Miscellaneous Topics

Miscellaneous Topics

This topic covers various important concepts in C++ programming, particularly focusing on arrays, pointers, memory allocation, and error handling. Understanding these concepts is essential for writing efficient and safe C++ programs.

Different Kinds of Arrays

In C++, arrays can hold primitive types, such as int, char, and byte. They can also hold objects, which allows for backward compatibility with C's arrays of structs. However, arrays of objects should only be used for uniform collections of identical structures. For collections of similar polymorphic objects, it is better to use containers like QList (from Qt) or vector (from the Standard Template Library or STL). These containers are dynamic arrays that manage memory allocation and deallocation automatically.

Example of Using Arrays

#include <iostream>
using namespace std;

int main() {
    int numbers[5] = {1, 2, 3, 4, 5};
    for (int i = 0; i < 5; i++) {
        cout << numbers[i] << " ";
    }
    return 0;
}

Valid Pointer Operations

Pointers are variables that store memory addresses. You can perform several operations with pointers:

Pointer Creation

A pointer can be created from:

• A constant pointer, such as an array name.
• An address obtained using the address-of operator &.
• A value from a dynamic memory allocation operator, such as new.

Pointer Assignment

You can assign one pointer to another if they are of the same type or a derived type. A void* pointer can be assigned to any type without casting. However, a non-void* pointer requires an explicit cast when assigned to a different type.

Pointer Arithmetic

You can increment or decrement pointers, or add/subtract integers. Pointer arithmetic is only valid within the bounds of the array, except for one position beyond the end of the array.

Example of Pointer Arithmetic

#include <iostream>
using namespace std;

int main() {
    int arr[3] = {10, 20, 30};
    int* p = arr;
    cout << *(p + 1) << " "; // Outputs 20
    return 0;
}

Pointer Comparison

Pointers can be compared using relational operators, such as == and !=.

Indirection and Indexing

You can access the value pointed to by a pointer using the indirection operator *. Additionally, you can use the array index operator with pointers, which the compiler interprets as *(p + i).

Example of Indirection

#include <iostream>
using namespace std;

int main() {
    int value = 42;
    int* ptr = &value;
    cout << *ptr << " "; // Outputs 42
    return 0;
}

Handling Memory Allocation Failures

new, it may fail if there is not enough memory available. The C++ standard specifies that new should throw a bad_alloc exception if it fails. You can handle this exception using a try-catch block.

Example of Handling Memory Allocation Failures

#include <iostream>
#include <new>
using namespace std;

void memoryEater() {
    double* ptr;
    try {
        ptr = new double[50000000];
    } catch (bad_alloc& e) {
        cerr << "Memory allocation failed: " << e.what() << endl;
    }
}

int main() {
    memoryEater();
    return 0;
}

Using set_new_handler()

You can also specify a custom function to be called when new fails by using set_new_handler(). This allows you to define what should happen in case of a memory allocation failure.

Example of set_new_handler()

#include <iostream>
#include <cstdlib>
#include <new>
using namespace std;

void out_of_store() {
    cerr << "Memory allocation failed. Exiting..." << endl;
    exit(1);
}

int main() {
    set_new_handler(out_of_store);
    double* ptr = new double[50000000];
    return 0;
}

Summary

• Arrays can hold primitive types and objects, but use containers for dynamic memory management.
• Pointers allow for various operations, including creation, assignment, arithmetic, and comparison.
• Handle memory allocation failures using exceptions and custom handlers.

Check your understanding

1. What are the differences between arrays and containers in C++?
2. Explain how pointer arithmetic works and provide an example.
3. What should you do when a memory allocation fails?
4. How can you use set_new_handler() in your programs?