Lambda constructor shows uneccesary copies #347
Description
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