September 9, 2023

Constants (const)

Posted on September 9, 2023  •  9 minutes  • 1715 words

In C++, the const keyword is used to declare a constant or to specify that a variable or function parameter should not be modified.

It can be applied to

  1. variables
  2. function parameters
  3. member functions
  4. member variables of classes

Here’s how const is used in various contexts:

1. Constant Variables:

Constant variables, often referred to as constants, are variables in C++ whose values cannot be changed after they have been initialized.

Constants are used when you want to define a value that should remain fixed throughout the program’s execution.

Here are some key points to understand about constant variables in C++:

1. Initialization:

Constants must be initialized at the time of declaration. Once a value is assigned to a constant, it cannot be changed later in the program.

// Declaration and initialization of a constant integer
const int constVariable = 10;

2. Data Types:

Constants can be of any data type supported in C++, including

  1. integers
  2. floating-point numbers
  3. characters
  4. user-defined types (objects and structures).
const double PI = 3.14159265;
const char GRADE = 'A';

3. Scope:

Constants have block or file scope, depending on where they are declared.

If declared at the global level (outside of any function), they have file scope and are accessible throughout the entire file.

If declared within a function, they have block scope and are accessible only within that function.

// File-scope constant
const int globalConst = 100; 

int main() {
    // Block-scope constant
    const int localConst = 42; 
    // ...
    return 0;
}

4. Value Substitution:

At compile time, the C++ compiler replaces references to constant variables with their actual values. This substitution helps optimize the code for performance.

2. Constant Pointers:

Constant pointers in C++ are pointers that point to a memory location, but they themselves cannot be used to modify the value at that location.

They are declared using the const keyword in combination with the pointer type.

Constant pointers are essential for creating const-correct code and ensuring that the data they point to remains unaltered.

There are two main types of constant pointers:

1. Pointer to a Constant (const pointer):

  1. A pointer that cannot modify the data it points to.
  2. Declared using const before the pointer’s data type.
// Pointer to a constant integer
const int* ptr;

Here, ptr is a pointer to an integer whose value cannot be changed through ptr. However, you can change ptr to point to a different integer.

int x = 42;
ptr = &x; // OK, ptr points to the address of 'x'
*ptr = 10; // Error, cannot modify 'x' through ptr

2. Constant Pointer to a Non-Constant (pointer to const):

  1. A pointer that can modify the data it points to but cannot change the address it points to.
  2. Declared using const after the pointer’s data type.
// Constant pointer to a non-constant integer
int const* ptr;

In this case, you can change the value of the integer that ptr points to, but you cannot change ptr to point to a different memory location.

int x = 42;
ptr = &x; // OK, ptr points to the address of 'x'
*ptr = 10; // OK, can modify 'x' through ptr
// ptr = &y; // Error, cannot change ptr to point to another address

3. Constant Pointer to a Constant (const pointer to const):

  1. A pointer that cannot modify either the data it points to or the address it points to.
  2. Declared with const both before and after the pointer’s data type.
// Constant pointer to a constant integer
const int* const ptr;

With this type of pointer, you cannot change either the value of the integer or the memory location it points to.

int x = 42;
ptr = &x; // Error, cannot change ptr to point to 'x'
*ptr = 10; // Error, cannot modify 'x' through ptr

Constant pointers are useful in situations where you want to ensure that the data being pointed to remains unaltered.

They are particularly valuable in function parameters, where you can specify whether the function will modify the data or not. For example:

void printValue(const int* valuePtr) {
    // This function cannot modify the value pointed to by valuePtr
    // It's a good practice when you want to pass data by reference but not allow modification
    // ...
}

Using constant pointers and const-correctness helps improve code safety and readability, as it clearly communicates your intentions regarding data modification.

3. Constant Variables Through References:

Constant variables through references in C++ refer to references that are declared as constants, meaning that you cannot use them to modify the data they refer to.

This concept is often used to pass data to functions by reference while ensuring that the function doesn’t modify the original data.

You declare a constant reference using the const keyword in front of the reference type.

Here’s how it works:

1. Declaring a Constant Reference:

You declare a constant reference by using the const keyword before the reference declaration. For example:

const int& constRef = someInteger;

In this case, constRef is a constant reference to an integer.

