Lambda constructor shows uneccesary copies

[andreasfertig]

I was made aware of the following example by @fenbf:

#include <string>

int main() {
std::string str {"Hello World"};
auto foo = [str]() { };
}

C++ Insights currently shows this transformation:

#include <string>

int main()
{
  std::string str = std::basic_string<char, std::char_traits<char>, std::allocator<char> >{"Hello World", std::allocator<char>()};
    
  class __lambda_5_12
  {
    public: 
    inline /*constexpr */ void operator()() const { }
    
    private: 
    std::basic_string<char, std::char_traits<char>, std::allocator<char> > str;
    public: 
    __lambda_5_12(std::basic_string<char, std::char_traits<char>, std::allocator<char> > _str)
    : str{_str}
    {}
  };
  
  __lambda_5_12 foo = __lambda_5_12{std::basic_string<char, std::char_traits<char>, std::allocator<char> >(str)};
}

There the constructor of __lambda_5_12 takes the arguments by copy. However, the Standard requires direct-initialization.
This is an area where the compiler is mightier than what developers can express in C++. The best option is to taken the parameters by reference or const reference. Some cases where variables are moved into captures still create an additional move.

Andreas