Why Does Post-Increment in `std::cout` Lead to Undefined Behavior?

Post-Increment's Behavior in std::cout

The provided C code snippet showcases the intricate behavior of post-increment when used with the std::cout output operator. The code's execution yields unexpected results due to undefined behavior.

When post-increment is applied to a variable within a std::cout expression, the evaluation and side effects of the argument computations are unsequenced, meaning they are not performed in any specific order. In this particular case, both the post-increment and post-decrement operators are applied to the same variable, resulting in undefined behavior.

According to the C standard, when there is an unsequenced side effect on a scalar object relative to another side effect or value computation involving the same object, the behavior is undefined. This means that the compiler and runtime have complete freedom in handling such situations, with a wide range of possible outcomes, including terminating the execution or producing unpredictable results.

Therefore, relying on the output of code with undefined behavior is dangerous and should be avoided. The compiler should flag any code that exhibits undefined behavior as an error, and programmers should adhere to standard practices and only use well-defined operations to ensure reliable code execution.

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