C++ Tutorial/Language Basics/block scope variable
Содержание
- 1 global and block scope
- 2 global variables across functions
- 3 Inner block variable scope
- 4 Names in inner scopes can hide names in outer scopes.
- 5 scope code block
- 6 Using the scope resolution operator: "::"
- 7 Using the scope resolution operator (2)
- 8 Variable block scope
- 9 Variables can be local to a block
global and block scope
<source lang="cpp">#include <iostream.h> int n=0; //Global main() {
int n = 1;
{
int n = 2 ;
{
int n = 3;
cout << "In inner n=" <<n<<endl;
cout << "Global n=" << ::n <<endl;
}
cout << "In outter n=" <<n<<endl;
cout << "Global n=" <<::n<<endl;
}
cout << "In main() n=" << n<<endl;
return 0 ;
}</source>
In inner n=3 Global n=0 In outter n=2 Global n=0 In main() n=1
global variables across functions
<source lang="cpp">#include <iostream> using namespace std; int glob = 10; // global variable void access_global(); void hide_global(); void change_global(); int main() {
cout << "In main() glob is: " << glob << "\n\n"; access_global(); hide_global(); cout << "In main() glob is: " << glob << "\n\n"; change_global(); cout << "In main() glob is: " << glob << "\n\n"; return 0;
} void access_global() {
cout << "In access_global() glob is: " << glob << "\n\n";
} void hide_global() {
int glob = 0; // hide global variable glob cout << "In hide_global() glob is: " << glob << "\n\n";
} void change_global() {
glob = -10; // change global variable glob cout << "In change_global() glob is: " << glob << "\n\n";
}</source>
Inner block variable scope
<source lang="cpp">#include <iostream> using namespace std;
int main() {
int i;
int j;
i = 10;
j = 100;
if(j > 0) {
int i; // this i is separate from outer i
i = j / 2;
cout << "inner i: " << i << "\n";
}
cout << "outer i: " << i << "\n";
return 0;
}</source>
inner i: 50 outer i: 10
Names in inner scopes can hide names in outer scopes.
<source lang="cpp">#include <iostream>
- include <ostream>
int main() {
for (int i = 0; i < 10; ++i)
{
int x = 2;
if (x < i)
{
double x = 3.4;
std::cout << x;
}
std::cout << x;
}
//std::cout << x; // Error: no x declared in this scope
}</source>
2223.423.423.423.423.423.423.42
scope code block
<source lang="cpp">#include <iostream> using namespace std; void func(); int main() {
int var = 5;
cout << "In main() var is: " << var << "\n\n";
func();
cout << "Back in main() var is: " << var << "\n\n";
{
cout << "In main() in a new scope var is: " << var << "\n\n";
cout << "Creating new var in new scope.\n";
int var = 10;
cout << "In main() in a new scope var is: " << var << "\n\n";
}
cout << "At end of main() var is: " << var << "\n";
return 0;
} void func() {
int var = -5; // local variable in func() cout << "In func() var is: " << var << "\n\n";
}</source>
Using the scope resolution operator: "::"
<source lang="cpp">#include <iostream> using std::cout; using std::endl; int count1 = 100; int main() {
int count1 = 10;
int count3 = 50;
cout << endl << "Value of outer count1 = " << count1;
cout << endl << "Value of global count1 = " << ::count1;
{
int count1 = 20;
int count2 = 30;
cout << endl << "Value of inner count1 = " << count1;
cout << endl << "Value of global count1 = " << ::count1;
count3 += count2;
}
cout << endl
<< "Value of outer count1 = " << count1
<< endl
<< "Value of outer count3 = " << count3;
cout << endl;
return 0;
}</source>
Value of outer count1 = 10 Value of global count1 = 100 Value of inner count1 = 20 Value of global count1 = 100 Value of outer count1 = 10 Value of outer count3 = 80
Using the scope resolution operator (2)
<source lang="cpp">#include <iostream>
- include <ostream>
namespace n {
struct counter {
static int n;
};
double n = 2.8;
} int n::counter::n = 7; int main() {
int counter = 0; int n = 10; std::cout << n::counter::n; std::cout << n::n; std::cout << x.n; std::cout << n; std::cout << counter;
}</source>
Value of outer count1 = 10 Value of global count1 = 100 Value of inner count1 = 20 Value of global count1 = 100 Value of outer count1 = 10 Value of outer count3 = 80
Variable block scope
<source lang="cpp">#include <iostream> using std::cout; using std::endl; int main() {
int count1 = 10;
int count3 = 50;
cout << endl << "Value of outer count1 = " << count1;
{
int count1 = 20;
int count2 = 30;
cout << endl << "Value of inner count1 = " << count1;
count1 += 3;
count3 += count2;
}
cout << endl
<< "Value of outer count1 = " << count1
<< endl
<< "Value of outer count3 = " << count3;
cout << endl;
return 0;
}</source>
Value of outer count1 = 10 Value of inner count1 = 20 Value of outer count1 = 10 Value of outer count3 = 80
Variables can be local to a block
<source lang="cpp">#include <iostream> using namespace std;
int main() {
int x = 19; // x is known to all code.
if(x == 19) {
int y = 20;
cout << "x + y is " << x + y << "\n";
}
// y not known here.
return 0;
}</source>
x + y is 39
