use declaration to define a variable

A use declaration in C++ allows you to bring a specific symbol or identifier from a namespace into the current scope. This can be used to simplify the code and make it more readable by avoiding the need to fully qualify the symbol with its namespace every time it is used.

To define a variable using a use declaration in C++, follow these steps:

Include the necessary header file: Before using a symbol or identifier from a namespace, you need to include the header file that contains the namespace definition. This can be done using the #include directive.
Declare the use declaration: To bring a symbol from a namespace into the current scope, you need to declare a use declaration. This is done using the using keyword followed by the symbol or identifier you want to use, and then the namespace it belongs to. For example, if you want to use the std::cout symbol from the std namespace, you would declare a use declaration like this: using std::cout;.
Define the variable: Once the use declaration is in place, you can define the variable using the symbol or identifier you brought into scope. This is done by specifying the variable's type, followed by its name, an optional initializer, and a semicolon. For example, if you want to define an integer variable named myVariable, you can do it like this: int myVariable = 42;.

Here's an example that puts it all together:

#include <iostream> // Step 1: Include the necessary header file

using std::cout; // Step 2: Declare the use declaration

int main() {
    int myVariable = 42; // Step 3: Define the variable
    cout << "The value of myVariable is: " << myVariable << std::endl;
    return 0;
}

In this example, the std::cout symbol from the std namespace is brought into the current scope using a use declaration. Then, a variable named myVariable of type int is defined and assigned the value 42. The cout symbol is used to print the value of myVariable to the console.

Note that the use declaration only brings the specified symbol into the current scope, not the entire namespace. So, in this example, you can use cout without having to qualify it with std:: every time.