问题描述
我知道VLA是C99的一部分，但不是C ++ 0x。我的问题是为什么和如何 g ++ 将编译代码与VLAs。我在下面写了一个测试用例：
test.c
#includestdio.h
int main（int argc，char * argv []）{
int i;
int array [argc];
for（i = 0; i array [i] = i;
for（i = 0; i printf（％d，i）
puts（）;
return 0;
}
我也将它复制到一个名为 cpp 。以下所有编译：
$ gcc test.c -o test // expected
$ g ++ test.cpp - o测试//不期望
$ g ++ test.cpp -o test -x c ++ //真的不期望
$ g ++ test.cpp -o test -std = c ++ 11 //真的不期望
我已阅读SO问题和但是我找不到为什么 g ++ 会做这个或者怎么做的答案是的。这似乎是非常不可预测的行为，因为标准没有指定VLA。 g ++ 将此识别为C代码，并使用 cc1 编译？有没有办法强制 g ++ 坚持标准？
此外， ：
test2.cpp
#includestdio.h
#include< vector>
int main（int argc，char * argv []）{
int i;
int array [argc];
for（i = 0; i array [i] = i;
for（i = 0; i printf（％d，i）
puts（）;
std :: vector< decltype（array）>温度
return 0;
}
然后无法编译：
$ g ++ test2.cpp -o test2 -std = c ++ 11
test2.cpp：在函数'int main（int，char **）' ：
test2.cpp：16：32：error：'int [（（sizetype）（（ssizetype）argc）+ -1））+ 1）]'是一个可变修改类型
std :: vector< decltype（array）>温度
^
test2.cpp：16：32：error：试图实例化'template< class> class std :: allocator'
test2.cpp：16：32：error：模板参数2无效
test2.cpp：16：38：error： $ b std :: vector< decltype（array）>温度
这个错误似乎意味着我仍然可以声明VLA，我只是不能作为 decaltype 的参数。这使得一切看起来很陌生。我可以理解明显的答案，这是错误的，所以只是不要这样做或者，如果你使用C ++，使用std :: vector，但这似乎是一个结束游戏类型的答案，
我无法在Visual Studio上编译以进行比较，因为我目前无法访问它。
系统信息，如果有区别：
$ lsb_release -a
LSB模块。
经销商ID：Ubuntu
描述：Ubuntu 14.04.1 LTS
发行版：14.04
代号：trusty
$ g ++ -v
内置规格。
COLLECT_GCC = g ++
COLLECT_LTO_WRAPPER = / usr / lib / gcc / x86_64-linux-gnu / 4.8 / lto-wrapper
目标：x86_64-linux-gnu
配置为：。 ./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 --unhout-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
线程模型：posix
gcc版本4.8.2（Ubuntu 4.8.2-19ubuntu1）
VLAs是C ++模式下的gcc（和clang）编译器扩展，记录。默认情况下会启用这些扩展。
如果使用 -pedantic 编译，您应该会看到警告。
有趣的是，你不会使用gcc4.9和 -std = c ++ 14 或 -std = c ++ 1y 。当他们解释时，他们删除了 -Wvla 警告在C ++ 14模式，因为VLA曾经在标准的草案。它们实际上并没有在最终版本的C ++ 14中实现，并且可能出现在C ++ 17中。
clang在C ++ 14模式下仍然警告，它应该。
编辑： gcc 5.1在C ++ 14模式警告， $ b
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)
VLAs are a gcc (and clang) compiler extension in C++ mode, documented here. These extensions are enabled by default.
You should see a warning if you compile with -pedantic.
Funnily, you will not with gcc4.9 and -std=c++14 or -std=c++1y. As they explain here, they removed the -Wvla warning in C++14 mode because VLAs were once in the draft for the standard. They did not actually make it in the final version of C++14 and might appear in C++17.
clang still warns in C++14 mode, as it should.
Edit: gcc 5.1 warns in C++14 mode too, so that got updated correctly.
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