C++ Coding Standards: 101 Rules, Guidelines, and Best Practices

31. Don’t write code that depends on the order of evaluation of function arguments

Summary

Keep (evaluation) order: The order in which arguments of a function are evaluated is unspecified, so don’t rely on a specific ordering.

Discussion

In the early days of C, processor registers were a precious resource, and compilers were hard pressed to allocate them efficiently for complex expressions in high-level languages. To allow generation of faster code, the creators of C gave the register allocator an extra degree of freedom: When calling a function, the order of evaluation of its arguments was left unspecified. That motivation is arguably less strong with today’s processors, but the fact remains that the order of evaluation is unspecified in ...