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().
CPP
#include <iostream>
using namespace std;
void fun()
{
class Test {
};
}
int main() { return 0; }
|
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().
CPP
#include <iostream>
using namespace std;
void fun()
{
class Test {
};
Test t;
Test* tp;
}
int main()
{
Test t;
Test* tp;
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:
CPP
#include <iostream>
using namespace std;
void fun()
{
class Test
{
public :
void method()
{
cout << "Local Class method() called" ;
}
};
Test t;
t.method();
}
int main()
{
fun();
return 0;
}
|
Output
Local Class method() called
Program 2:
CPP
#include <iostream>
using namespace std;
void fun()
{
class Test
{
public :
void method();
};
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:
CPP
#include <iostream>
using namespace std;
void fun()
{
class Test
{
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:
CPP
#include <iostream>
using namespace std;
void fun()
{
class Test
{
public :
static void method()
{
cout << "Local Class method() called" ;
}
};
Test::method();
}
int main()
{
fun();
return 0;
}
|
Output
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:
CPP
#include <iostream>
using namespace std;
void fun()
{
static int x;
enum { i = 1, j = 2 };
class Test {
public :
void method()
{
cout << "x = " << x
<< endl;
cout << "i = " << i
<< endl;
}
};
Test t;
t.method();
}
int main()
{
fun();
return 0;
}
|
Program 2:
CPP
#include <iostream>
using namespace std;
void fun()
{
int x;
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.
CPP
#include <iostream>
using namespace std;
int x;
void fun()
{
class Test1 {
public :
Test1() { cout << "Test1::Test1()" << endl; }
};
class Test2 {
Test1 t1;
public :
void method()
{
cout << "x = " << x << endl;
}
};
Test2 t;
t.method();
}
int main()
{
fun();
return 0;
}
|
Output
Test1::Test1()
x = 0
Must Read: Nested Classes in C++
Last Updated :
16 Nov, 2021
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