C++ Type Conversion Operators

Using named casts is considered a safer and reliable method of type conversion in C++ compared to other methods. In C++, there are four main named type-casting expressions:

• static_cast
• dynamic_cast
• const_cast
• reinterpret_cast

C++ static_cast

We use static_cast for standard type conversions, such as converting from float to int.

Let's look at an example.

#include <iostream>
using namespace std;

int main() {
    float my_float = 3.14;
    
    // convert float to int
    int my_int = static_cast<int>(my_float);
    cout << "Float: " << my_float << " -> Int: " << my_int << endl;

    return 0;
}

Output

Float: 3.14 -> Int: 3

The static_cast<int>(my_float) expression converts the float value stored in my_float to an integer and stores it in my_int.

Because floating-point values can have decimal components and integers cannot, this conversion removes the decimal part, converting 3.14 to 3.

C++ dynamic_cast

We mainly use dynamic_cast for polymorphic type conversions, especially when dealing with inheritance hierarchies. It's typically used in scenarios where a base class pointer needs to be converted to a derived class pointer.

Let's look at an example.

#include <iostream>
using namespace std;

class Base {
public:
    // base class print function
    virtual void print() {
        cout << "Base class" << endl;
    }
};

class Derived : public Base {
public:
    // derived class print function overriding the base class
    void print() override {
        cout << "Derived class" << endl;
    }
};

int main() {
    // create a Base class pointer 
    // pointing to a Derived object
    Base *base_ptr = new Derived();
    
    // use dynamic_cast to cast the 
    // Base pointer to a Derived pointer
    Derived *derived_ptr = dynamic_cast<Derived*>(base_ptr);
    
    // call the print function through the derived pointer
    if (derived_ptr) {
        derived_ptr->print();
    }

    // delete the dynamically allocated object
    delete base_ptr;

    return 0;
}

Output

Derived class

In this example,

• We defined a base class Base and a derived class Derived, where Derived inherits from Base.
• In main(), we created a pointer base_ptr of type Base* pointing to a Derived object.
• We then used dynamic_cast to cast base_ptr to a Derived* pointer and assign it to derived_ptr.
• Finally, we called the print() function through derived_ptr, which means that we can access the Derived class function through the Base class pointer after the dynamic cast.

Note: If the base class pointer doesn't point to an object of the derived class, dynamic_cast returns nullptr.

C++ const_cast

We use const_cast to cast away the const qualifier from a variable.

One common scenario where we can use const_cast is when working with third-party libraries that have functions which take non-const pointers as arguments, but we need to pass in const data.

Let's look at an example.

#include <iostream>
using namespace std;

// function that takes a non-const pointer
void modify_data(int* data) {
    // modify the data
    *data *= 2;
}

int main() {
    int x = 10;
    
    // a const pointer for variable x
    const int* ptr = &x;

    // use const_cast to 
    // remove const qualifier and allow modification
    int* mutable_ptr = const_cast<int*>(ptr);

    // call the function
    modify_data(mutable_ptr);

    // value is modified successfully
    cout << "Modified value: " << x << endl;

    return 0;
}

Output

Modified value: 20

In this example, we used the const_cast expression to remove the const qualifier from a pointer to pass it through a function that takes non-const pointer as an argument.

Here,

• modify_data(int* data): is a function that takes non-const pointer as an argument
• ptr: is a const pointer that we need to pass to modify_data
• const_cast<int*>(ptr): casts ptr as a non-const pointer and assigns it to mutable_ptr

C++ reinterpret_cast

reinterpret_cast is used to convert one pointer type to another pointer type or one reference type to another reference type.

Unlike static_cast, reinterpret_cast doesn't actually convert the data types but reinterprets one pointer type as another at compile time.

Let's look at an example.

#include <iostream>
using namespace std;

int main() {
    // create an integer variable
    int x = 67;

    // pointer to an integer
    int* ptr_to_int = &x;

    // reinterpret the pointer to an integer 
    // as a pointer to char
    char* ptr_to_char = reinterpret_cast<char*>(ptr_to_int);

    // dereference the double pointer
    // originally holding an integer as if it contains a double
    cout << "Dereferencing ptr_to_char: " << *ptr_to_char << endl;

    return 0;
}

Output

Dereferencing ptr_to_char: C

Here, we used reinterpret_cast to reinterpret the pointer ptr_to_int, which originally pointed to an integer, as a pointer to a character.

This means that the memory location pointed by ptr_to_int still holds an integer value. But it will be treated as a character.