opposed to it, a local variable is declared within the body of a function
or a block. Its visibility/availability is limited to the block enclosed in curly braces { } where
it is declared. More precisely, the scope of a variable is from its definition to the end of the
enclosing braces. Recalling the example of output streams
int main ()
{
std::ofstream myfile(”example.txt”);
myfile ≪ ”Writing this to a file. ” ≪ std::endl;
return 0;
}
the scope of myfile is the from its definition to the end of function main. If we would write:
int main ()
{
int a= 5;
{
std::ofstream myfile(”example.txt”);
myfile ≪ ”Writing this to a file. ” ≪ std::endl;
}
myfile ≪ ”a is ” ≪ a ≪ std::endl; // error
return 0;
}
then the second output is not valid because myfile is out of scope. The program would not
compile and the compiler would tell you something like “myfile is not defined in this
scope”.
Hiding: If variables with the same name exist in different scopes then only variable is visible
the others are hidden. A variables in an inner scope hides all variables in outer scopes. For
instance: 3
3 TODO: Picture would be nice.