C++11 - Parameter pack question

[Sidney Marshall]

Hello,

The following code compiles and runs correctly but gives a warning:

----------------------------------------
#include<iostream>

using namespace std;

template<class T>
T sum(T value) {
return value;
}

template<class T, class ... U>
auto sum(T value, U ... values) {
return value + sum(values ...);
}

int main() {
cout << sum(1, 2, 4.6) << endl;
cout << sum(1, "abcde", 2) << endl;
}
--------------------------------------

$ g++ -std=c++11 pack.cpp
pack.cpp:11:31: warning: ‘sum’ function uses
‘auto’ type specifier without trailing return type [enabled by default]
auto sum(T value, U ... values) {

$ ./a.exe
7.6
de
-------------------------------------------

All of my attempts to use a trailing return type
failed (and I believe should have failed according to the standard).

My two questions:

1. What possible user mistake is the warning worried about?

2. How can I write my code with better style so that there is no warning?

Thank you.

--Sidney Marshall

[Marc Glisse]

Post by Sidney Marshall
----------------------------------------
#include<iostream>
using namespace std;
template<class T>
T sum(T value) {
return value;
}
template<class T, class ... U>
auto sum(T value, U ... values) {
return value + sum(values ...);
}
int main() {
cout << sum(1, 2, 4.6) << endl;
cout << sum(1, "abcde", 2) << endl;
}
--------------------------------------
$ g++ -std=c++11 pack.cpp
pack.cpp:11:31: warning: â??sumâ?? function uses â??autoâ?? type specifier
without trailing return type [enabled by default]
auto sum(T value, U ... values) {

The warning should probably specify that this is a C++14 feature. You
should be compiling with -std=c++1y.

[Sidney Marshall]

Post by Marc Glisse

Post by Sidney Marshall
----------------------------------------
#include<iostream>
using namespace std;
template<class T>
T sum(T value) {
return value;
}
template<class T, class ... U>
auto sum(T value, U ... values) {
return value + sum(values ...);
}
int main() {
cout << sum(1, 2, 4.6) << endl;
cout << sum(1, "abcde", 2) << endl;
}
--------------------------------------
$ g++ -std=c++11 pack.cpp
pack.cpp:11:31: warning: â??sumâ?? function
uses â??autoâ?? type specifier without trailing
return type [enabled by default]
auto sum(T value, U ... values) {

The warning should probably specify that this is
a C++14 feature. You should be compiling with -std=c++1y.

You are correct - I didn't read section 7.1.6.4
paragraphs 2 and 5 of the standard correctly. Thank you.

I guess I can't write this function in C++11 this way.

Post by Marc Glisse
--
Marc Glisse

--Sidney Marshall