nvcc's pre-processor breaks boost/lexical_cast.hpp

[Rich Wareham <rjw57@…>]

The nvcc compiler fails to compile a trivial file which just #includes the boost/lexical_cast.hpp header. g++ compiles the file correctly.

rjw57@spica:~$ cat foo.cpp
#include <boost/lexical_cast.hpp>
rjw57@spica:~$ cat foo.cu
#include <boost/lexical_cast.hpp>
rjw57@spica:~$ /usr/bin/g++-4.4 -c foo.cpp
rjw57@spica:~$ /usr/bin/nvcc --compiler-bindir=/usr/bin/g++-4.4 -c foo.cu
/usr/include/boost/lexical_cast.hpp(352): error: expected an expression

1 error detected in the compilation of "/tmp/tmpxft_00001520_00000000-4_foo.cpp1.ii".

The problem is with nvcc's pre-processor which can be observed by using -E and grepping for // == 1 in the output:

rjw57@spica:~$ /usr/bin/g++-4.4 -E -o foo-gcc.i foo.cpp
rjw57@spica:~$ /usr/bin/nvcc --compiler-bindir=/usr/bin/g++-4.4 -E -o foo-nvcc.i foo.cu
rjw57@spica:~$ ack-grep '\/\/ == 1' foo*.i
foo-nvcc.i
73304:            static const std::size_t value = std::numeric_limits<Source>::is_signed + std::numeric_limits<Source>::is_specialized + // == 1 std::numeric_limits<Source>::digits10 * 2;

The nvcc pre-processor does not strip comments from the input to the compiler and appears to be removing new-lines. The gcc compiler does strip the comments.

This is possibly a bug in nvcc but I suggest a workaround could be put into boost itself without too much pain. Also I suspect it'd take a standards lawyer to work out who is in the wrong here.

The troublesome line (number 345 in 1.4.2) is in the boost/lexical_cast.hpp file. This problem is still present in the SVN trunk.

...
        template<class Source>
        struct lcast_src_length_integral
        {
#ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS
            BOOST_STATIC_CONSTANT(std::size_t, value =
                  std::numeric_limits<Source>::is_signed +
                  std::numeric_limits<Source>::is_specialized + // == 1
                  std::numeric_limits<Source>::digits10 * 2
              );
#else
            BOOST_STATIC_CONSTANT(std::size_t, value = 156);
            BOOST_STATIC_ASSERT(sizeof(Source) * CHAR_BIT <= 256);
#endif
        };
...

A suggested workaround is removing the C++-style comment or replacing it with a C-style one?

[Rich Wareham <rjw57@…>]

My local friendly standards lawyer points out it is a problem with nvcc (comments should be stripped before processing macros apparently) but nonetheless, it seems that boost could defensively code around this issue.

[Antony Polukhin]

(In [73707]) FIxes #5631 Workaround for nvcc bug

[Antony Polukhin]

Great thanks for testing on nvcc!
Sorry for not responding sooner. If you find any new bugs in lexical_cast, report them and put my name in "Owned by:" field.

Does nvcc now fails on some other //? Does now lexical_cast.hpp compiles on nvcc?