Can the Conditional Operator Assign to an Lvalue in C vs. C ?

The Nuances of the Conditional Operator in C vs C

The conditional operator, or ternary operator, denoted by the ?: syntax, plays a crucial role in both C and C . However, a subtle but significant difference exists between the two languages regarding how this operator treats lvalues.

Lvalue Behavior

In C, the conditional operator cannot assign a value to an lvalue (left-hand value), a variable or object that can be modified or assigned to. For instance, the following code is invalid in C:

<code class="c">(true ? a : b) = 1;</code>

Instead, one must resort to an if-else statement or use a pointer to assign the value:

<code class="c">*(true ? &a : &b) = 1;</code>

Precedence and Associativity

Another key difference concerns the precedence and associativity of the conditional operator relative to the assignment operator (=). In C , both the ?: and = operators have equal precedence and group right-to-left. This implies that the following code is valid:

<code class="cpp">(true ? a = 1 : b = 2);</code>

However, in C, the lack of equal precedence would require parentheses around the second expression:

<code class="c">(true ? a = 1 : (b = 2));</code>

Implications in Practice

This difference in behavior has implications in various scenarios, such as setting values of class members in constructors or initializing references to objects. C programmers must be aware of the lvalue restriction in C and employ alternative approaches when working with lvalues in conditional expressions.

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