postincrement operation in same statement

Question

the question is to understand how the standards define or allow to handle these situations and what would be behaviour in this particular case wherein the variable undergoing post/pre increment is used in same statement as that of expression, when it is being used as argument to function call.

take for example following sample code

char a[SZ];

which of the following would be correct?

strlcpy(&a[i++],"Text",SZ-i-1);

strlcpy(&a[i++],"Text",SZ-i);

if the

"," comma

would used for computation of i++ or

";" semicolon

??

Answer 1

In this case, since the "comma separated expressions" are parameters of a function (strlcpy), the order of evaluation of the expressions is unspecified, even in C++17. However, C++17 guarantees that expression evaluation won't be interleaved between arguments, so that each expression is fully formed before forming another one. So, in your strlcpy(&a[i++],"Text",SZ-i), you cannot rely on the value of i: it could exhibit a different behavior depending on your implementation. Though since it's not undefined behavior, you know it's either going to be the old value of i, or the old value plus one.