§2024-12-02

¶namespace

In C++, a namespace is used to organize and manage code to avoid naming conflicts. A namespace allows you to group related classes, functions, variables, and other entities under a common name. It helps prevent name clashes, particularly when combining code from different libraries or modules that might use the same names for variables or functions.

For example, the standard C++ library functions and types are defined inside the std namespace, so to use them, you either need to explicitly specify std:: or use a using directive.

#include <iostream>

namespace myNamespace {
    int x = 10;
    void display() {
        std::cout << "Hello from myNamespace!" << std::endl;
    }
}

int main() {
    // Accessing the variable and function from the namespace
    std::cout << myNamespace::x << std::endl;  // Outputs: 10
    myNamespace::display();  // Outputs: Hello from myNamespace!
    return 0;
}

Libraries in C++ are collections of precompiled code that you can link to your program. A namespace is a way to organize the names of the entities in the code, which can come from any part of the program, including libraries.

In summary, a namespace in C++ is a mechanism for avoiding name collisions and organizing code, not a library.

In C++, the statement using namespace std; is a directive that tells the compiler to use the standard namespace (std) without requiring you to prefix every standard library component with std::. This helps avoid repeatedly writing std:: before common standard library types, functions, and objects.

For example:

#include <iostream>

int main() {
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

#include <iostream>
using namespace std;

int main() {
    cout << "Hello, World!" << endl;
    return 0;
}

In the second example, the using namespace std; line allows you to write cout and endl directly without the std:: prefix. Advantages:

- Less Typing: You don't need to type std:: every time you use something from the standard library (e.g., cout, cin, vector, etc.).
- Simpler Code: It makes the code cleaner and more concise, especially for smaller programs.

Disadvantages:

Name Conflicts: If you use multiple libraries that have the same names for functions or classes, using using namespace std; could lead to ambiguities or conflicts.

For example, if you include another library that also has a class or function called sort, you may run into issues because the compiler won't know whether you mean std::sort or the other sort.

Larger Projects: In larger projects or when writing code that will be part of a library, it's better to avoid using using namespace std; to prevent naming conflicts and maintain clarity.

¶Best Practices:

- In small, simple programs or examples, it's fine to use using namespace std; because the risk of conflicts is minimal.
- In larger, more complex programs, it's often recommended to avoid using namespace std; and instead explicitly use the std:: prefix to maintain clarity and avoid potential conflicts.

¶ C++ User Input

#include <iostream>
using namespace std;
int main() {
    int x; 
    cout << "Type a number: "; // Type a number and press enter
    cin >> x; // Get user input from the keyboard
    cout << "Your number is: " << x; // Display the input value 

    return 0;
}

¶Basic Data Types

The data type specifies the size and type of information the variable will store:

Data Type Size Description

int myNum = 1000;
double myNum = 19.99;
float f1 = 35e3;
double d1 = 12E4;
bool isCodingFun = true;

// Online C++ compiler to run C++ program online
#include <iostream>
#include <string>   // g++ might have implicit declaration
using namespace std;
int main() {
    bool isCodingFun = true;
    cout << isCodingFun << "\n" ; // 1
    int myNum = 1000;
    cout << myNum << "\n";
    double myDoubleNum = 19.99;
    cout << myDoubleNum << "\n";
    float f1 = 35e3;
    cout << f1 << "\n";
    double d1 = 12E4;
    cout << d1 << "\n";
    string greeting = "Hello"; 
    cout << greeting << "\n";
    return 0;
}

/* output 1 1000 19.99 35000 120000 Hello /*

¶User Input Strings

string firstName;
cout << "Type your first name: ";
cin >> firstName; // get user input from the keyboard
cout << "Your name is: " << firstName;

// Type your first name: John
// Your name is: John

However, cin considers a space (whitespace, tabs, etc) as a terminating character, which means that it can only store a single word (even if you type many words):

¶C-style strings are created with the char type instead of string.

#include <stdio.h>

int main() {
    // Declare a C-style string (character array)
    char str[] = "Hello, World!";
    
    // Print the string
    printf("%s\n", str);
    
    return 0;
}