What is the effect of using the `const` keyword with a variable?

The const keyword in C++ is used to indicate that a variable's value is constant and cannot be modified after it has been initialized. This means that once a value is assigned to a variable declared with const, any attempt to change this value will result in a compilation error. This feature is useful for various programming scenarios, such as defining constants that should not change, helping to prevent accidental modification of values throughout a program, and increasing code readability by making the intent clear that certain variables are meant to remain unchanged.

Using const can also improve program safety and maintainability, as it allows programmers to enforce that certain values remain fixed for the duration of their scope. This plays a significant role in ensuring that the logic of the program is followed consistently, as any attempt to change a const variable will be caught by the compiler, highlighting potential logical errors early in the development process.

In contrast, the other options do not accurately represent the effects of using const. For example, a const variable cannot be modified after initialization (which is why the correct answer emphasizes this point), it does not dictate visibility within its scope or restrict usage to header files specifically.