2. Cannot Modify Referenced Data:

A constant reference ensures that you cannot modify the data it refers to. Attempting to do so will result in a compilation error. For example:

const int& constRef = someInteger;
// constRef++; // Error: Cannot modify 'someInteger' through 'constRef'

3. Use in Function Parameters:

Constant references are often used in function parameters to indicate that the function won’t modify the original data.

This is a common practice to improve performance by avoiding unnecessary copying of data.

void printValue(const int& value) {
    // This function cannot modify 'value'
    // It's a good practice when you want to pass data by reference but not allow modification
    // ...
}

By passing the argument as a constant reference, you ensure that the function printValue won’t accidentally modify the value being passed.

4. Lifetime Extension:

When you create a constant reference to a temporary object, C++ extends the lifetime of the temporary to match that of the constant reference.

This is a safety feature to prevent accessing invalid memory.

const int& tempRef = 42; // Valid, temporary's lifetime is extended

5. Use Cases:

  1. Constant references are often used when passing large objects or containers to functions to avoid the overhead of copying the data.
  2. They are useful when you want to enforce that a function doesn’t modify its arguments.
  3. They can be used to iterate over and read elements of a container without modifying them.
void printElements(const std::vector<int>& vec) {
    for (const int& element : vec) {
        // Read-only access to elements of 'vec'
        // ...
    }
}

4. Constant Member Functions:

Constant member functions in C++ are functions declared within a class that are marked as const.

When a member function is declared as const, it signifies that the function will not modify the state of the object it is called on.

In other words, it promises not to modify any non-static member variables of the class. This concept is an important aspect of const-correctness in C++, which ensures that objects are not altered unintentionally.

Here are some key points about constant member functions:

1. Syntax:

You declare a constant member function by adding the const keyword at the end of the function declaration in the class definition.

For example:

class MyClass {
public:
    int getValue() const {
        return myVar;
    }
private:
    int myVar;
};

In this example, getValue() is a constant member function because of the const keyword.

2. No Modifications Allowed:

Within a constant member function, you cannot modify the non-static member variables of the class. Attempting to do so will result in a compilation error.

class MyClass {
public:
    int getValue() const {
        // myVar++; // Error: Cannot modify 'myVar' in a const member function
        return myVar;
    }
private:
    int myVar;
};

3. Overloading Non-Const and Const Member Functions:

You can overload a member function by providing both a non-const and a const version of it.

This allows you to have different behavior based on whether the object is const or non-const.

class MyClass {
public:
    int getValue() {
        // This is the non-const version
        return myVar;
    }

    int getValue() const {
        // This is the const version
        return myVar;
    }
private:
    int myVar;
};

5. Constant Member Variables:

Constant member variables in C++ are class data members that are declared as constants. Once they are initialized, their values cannot be modified throughout the lifetime of an object of that class.

Here are some key points about constant member variables in C++:

1. Declaration and Initialization:

Constant member variables are declared within a class just like regular member variables. However, they must be initialized at the time of declaration or in the class’s constructor initializer list.

class MyClass {
public:
    // Constant member variable with an initial value
    const int myConstant = 42; 
};

2. Cannot Be Modified:

Once a constant member variable is assigned a value, it cannot be changed, even within member functions of the class.

Any attempt to modify it will result in a compilation error.

class MyClass {
public:
    const int myConstant = 42;

    void modifyConstant() {
        // myConstant = 10; // Error: Cannot modify a constant member variable
    }
};

3. Initialization in Constructor:

If you need to initialize a constant member variable based on constructor parameters or more complex logic, you can use the member initializer list in the constructor:

class MyClass {
public:
    const int myConstant;

    MyClass(int value) : myConstant(value) {
        // Constructor initializer list sets the value of myConstant
    }
};

6. Constant Function Parameters:

Constant function parameters in C++ are parameters of functions (either member functions or standalone functions) that are marked as const.

When a parameter is declared as const, it indicates that the function will not modify the value of the parameter within its body.

This is a way to specify that the function is a read-only operation and will not change the state of the parameter passed to it.

void myFunction(const int x) {
    // 'x' is a constant parameter; it cannot be modified here
    // ...
}

Using const helps make your code more robust and self-documenting because it conveys your intentions regarding variable mutability to both the compiler and other developers.

It is an essential concept in C++ for creating const-correct and safe code.

Follow me

I work on everything coding and share developer memes