I discovered (with g++ 6.3) that I can use the [[deprecated]] attribute to deprecate a typedef ([[deprecated]] typedef longname shortname;) but not a using declaration ([[deprecated]] shortname = longname;).
So I was wondering, is this intended (if yes, why?)? Or is this a bug?
This as a MWE:
#include <vector>
namespace our {
using vector = std::vector<float>;
}
//[[deprecated("use the namespace 'our'")]] using myvector = std::vector<float>;
[[deprecated("use the namespace 'our'")]] typedef std::vector<float> myvector;
int main() {
myvector a;
return a.size();
}
gives depending on which of the lines is commented in the following warning (intended):
g++ -march=native -std=c++11 -m64 -O2 -g -Wextra -Wall -Wshadow -lstdc++ -m64 -g -march=native -flto main.cpp -o main
main.cpp: In function ‘int main()’:
main.cpp:11:12: warning: ‘myvector’ is deprecated: use the namespace 'our' [-Wdeprecated-declarations]
myvector a;
^
main.cpp:8:70: note: declared here
[[deprecated("use the namespace 'our'")]] typedef std::vector<float> myvector;
^~~~~~~~
or the following error (unintended)
g++ -march=native -std=c++11 -m64 -O2 -g -Wextra -Wall -Wshadow -lstdc++ -m64 -g -march=native -flto main.cpp -o main
main.cpp:7:43: error: expected unqualified-id before ‘using’
[[deprecated("use the namespace 'our'")]] using myvector = std::vector<float>;
^~~~~
main.cpp: In function ‘int main()’:
main.cpp:11:3: error: ‘myvector’ was not declared in this scope
myvector a;
^~~~~~~~
main.cpp:12:10: error: ‘a’ was not declared in this scope
return a.size();
^
The [[deprecated]] statement can not be placed at that position in the using statement (though I think in the line before would be nice).
Out of these
//using [[deprecated("use the namespace 'our'")]] myvector = std::vector<float>;
using myvector [[deprecated("use the namespace 'our'")]] = std::vector<float>;
//using myvector = [[deprecated("use the namespace 'our'")]] std::vector<float>;
//[[deprecated("use the namespace 'our'")]] typedef std::vector<float> myvector;
The first won't compile, clang gives a nicer error message in this case
clang++ -Weverything -std=c++14 -O2 -g main.cpp -o main
main.cpp:4:18: warning: alias declarations are incompatible with C++98 [-Wc++98-compat]
using vector = std::vector<float>;
^
main.cpp:7:11: warning: C++11 attribute syntax is incompatible with C++98 [-Wc++98-compat]
using [[deprecated("use the namespace 'our'")]] myvector = std::vector<float>;
^
main.cpp:7:11: error: an attribute list cannot appear here
using [[deprecated("use the namespace 'our'")]] myvector = std::vector<float>;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[[deprecated("use the namespace 'our'")]]
main.cpp:7:64: warning: alias declarations are incompatible with C++98 [-Wc++98-compat]
using [[deprecated("use the namespace 'our'")]] myvector = std::vector<float>;
^
main.cpp:13:3: warning: 'myvector' is deprecated: use the namespace 'our' [-Wdeprecated-declarations]
myvector a;
^
main.cpp:7:53: note: 'myvector' has been explicitly marked deprecated here
using [[deprecated("use the namespace 'our'")]] myvector = std::vector<float>;
^
main.cpp:14:10: warning: implicit conversion loses integer precision: 'size_type' (aka 'unsigned long') to
'int' [-Wshorten-64-to-32]
return a.size();
~~~~~~ ^~~~~~~~
5 warnings and 1 error generated.
So this points out that the attribute is "just in the wrong place" (and interestingly the deprecation warning gets later still printed as desired!)
The third prints a warning for gcc (though not what we want) warning: ignoring attributes applied to class type ‘std::vector<float>’ outside of definition [-Wattributes] or and error for clang 'deprecated' attribute cannot be applied to types.
The second is what we want.