From C to C++

const: Const is C++ version of final in Java. Const can be used at many places.

• const variable
• any variable be it global, local, or in class if declared const becomes un-modifiable
• const member function
• Method in a class declared as const cannot modify variable inside
• Declaration: void display() const; // remember const is after the brackets
• Definition: void classname::display() const { ... }
• const object
• Can't modify any of the data member except ones declared as mutable
• Can call only const methods as they are the only ones guaranteed not to modify object state

Use it often. Makes code more safer.

Example:

const char* name = "shreyas"; // string is constant pointer is not (read R to L)

name = "dhara";

*name = 'S' // Error

char const *location = "hyderabad"; // same as above

char * const weather = "hot"; // pointer is constant string is not

weather = "cool"; // Error

*weather = 'H';

const char * const weather = "hot"; // Both pointer and string it points to are constant

weather = "cool"; // Error

*weather = 'H'; // Error

Coonst and reference: If variable is declared const it's reference must also be declare const. Vice versa is not true

Example:

const int i = 10;

const int &ref = i;

int &err = i; // gives error

int j = 20;

const int &cref = j;

Note. if you return const string like "hello" your method declaration should be

const char* getname() {
return "hello";

}

mutable: Mutable let's you modify the member of const object. By definition const object is non-modifiable and you can only call const methods on them. By setting it mutable you can modify member variable of const object.

Example:

class Sample {

private:

mutable int num;

public:

// It is ironic that we have to declare method as const

// that actually modifies the variable

void setNum(int n) const {

num = n;

}

};

...

Sample s;

s.setNum(20);

namespace: It is similar to java's package.

Example:

namespace sample {

// Nested namespace is allowed

namespace nested {

}

class Base {

};

void display();

} // namespace sample

// Definition outside namespace

void sample::display() {

cout << "outside namespace";

}

// We can give alternate name to a namespace

namespace very_very_long_namespace {

class Trial {

};

}

namespace vvln = very_very_long_namespace;

int main() {

sample::Base base;

// Alternate way

using namespace sample;

Base base;

// Error cannot have namespace at global level

// namespce local {

// }

}