To allow for chaining. Consider a statement like: a.f(foo(1)).g(foo(2)).arr[foo(3)].h(foo(4)); In C++14, the foo() calls happened in unspecified order. C+

To allow for chaining. Consider a statement like: a.f(foo(1)).g(foo(2)).arr[foo(3)].h(foo(4)); In C++14, the foo() calls happened in unspecified order. C++17 guarantees that "a(b)" and "a[b]" evaluate "a" first, which effectively means the foo() calls will now happen in order (1 to 4).