Lambda returning decltype(auto) vrs auto

[JUANDENT]

Hi,

What is the difference between returning decltype(auto), auto and not specifyiung return type at all in a lambda?

I have this code:

template<typename First, typename... Args>
void printLambdaDecltype(const First& f, const Args& ...args)
{
	cout << f;
	(cout << ... << [](const auto& arg) -> decltype(auto)
	{
			cout << ' ';
			return arg;
	}(args)) << endl;

}

template<typename First, typename... Args>
void printLambdaAuto(const First& f, const Args& ...args)
{
	cout << f;
	(cout << ... << [](const auto& arg) -> auto
	{
			cout << ' ';
			return arg;
	}(args)) << endl;

}

template<typename First, typename... Args>
void printLambdaNoReturnSpecified(const First& f, const Args& ...args)
{
	cout << f;
	(cout << ... << [](const auto& arg)
	{
			cout << ' ';
			return arg;
	}(args)) << endl;

}

?
Regards,
Juan

[JUANDENT]

I think the decltype(auto) lambda returns const auto&;
the auto lambda returns just auto (no reference nor constant) - but a copy;
the not specified returns the same as auto (will return a copy of arg)

[helios]

You asked this already: http://www.cplusplus.com/forum/general/188645/

EDIT: In fact, this is the third time you ask this question: http://www.cplusplus.com/forum/general/186478/
Maybe try using the search function?

[seeplus]

auto never deduces to a reference. decltype(auto) can deduce to a reference. It returns a prvalue type (auto) if return value is of type rvalue and an lvalue type (auto &) if the return value is of type lvalue.

It's used with a templated function when the return is based upon the template type. Basically, value in, value out - ref in, ref out.

[JUANDENT]

yes I had asked something similar. In this case I am asking for a particular situation, to wit:

Is the following true:

I think the decltype(auto) lambda returns const auto&;
the auto lambda returns just auto (no reference nor constant) - but a copy;
the not specified returns the same as auto (will return a copy of arg)

?

[Cubbi]

the return types of those three lambdas are const T&, T, and T.

[JUANDENT]

thanks!!