Final answer:
The error message "Expression must be a modifiable lvalue" occurs in programming when you try to assign a value to something that cannot be altered, such as a constant. An lvalue refers to a memory location that can be modified, and the error suggests you are trying to change a non-modifiable lvalue.
Step-by-step explanation:
The error message "Expression must be a modifiable lvalue" typically occurs in the context of programming, particularly in languages like C, C++, and similar languages. An lvalue is an expression that refers to a memory location and allows us to take the address of that location with the & operator. The term modifiable means that the lvalue is allowed to be changed or assigned a new value. Errors involving non-modifiable lvalues generally arise when you attempt to assign a value to an expression that does not refer to a mutable memory location, such as a constant or a function returning a value by value rather than by reference.
For example, consider the following code:
const int x = 10;
x = 20; // Error: x is a non-modifiable lvalue
In this example, x is a constant and therefore cannot be modified, thus attempting to assign a new value to x will result in the aforementioned error message.