'round' is not a member of 'std'

Question

I'm try to compile this code:

#include <cmath>
double gravity (double level) {
    return 0.02 * std::round(level);
}

But GCC is telling me:

error: 'round' is not a member of 'std'

I know I've used the round function many times in ISO C++98 before. Unusually, round and ::round both work.

What gives?

Update: I was compiling with g++ -std=c++98 -Wall -pedantic. Switching to std=c++0x works.

But why do the unqualified/anonymous round and ::round both work if std::round doesn't?

Answer 1

The std::round functions are C++11, so you would need to compile with C++11 or a more recent standard enabled.

Answer 2

I've done a bit of research, here's what I've found:

round is defined in ISO C++11, as it contains the ISO C99 standard library.

round is not part of the ISO C++98, which uses the ISO C90 standard library.

That's why it's not in namespace std for C++98.

But g++ is (incorrectly) including the C99 headers, even when compiled with -std=c++98 -pedantic, which should disable all non-standard stuff:

GNU_SOURCE is defined by G++ and ... it implies _USE_ISOC99

(from http://gcc.gnu.org/ml/gcc/2002-09/msg00580.html)

This is why ::round works.

This apparently is a bug in GCC: Why does GCC allow use of round() in C++ even with the ansi and pedantic flags?

Other C++ compilers may not provide a round function (since it's not required by the standard), so I should define my own.