'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.

Answer 3

how about

float myRoundFunc(float toRound)
{
  return std::ceil(toRound - 0.5);
}

That way there is no comparison and it will round properly.

Answer 4

old topic but maybe usefull for the ones searching in google:

//TODO: C++11 got round() function defined in math.h
float distance = sqrt((float)(plus_x * plus_x + plus_y * plus_y));
if (distance >= floor(distance)+0.5) distance = ceil(distance);
else distance = floor(distance);

Answer 5

In the C++03 standard, the normative reference for the standard C library is ISO/IEC 9899:1990 (C90), which does not include the round function. In C++11, it is ISO/IEC 9899:1999 (C99), which does include the round function.

The contents of <cmath> in each C++ standard are the same as math.h from the corresponding standard C library except for a few changes (§26.8/4 in C++11):

The contents of these headers [<cmath> and <cstdlib>] are the same as the Standard C library headers <math.h> and <stdlib.h> respectively [...]

They are, however, in namespace scope of std (§17.6.1.2/4 in C++11):

In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope (3.3.6) of the namespace std.

So, to wrap it up, the round function is not available in C++03, so compile with the -std=c++0x option and refer to it as std::round.