A class declared inside a function becomes local to that function and is called Local Class in C++.
- A local class name can only be used locally i.e., inside the function and not outside it.
- The methods of a local class must be defined inside it only.
- A local class can have static functions but, not static data members.
For example, in the following program, Test is a local class in fun().
// C++ program without any compilation error // to demonstrate a Local Class #include <iostream> using namespace std;
// Creating the class void fun()
{ // local to fun
class Test {
// members of Test class
};
} // Driver Code int main() { return 0; }
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Following are some interesting facts about Local Classes in C++:
1) A local class type name can only be used in the enclosing function.
For example, in the following program, declarations of t and tp are valid in fun(), but invalid in main().
// A program without any compilation error to demonstrate // that a local class type name can only be used // in the enclosing function #include <iostream> using namespace std;
void fun()
{ // Local class
class Test {
// Body
};
Test t; // Fine
Test* tp; // Fine
} int main()
{ Test t; // Error
Test* tp; // Error
return 0;
} |
2) All the methods of Local classes must be defined inside the class only. For example, program 1 works fine and program 2 fails in the compilation.
Program 1:
// C++ program without any compilation error to demonstrate // that all the methods of Local classes must be defined // inside the class only #include <iostream> using namespace std;
void fun()
{ class Test // local to fun
{
public :
// Fine as the method is defined
// inside the local class
void method()
{
cout << "Local Class method() called" ;
}
};
Test t;
t.method();
} int main()
{ fun();
return 0;
} |
Local Class method() called
Program 2:
// C++ program with compilation error to demonstrate that // all the methods of Local classes must be defined inside // the class only #include <iostream> using namespace std;
void fun()
{ class Test // local to fun
{
public :
void method();
};
// Error as the method is defined outside the local
// class
void Test::method() { cout << "Local Class method()" ; }
} int main() { return 0; }
|
Output
Compiler Error: In function 'void fun()': error: a function-definition is not allowed here before '{' token
3) A Local class cannot contain static data members. It may contain static functions though. For example, program 1 fails in compilation, but program 2 works fine.
Program 1:
// A program with compilation error to demonstrate that // a Local class cannot contain static data members #include <iostream> using namespace std;
void fun()
{ class Test // local to fun
{
static int i;
};
} int main() { return 0; }
|
Output
Compiler Error: In function 'void fun()': error: local class 'class fun()::Test' shall not have static data member 'int fun()::Test::i'
Program 2:
// C++ program without any compilation error to demonstrate // that a Local class cannot contain static data members #include <iostream> using namespace std;
void fun()
{ class Test // local to fun
{
public :
static void method()
{
cout << "Local Class method() called" ;
}
};
Test::method();
} int main()
{ fun();
return 0;
} |
Local Class method() called
4) Member methods of the local class can only access static and enum variables of the enclosing function. Non-static variables of the enclosing function are not accessible inside local classes. For example, program 1 compiles and runs fine. But, program 2 fails in the compilation.
Program 1:
// C++ program without any compilation error to demonstrate // that member methods of local class can only access static // and enum variables of the enclosing function #include <iostream> using namespace std;
void fun()
{ static int x;
enum { i = 1, j = 2 };
// Local class
class Test {
public :
void method()
{
cout << "x = " << x
<< endl; // fine as x is static
cout << "i = " << i
<< endl; // fine as i is enum
}
};
Test t;
t.method();
} int main()
{ fun();
return 0;
} |
x = 0 i = 1
Program 2:
// C++ program with compilation error to demonstrate that // member methods of local class can only access static // and enum variables of the enclosing function #include <iostream> using namespace std;
void fun()
{ int x;
// Local class
class Test {
public :
void method() { cout << "x = " << x << endl; }
};
Test t;
t.method();
} int main()
{ fun();
return 0;
} |
Error:
prog.cpp: In member function ‘void fun()::Test::method()’:
prog.cpp:14:43: error: use of local variable with automatic storage from containing function
void method() { cout << “x = ” << x << endl; }
^
prog.cpp:9:9: note: ‘int x’ declared here
int x;
^
5) Local classes can access global types, variables, and functions. Also, local classes can access other local classes of the same function. For example, the following program works fine.
// C++ program without any compilation error to demonstrate // that Local classes can access global types, variables and // functions #include <iostream> using namespace std;
int x;
void fun()
{ // First Local class
class Test1 {
public :
Test1() { cout << "Test1::Test1()" << endl; }
};
// Second Local class
class Test2 {
// Fine: A local class can use other local classes
// of same function
Test1 t1;
public :
void method()
{
// Fine: Local class member methods can access
// global variables.
cout << "x = " << x << endl;
}
};
Test2 t;
t.method();
} int main()
{ fun();
return 0;
} |
Test1::Test1() x = 0
Must Read: Nested Classes in C++