for vs if vs while中的C ++ 17结构化绑定声明

Question

When I compile this code:

std::tuple<int, int> array[] = {std::make_tuple(1, 2), std::make_tuple(1, 2),
                                std::make_tuple(1, 2), std::make_tuple(1, 2)};
for (auto[a, b] : array) {
  printf("%u %u", a, b);
}

if (auto[a, b] = std::forward_as_tuple(1, 2); b != 0xff) {
  printf("%u %u", a, b);
}

while (auto[a, b] = std::make_tuple(1, 2); b != 0xff) {
  printf("%u %u", a, b);
}

With:

clang++ -std=c++1z

I get the following errors:

main2.cpp:76:14: error: decomposition declaration not permitted in this context
  while (auto[a, b] = std::make_tuple(1, 2); b != 0xff) {
             ^~~~~~
main2.cpp:76:46: error: use of undeclared identifier 'b'
  while (auto[a, b] = std::make_tuple(1, 2); b != 0xff) {
                                             ^
2 errors generated.

Why is auto[a, b] = std::forward_as_tuple(1, 2); b != 0xff auto[a, b] = std::forward_as_tuple(1, 2); b != 0xff supported in an if but not in a while ? Is there some technical reason or is it a "that's just the way it is" reason?

Answer 1

According to the latest draft standard for C++, the while loop does not in fact have an optional init-statement the kind that if and switch gained in C++17.

The formal syntax is:

while ( condition ) statement

In conclusion, the structured binding is not the issue here. Check this segment of the draft for reference.