C/C++ Ternary Operator - Some Interesting Observations

Predict the output of following C++ program. One would expect the output will be same in both the print statements. However, the output will be, Why the second statement printing 49? Read on the ternary expression. Ternary Operator (C/C++): A ternary operator has the following form, exp₁ ? exp₂ : exp₃ The expression exp₁ will be evaluated always. Execution of exp₂ and exp₃ depends on the outcome of exp₁. If the outcome of exp₁ is non zero exp₂ will be evaluated, otherwise exp₃ will be evaluated. Side Effects: Any side effects of exp₁ will be evaluated and updated immediately before executing exp₂ or exp₃. In other words, there is sequence point after the evaluation of condition in the ternary expression. If either exp₂ or exp₃ have side effects, only one of them will be evaluated. Return Type: It is another interesting fact. The ternary operator has return type. The return type depends on exp₂, and convertibility of exp₃ into exp₂ as per usual\overloaded conversion rules. If they are not convertible, the compiler throws an error. See the examples below, The following program compiles without any error. The return type of ternary expression is expected to be float (as that of exp₂) and exp₃ (i.e. literal zero - int type) is implicitly convertible to float. The following program will not compile, because the compiler is unable to find return type of ternary expression or implicit conversion is unavailable between exp₂ (char array) and exp₃ (int). The following program *may* compile, or but fails at runtime. The return type of ternary expression is bounded to type (char *), yet the expression returns int, hence the program fails. Literally, the program tries to print string at 0th address at runtime.

We can observe that exp₂ is considered as output type and exp₃ will be converted into exp₂ at runtime. If the conversion is implicit the compiler inserts stubs for conversion. If the conversion is explicit the compiler throws an error. If any compiler misses to catch such error, the program may fail at runtime. Best Practice: It is the power of C++ type system that avoids such bugs. Make sure both the expressions exp₂ and exp₃ return same type or atleast safely convertible types. We can see other idioms like C++ convert union for safe conversion. We will be happy to learn and update from other geeks.