auto

Type Deduction

C++ auto 的 type deduction 規則和 template type deduction ㄧ樣，應該說，完全就是一種 mapping，除了一個例外：

the only real difference between auto and template type deduction is that auto assumes that a braced initializer represents a std::initializer_list, but template type deduction doesn’t.

當牽涉到 std::initializer_list 時：

int main() {
  auto x = {1, 2, 3}; // error: cannot deduce type of initializer list because std::initializer_list was not found; include <initializer_list>
  return 0;
}

#include <initializer_list>
int main() {
  auto x = {1, 2, 3}; // success!
  return 0;
}

The compiler sees it ({1, 2, 3}) as std::initializer_list<T>, and then try to deduce T, in this case, a int:

#include <initializer_list>
 
int main()
{
  std::initializer_list<int> x = std::initializer_list<int>{1, 2, 3};
  return 0;
}
 

However, the following case failed, because the compiler fails to deduce type T:

#include <initializer_list>
 
int main() {
  auto x = {1, 2, 3.23}; // error: deduced conflicting types ('int' vs 'double') for initializer list element type
  return 0;
}

另外一個要注意的點是：

auto in a function return type or a lambda parameter implies template type deduction, not auto type deduction.

所以

#include <iostream>
#include <vector>
#include <initializer_list>
 
auto func2() {
  return {1, 2, 3};
};
 
int main()
{	
  std::vector<int> v{};
  auto func = [&v](const auto& V){v = V;};
  func({1,2,3}); // error: no matching function for call to object of type lambda
  
  auto x = func2(); // error: cannot deduce return type from initializer list
}