编译器与数组分配的不同行为

问题描述

我最近发现一个有趣的行为g ++与MSVC ++ 2008相比。考虑这个小程序：

I recently found a interesting behaviour of g++ when compared with MSVC++ 2008. Consider this tiny program:

#include <cstdlib>

const int ARR_LENGTH = 512;

void doSomething( int iLen );

int main( int argc, char** argv )
{
    doSomething( ARR_LENGTH );
    return 0;
}

void doSomething( int iLen )
{
    int iTest[iLen];
    return;
}

它会编译吗？你有什么感想？根据我对C（或C ++）的知识，这不应该编译，因为我可以调用函数doSomething（）与任何我想要的整数，因此iTest数组的大小不能在编译时确定。然而，当我尝试用g ++编译这个，它的工作正常。现在我可以理解这里可能发生了什么 - 编译器注意到，我调用这个函数只有一次传递编译时常量作为参数。一些严重的优化在这里...但是当我试图编译这个使用MSVC ++ 2008，我得到这个：

Will it compile? What do you think? According to my knowledge of C (or C++ for that matter), this should NOT compile, since i can call the function doSomething() with any integer i want, so the size of iTest array cannot be determined at compile time. However, when i try to compile this with g++, it works just fine. Now i can understand what probably happened here - the compiler noticed that i call this function only once passing a compile-time constant as a parameter. Some serious optimizations going on here... But when i try to compile this using MSVC++ 2008, i get this:

1>c:\prj\test\test.cpp(15) : error C2057: expected constant expression
1>c:\prj\test\test.cpp(15) : error C2466: cannot allocate an array of constant size 0
1>c:\prj\test\test.cpp(15) : error C2133: 'iTest' : unknown size

我的问题是如何符合语言的定义（C标准（C ++标准））？这是很好的g ++做这样的优化（在这种情况下很容易看到，但第一次，我遇到它，它是在一个大项目，它没有什么意义一见钟情）。

My question is: how does this comply with the definition of the language (the C standard (C++ standard))? Is it just fine for g++ to do such an optimization (which in this case is easy to see, but the first time i encountered it, it was in a large project and it did not make much sense at first sight).

推荐答案

C99（C标准的最新版本）允许动态大小的数组。
但是，Visual Studio（仅支持C89）不支持此功能。

C99 (the most recent version of the C standard) does allow dynamically sized arrays.However, the feature is not supported by Visual Studio (which only implements C89 support)

在C ++中，它不是，可能永远不会有效。

In C++ it is not, and probably will never be, valid.

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