Video Link: C++ Weekly - Ep 24 C++17’s Structured Bindings

Verification Case

结构化绑定是一个比较有意思的功能，从某个角度来看，使用{}可以将数据打包成tuple/array/class等，结构化绑定便是使用auto []将以打包的数据展开。举个最常见的例子[1]：

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int main() {
  std::set<std::string> myset;
  if (auto it = myset.insert("Hello"); it->second) 
    std::cout << "insert is successful. The value is " << std::quoted(*iter->first) << '\n';
  else
    std::cout << "The value " << std::quoted(*iter->first) << " already exists in the set\n";
}

使用结构化绑定具名iter、success，代码表述能力更强，也更为精炼。

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int main() {
  std::set<std::string> myset;
  if (auto [iter, success] = myset.insert("Hello"); success) 
    std::cout << "insert is successful. The value is " << std::quoted(*iter) << '\n';
  else
    std::cout << "The value " << std::quoted(*iter) << " already exists in the set\n";
}

结构化绑定array

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TEST(StructuredBindingDeclarationArray, Value) {
  int a[2] = {1, 2};
  auto [x, y] = a;
  ASSERT_EQ(x, 1);
  ASSERT_EQ(y, 2);
  ASSERT_NE(&x, a);
  ASSERT_NE(&y, std::next(a));
}

将数组中的每个元素具名绑定。上述为值绑定，引用绑定可以定义为auto& [x, y] = a;或万能引用auto&& [x, y] = a;，const引用绑定添加const修饰符即可，如const auto& [x, y] = a;。[前的所有修饰符便像是修饰[]中的每个元素一样，这也是通常性的思维。

结构化绑定struct/tuple/pair…

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std::tuple<float&, int, char&&> baseline()；
TEST_F(StructuredBindingDeclarationTuple, RValue) {
  auto [x, y, z] = this->baseline();
  ASSERT_TRUE((std::is_lvalue_reference_v<decltype(x)>));
  ASSERT_FALSE((std::is_rvalue_reference_v<decltype(x)>));
  ASSERT_FALSE((std::is_lvalue_reference_v<decltype(y)>));
  ASSERT_FALSE((std::is_rvalue_reference_v<decltype(y)>));
  ASSERT_FALSE((std::is_lvalue_reference_v<decltype(z)>));
  ASSERT_TRUE((std::is_rvalue_reference_v<decltype(z)>));
  ASSERT_EQ(&x, &tx);
  ASSERT_NE(&y, &ty);
  ASSERT_EQ(y, 2);
}

其中值类型绑定时，const以及引用特性与绑定array场景下完全一致。而引用类型(lvalue/rvalue)绑定时，const以及引用特性均无影响，即其类型在结构定义时确定，后续结构化绑定的所有修饰符均对其无效。如：std::tuple<float&, int, char&&>，无论是auto [x, y, z]、auto& [x, y, z]亦或是const auto& [x, y, z]，x的type始终为非const的左值引用，z的type始终为非const的右值引用，只有y会受影响(同结构化绑定array)。

注：完整的测试用例可以参考https://gitee.com/dreamonlysh/tony/blob/master/support/estd/unittest/structured_binding_declaration_test.cpp

参考资料

[1] Structured binding declaration. https://en.cppreference.com/w/cpp/language/structured_binding