Question

I want to trim a string in C++ with this code:

std::string str("  Trim test   ");
str.erase(                                 /* 1 */
          0,                               /* 2 */
          str.find_first_not_of(" ")       /* 3 */
         )                                 /* 4 */
   .erase(                                 /* 5 */
          str.find_last_not_of(" ") + 1,   /* 6 */
          std::string::npos                /* 7 */
         );                                /* 8 */

Does the standard allow line #6 to be calculated before line #1 is executed, so that when #5 is finally called, the argument might no longer be valid?

Answer 1

Yes, the standard allow line #6 to be calculated before line #1 is executed.

From the Wikipedia page on sequence points:

[Sequence points occur] Before a function is entered in a function call. The order in which the arguments are evaluated is not specified, but this sequence point means that all of their side effects are complete before the function is entered. In the expression f(i++) + g(j++) + h(k++), f is called with a parameter of the original value of i, but i is incremented before entering the body of f. Similarly, j and k are updated before entering g and h respectively. However, it is not specified in which order f(), g(), h() are executed, nor in which order i, j, k are incremented. Variables j and k in the body of f may or may not have been already incremented. Note that a function call f(a,b,c) is not a use of the comma operator and the order of evaluation for a, b, and c is unspecified.

You should rewrite it as

str.erase(0, str.find_first_not_of(" "));           

str.erase(str.find_last_not_of(" ") + 1, std::string::npos);