Why an lvalue is considered an expression ?

[pieroborrelli]

I'm using the book Programming -- Principles and Practice Using C++ by Bjarne Stroustrup and I'm having some difficulty with the definition of expression. Here is an excerpt from the book:

The most basic building block of programs is an expression. An expression computes a value from a number of operands. The simplest expression is simply a literal value, such as 10, 'a', 3.14, or "Norah". Names of variables are also expressions.

Then the author begins to talk about about lvalues and rvalues. I can't understand why a variable name is considered an expression, because it has neither operands nor operators. What do lvalues and rvalues have to do with that? How can be the name of a variable be an expression in a statement like a = 10?

[Peter87]

It makes it easier to describe the parsing rules if you consider everything that gives you a value an expression.

Addition could be seen as two expressions with a + sign in between:
1. exp + exp

If a variable name was not an expression you would have to have a few extra rules:
1. exp + exp
2. variable + exp
3. exp + variable
4. variable + variable

And if you don't consider literals expressions you would have to have even more rules.

[pieroborrelli]

It makes it easier to describe the parsing rules if you consider everything that gives you a value an expression.

Addition could be seen as two expressions with a + sign in between:
1. exp + exp

If a variable name was not an expression you would have to have a few extra rules:
1. exp + exp
2. variable + exp
3. exp + variable
4. variable + variable

And if you don't consider literals expressions you would have to have even more rules.

@Peter87 why my book explain me the difference between lvalue and rvalue ?

[Peter87]

I'm no expert on lvalues and rvalues but as I understand it a lvalue is an expression that you can put on the left side of an assignment. rvalues can only be put on the right side of an assignment.

int a;
a = 1 + 2;

Here a is a lvalue because you can assign a value to it .
1 + 2 is a rvalue because you can't assign a value to it.

This will not work:

1 + 2 = a; // error! 

[Peter87]

An lvalue doesn't have to be just a variable name. It can be a function call returning a reference.

int& foo()
{
	static int bar = 0;
	return bar;
}

int main()
{
	foo() = 1; // sets the static variable bar in foo() to 1.
}

Another example is when assigning a value to an array element using the subscript operator or pointer syntax.

int main()
{
	int arr[] = {1, 2, 3};
	
	// Two ways of setting the third value of the array to 1:
	arr[2] = 1; // Using the subscript operator.
	*(arr + 2) = 1; // Using pointer syntax.
}

[pieroborrelli]

An lvalue doesn't have to be just a variable name. It can be a function call returning a reference.

@Peter87 why assigment is an expression if it doesn't compute any value ?

[Peter87]

An assigment returns a reference to the object that is being assigned to.

int a;
int b;
a = b = 1; // Assigns the value 1 to both a and b.
           // This is the same as writing: a = (b = 1);
           //                          or: b = 1; a = b;