Local Classes in C++

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

// 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; }

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

// 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:

CPP

// 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;
}

Output

Local Class method() called

Program 2:

CPP

// 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:

CPP

// 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:

CPP

// 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;
}

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

// 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;
}

Output

x = 0
i = 1

Program 2:

CPP

// 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.

CPP

// 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;
}

Output

Test1::Test1()
x = 0