How Does C Type Inference Affect the Lvalue/Rvalue Behavior of Conditional Expressions?

Type Inference in Conditional Expressions

The conditional operator (?:) is a powerful tool in C , allowing concise and expressive code. However, understanding the result type of this operator can be crucial for proper usage.

Consider the following code example:

int x = 1;
int y = 2;
(x > y ? x : y) = 100;

In this case, the first conditional operator evaluates to an rvalue of type int, as both operands are of type int. Hence, the assignment is valid.

However, in the following example:

int x = 1;
long y = 2;
(x > y ? x : y) = 100;

The conditional operator cannot be evaluated to an lvalue because the second and third operands are of different types. Consequently, an assignment to this expression is invalid:

error: lvalue required as left operand of assignment
      |     (x > y ? x : y) = 100;
      |     ~~~~~~~^~~~~~~~

To understand this behavior, we need to delve into the concept of value categories in C .

A conditional expression inherits its type and value category from its operands. If both operands are lvalues, the conditional expression is also an lvalue. If one of the operands is an rvalue, the conditional expression is an rvalue.

In the first example, both x and y are lvalues of the same type, so the conditional expression evaluates to an lvalue of type int. In the second example, x is an lvalue but y is an rvalue, so the conditional expression cannot be evaluated to an lvalue.

It's important to remember that the conditional operator's value category is determined at compile time. Even if the condition evaluates to false, the type and value category of the conditional expression must remain consistent. This is in accordance with the rule that the left-hand side of an assignment operator must be an lvalue.

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