I understand that VLAs are part of C99 but not C++0x. My question is then why and how g++ would compile code with VLAs. I wrote a test case below:
test.c
#include "stdio.h"
int main(int argc, char *argv[]) {
int i;
int array[argc];
for(i = 0; i < argc; i++)
array[i] = i;
for (i = 0; i < argc; i++)
printf("%d ", i);
puts("");
return 0;
}
I also copied it into a file called test.cpp. The following all compile:
$ gcc test.c -o test // expected
$ g++ test.cpp -o test // not expected
$ g++ test.cpp -o test -x c++ // really not expected
$ g++ test.cpp -o test -std=c++11 // really not expected
I have read the SO questions Variable length arrays in C++? and What is the difference between g++ and gcc? but I cannot find an answer as to why g++ would do this or how it does. This seems like extremely unpredictable behavior as the standard specifies no VLAs. Does g++ recognize this as C code and compile with cc1? Is there a way to force g++ to stick to the standard?
Additionally, I can break it with the following:
test2.cpp
#include "stdio.h"
#include <vector>
int main(int argc, char *argv[]) {
int i;
int array[argc];
for(i = 0; i < argc; i++)
array[i] = i;
for (i = 0; i < argc; i++)
printf("%d ", i);
puts("");
std::vector<decltype(array)> temp;
return 0;
}
Which then fails to compile:
$ g++ test2.cpp -o test2 -std=c++11
test2.cpp: In function ‘int main(int, char**)’:
test2.cpp:16:32: error: ‘int [(((sizetype)(((ssizetype)argc) + -1)) + 1)]’ is a variably modified type
std::vector<decltype(array)> temp;
^
test2.cpp:16:32: error: trying to instantiate ‘template<class> class std::allocator’
test2.cpp:16:32: error: template argument 2 is invalid
test2.cpp:16:38: error: invalid type in declaration before ‘;’ token
std::vector<decltype(array)> temp;
This error appears to mean that I am still allowed to declare VLAs, I just can't have them as an argument to decaltype. This makes everything seem much stranger. I can understand the obvious answer of, "It's wrong, so just don't do it" or, "if you're using C++, use std::vector" but that seems like an end-game type of answer and doesn't quite get to the root of the issue.
I cannot compile on Visual Studios to compare as I do not currently have access to it.
System information in case that is the difference:
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04.1 LTS
Release: 14.04
Codename: trusty
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.2-19ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)
