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C++17 in Detail: A Deep Dive

Be Careful With Braces When Returning

Explore how returning values with braces in C++17 affects move semantics, causing copies instead of moves. Understand the implications for types like std::unique_ptr, std::vector, and std::string, and learn to manage optional types effectively.

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You might be surprised by the following code:

C++ 17
#include <iostream>
#include <optional>
#include <string>
using namespace std;
std::optional<std::string> CreateString()
{
  std::string str {"Hello Super Awesome Long String"};
  return {str}; // this one will cause a copy 
  // return str; // this one moves
}
int main(){
  std::optional<std::string> ostr = CreateString();
  cout << *ostr << endl;
}

According to the Standard if you wrap a return value into braces {} then you prevent move operations from happening. The returned object will be copied only.

This is similar to the case with non-copyable types:

C++
std::unique_ptr<int> foo() {
  std::unique_ptr<int> p;
  return {p}; // uses copy of unique_ptr and so it breaks...
  // return p; // this one moves, so it's fine with unique_ptr 
}
...