Why don't complex-number literals work in clang? - c++

Question

When I run this code on ideone.com, it prints (2,3):
#include <iostream>
#include <complex>
int main() {
std::complex<double> val = 2 + 3i;
std::cout << val << std::endl;
return 0;
}
But when I use clang on macOS 10.11.6, I get no errors or warnings, but the output is (2,0):
$ clang --version
Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.6.0
$ clang -lc++ test.cpp && ./a.out
(2,0)
What happened to the imaginary part? Am I doing something wrong?

Answer 1

I believe for this first example the compiler is using a GNU extension:
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal number
suffixes as GNU extensions. When this option is turned off these
suffixes are treated as C++11 user-defined literal numeric suffixes.
This is on by default for all pre-C++11 dialects and all GNU dialects:
-std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...).
When I run it with clang I get (are you using -Wall -pedantic? :)):
warning: imaginary constants are a GNU extension
[-Wgnu-imaginary-constant]
Either way, your code is not standard compliant. To use C++14 literals make the code:
#include <iostream>
#include <complex>
using namespace std::complex_literals;
int main() {
std::complex<double> val = 2.0 + 3i;
std::cout << val << std::endl;
return 0;
}
From the documentation:
These operators are declared in the namespace
std::literals::complex_literals, where both literals and
complex_literals are inline namespaces. Access to these operators can
be gained with using namespace std::literals, using namespace
std::complex_literals, and using namespace
std::literals::complex_literals.